Kids Research Express

Lorentz contraction

2011-02-19T21:40:00.000-08:00

Lorentz contraction (lôr`ĕnts), in physics, contraction or foreshortening of a moving body in the direction of its motion, proposed by H. A. Lorentz on theoretical grounds and based on an earlier suggestion by G. F. Fitzgerald; it is sometimes called the Fitzgerald, or Lorentz-Fitzgerald, contraction. The Lorentz contraction hypothesis was put forward in an attempt to explain the negative result of the Michelson-Morley experiment of 1887 designed to demonstrate the earth's absolute motion through space (see ether; relativity). The hypothesis held that any material body is contracted in the direction of its motion by a factor 1−v²/c², where v is the velocity of the body and c is the velocity of light. Although the Lorentz contraction did not succeed entirely in reconciling the results of the Michelson-Morley experiment with classical theory, it did serve as the basis for the mathematics of Einstein's theory of relativity. The equations used in relativity theory to change from a coordinate system, or frame of reference, in which the observer is at rest to a second system that is moving at constant velocity with respect to the first system are known as the Lorentz transformation. The Lorentz transformation will result in a stationary observer recording an effect equivalent to the Lorentz contraction when observing an object in uniform motion relative to his system of coordinates. Einstein showed, however, that this effect is due not to the actual deformation of the body in question, as Lorentz had originally supposed, but to a change in the way space and time are measured.

Atomic clock

2011-02-19T20:54:00.001-08:00

Atomic clock, electric or electronic timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits. Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks. Much greater accuracy can be attained by using the oscillations of atoms or molecules. Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, the clock is controlled by a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices.

The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules. The error between a pair of such clocks, i.e., the difference in indicated time if both were started at the same instant and later compared, was typically about one second in three thousand years. In 1955 the first cesium-beam clock (a device that uses as a reference the exact frequency of the microwave spectral line emitted by cesium atoms) was placed in operation at the National Physical Laboratory at Teddington, England. It is estimated that such a clock would gain or lose less than a second in three million years. The U.S. standard is the NIST-F1, which went into service in 1999 and should neither gain nor lose a second in 20 million years. A fountain atomic clock, the NIST F-1 consists of a 3-foot vertical tube inside a taller structure. It uses lasers to cool cesium atoms, forming a ball of atoms that lasers then toss into the air, much like one would toss a tennis ball, creating a fountain effect. This allows the atoms to be observed for much longer than could be done with any previous clock.

Many of the world's nations maintain atomic clocks at standards laboratories, the time kept by these clocks being averaged to produce a standard called international atomic time (TAI). Highly accurate time signals from these standards laboratories are broadcast around the globe by shortwave-radio broadcast stations or by artificial satellites, the signals being used for such things as tracking space vehicles, electronic navigation systems, and studying the motions of the earth's crust. The accuracy of these clocks made possible an experiment confirming an important prediction of Einstein's theory of relativity. Prototypes of atomic clocks using atoms such as hydrogen or beryllium could be still thousands of times more accurate. For example, researchers at the U.S. National Institute of Standards and Technology have demonstrated an atomic clock based on an energy transition in a single trapped mercury ion (a mercury atom that is missing one electron) that has the potential to be up to 1,000 times more accurate than current atomic clocks.

Astronomy: Modern Techniques, Discoveries, and Theories

2011-02-19T06:14:00.000-08:00

Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition (see stellar structure and stellar evolution). The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars (see Hertzsprung-Russell diagram). With the construction of ever more powerful telescopes (see observatory), the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding (see Hubble's law). Using Cepheid variables as distance indicators, Harlow Shapley determined the size and shape of our galaxy, the Milky Way. During World War II Walter Baade defined two "populations" of stars, and suggested that an examination of these different types might trace the spiral shape of our own galaxy (see stellar populations). In 1951 a Yerkes Observatory group led by William W. Morgan detected evidence of two spiral arms in the Milky Way galaxy.

Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, have been formulated (see cosmology). Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In 1963, the moon passed in front of the radio source 3C-273, allowing Cyril Hazard to calculate the exact position of the source. With this information, Maarten Schmidt photographed the object's spectrum using the 200-in. (5-m) reflector on Palomar Mt., then the world's largest telescope. He interpreted the result as coming from an object, now known as a quasar, at an extreme distance and receding from us at a substantial fraction of the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell discovered a radio source a few hundred light years away featuring regular pulses at intervals of about 1 second with an accuracy of repetition of one-millionth of a second. This was the first discovered pulsar, a rapidly spinning neutron star emitting lighthouse-type beams of energy, the end result of the death of a star in a supernova explosion.

The discovery by Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of radio astronomy. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth's atmosphere make space-based observations necessary for infrared, ultraviolet, gamma-ray, and X-ray astronomy. The Surveyor and Apollo spacecraft of the late 1960s and early 1970s helped launch the new field of astrogeology. A series of interplanetary probes, such as Mariner 2 (1962) and 5 (1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager 1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn, Uranus, and Neptune; more recently, the Magellan probe to Venus (1990) and the Galileo probe to Jupiter (1995) have continued this line of research (see satellite, artificial; space probe). The Hubble Space Telescope, launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments.

Development of Modern Astronomy

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The Copernican Revolution

After the fall of Rome, European astronomy was largely dormant, but significant work was carried out by the Muslims and the Hindus. It was by way of Arabic translations that Greek astronomy reached medieval Europe. One of the great landmarks of the revival of learning in Europe was the publication (1543) by Nicolaus Copernicus (1473–1543) of his De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). According to the Copernican system, the earth rotates on its axis and, with all the other planets, revolves around the sun. The assertion that the earth is not the center of the universe was to have profound philosophical and religious consequences. Copernicus's principal claim for his new system was that it made calculations easier. He retained the uniform circular motion of the Ptolemaic system, but by placing the sun at the center, he was able to reduce the number of epicycles. Copernicus also determined the sidereal periods (time for one revolution around the sun) of the planets and their distance from the sun relative to the sun-earth distance (see astronomical unit).

Brahe and Kepler

The great astronomer Tycho Brahe (1546–1601) was principally an observer; a conservative in matters of theory, he rejected the notion that the earth moves. Under the patronage of King Frederick II, Tycho established Uraniborg, a superb observatory on the Danish island of Hveen. Over a period of 20 years (1576–97), he and his assistants compiled the most accurate and complete astronomical observations to that time. At his death his records passed to Johannes Kepler (1571–1630), who had been his last assistant. Kepler spent nearly a decade trying to fit Tycho's observations, particularly of Mars, into an improved system of heliocentric circular motion. At last, he conceived the idea that the orbit of Mars was an ellipse with the sun at one focus. This led him to the three laws of planetary motion that bear his name (see Kepler's laws).

Galileo's Telescope

Galileo Galilei (1564–1642) made fundamental discoveries in both astronomy and physics; he is perhaps best described as the founder of modern science. Galileo was the first to make astronomical use of the telescope. His discoveries of the four largest moons of Jupiter and the phases of Venus were persuasive evidence for the Copernican cosmology. His discoveries of craters on the moon and blemishes on the sun (sunspots) discredited the ancient belief in the perfection of the heavens. These findings were announced in The Sidereal Messenger, a small book published in 1610. Galileo's Dialogue on the Two Chief Systems of the World (1632) was an eloquent argument for the Copernican system over the Ptolemaic. However, Galileo was called before the Inquisition and forced to renounce publicly all doctrines considered contrary to Scripture.

Astrophysical Discoveries

Isaac Newton (1642–1727), possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and physics. His laws of motion and theory of universal gravitation provided a physical, dynamic basis for the merely descriptive laws of Kepler. Until well into the 19th cent., all progress in astronomy was essentially an extension of Newton's work. Edmond Halley's prediction that the comet of 1682 would return in 1758 was refined by A. C. Clairault, who included the perturbing effects of Jupiter and Saturn on the orbit to calculate the nearly exact date of the return of the comet. In 1781, William Herschel accidentally discovered a new plane!t, eventually named Uranus. Discrepancies between the observed and theoretical orbits of Uranus indicated the existence of a still more distant planet that was affecting Uranus's motion. J. C. Adams and U. J. J. Leverrier independently calculated the position where the new planet, Neptune, was actually discovered (1846). Similar calculations for a large "Planet X" led in 1930 to the discovery of Pluto, now classed as a dwarf planet.

By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of Leonhard Euler, J. L. Lagrange, P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered Ceres, the first of many asteroids. When Ceres was lost to view, C. F. Gauss applied the advanced gravitational techniques to compute the position where the asteroid was subsequently rediscovered. In 1838, F. W. Bessel made the first measurement of the distance to a star; using the method of parallax with the earth's orbit as a baseline, he determined the distance of the star 61 Cygni to be 60 trillion mi (about 10 light-years), a figure later shown to be 40% too large.

Ancient Astronomy

2011-02-19T05:48:00.000-08:00

Astronomy is the oldest of the physical sciences. In many early civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict future events. The first practical function of astronomy was to provide a basis for the calendar, the units of month and year being determined by astronomical observations. Later, astronomy served in navigation and timekeeping. The Chinese had a working calendar as early as the 13th cent. B.C. About 350 B.C., Shih Shen prepared the earliest known star catalog, containing 800 entries. Ancient Chinese astronomy is best known today for its observations of comets and supernovas. The Babylonians, Assyrians, and Egyptians were also active in astronomy. The earliest astronomers were priests, and no attempt was made to separate astronomy from astrology. In fact, an early motivation for the detailed study of planetary positions was the preparation of horoscopes.

Greek Innovations

The highest development of astronomy in the ancient world came with the Greeks in the period from 600 B.C. to A.D. 400. The methods employed by the Greek astronomers were quite distinct from those of earlier civilizations, such as the Babylonian. The Babylonian approach was numerological and best suited for studying the complex lunar motions that were of overwhelming interest to the Mesopotamian peoples. The Greek approach, on the contrary, was geometric and schematic, best suited for complete cosmological models. Thales, an Ionian philosopher of the 6th cent. B.C., is credited with introducing geometrical ideas into astronomy. Pythagoras, about a hundred years later, imagined the universe as a series of concentric spheres in which each of the seven "wanderers" (the sun, the moon, and the five known planets) were embedded. Euxodus developed the idea of rotating spheres by introducing extra spheres for each of the planets to account for the observed complexities of their motions. This was the beginning of the Greek aim of providing a theory that would account for all observed phenomena. Aristotle (384–322 B.C.) summarized much of the Greek work before him and remained an absolute authority until late in the Middle Ages. Although his belief that the earth does not move retarded astronomical progress, he gave the correct explanation of lunar eclipses and a sound argument for the spherical shape of the earth.

The Alexandrian School and the Ptolemaic System

The apex of Greek astronomy was reached in the Hellenistic period by the Alexandrian school. Aristarchus (c.310–c.230 B.C.) determined the sizes and distances of the moon and sun relative to the earth and advocated a heliocentric (sun-centered) cosmology. Although there were errors in his assumptions, his approach was truly scientific; his work was the first serious attempt to make a scale model of the universe. The first accurate measurement of the actual (as opposed to relative) size of the earth was made by Eratosthenes (284–192 B.C.). His method was based on the angular difference in the sun's position at the high noon of the summer solstice in two cities whose distance apart was known.

The greatest astronomer of antiquity was Hipparchus (190–120 B.C.). He developed trigonometry and used it to determine astronomical distances from the observed angular positions of celestial bodies. He recognized that astronomy requires accurate and systematic observations extended over long time periods. He therefore made great use of old observations, comparing them to his own. Many of his observations, particularly of the planets, were intended for future astronomers. He devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon.

Ptolemy (A.D. 85–165) applied the scheme of epicycles to the planets as well. The resulting Ptolemaic system was a geometrical representation of the solar system that predicted the motions of the planets with considerable accuracy. Among his other achievements was an accurate measurement of the distance to the moon by a parallax technique. His 13-volume treatise, the Almagest, summarized much of ancient astronomical knowledge and, in many translations, was the definitive authority for the next 14 centuries.

space shuttle

2011-02-18T15:41:00.000-08:00

Space shuttle, reusable U.S. space vehicle. Developed by the National Aeronautics and Space Administration (NASA), it consists of a winged orbiter, two solid-rocket boosters, and an external tank. As with previous spacecraft, the shuttle is launched from a vertical position. Liftoff thrust is derived from the orbiter's three main liquid-propellant engines and the boosters. After 2 min the boosters use up their fuel, separate from the spacecraft, and—after deployment of parachutes—are recovered following splashdown. After about 8 min of flight, the orbiter's main engines shut down; the external tank is then jettisoned and burns up as it reenters the atmosphere. The orbiter meanwhile enters orbit after a short burn of its two small Orbiting Maneuvering System (OMS) engines. To return to earth, the orbiter turns around, fires its OMS engines to reduce speed, and, after descending through the atmosphere, lands like a glider. Five different orbiters—Columbia, Challenger, Atlantis, Discovery, and Endeavour—have seen service; two have been lost in accidents.

Following four orbital test flights (1981–82) of the space shuttle Columbia, operational flights began in Nov., 1982. On Jan. 28, 1986, the Challenger exploded shortly after takeoff, killing all seven astronauts. The commission that investigated the disaster determined that the failure of the O-ring seal in one of the solid fuel rockets was responsible. Shuttle flights were halted until Sept., 1988, while design problems were corrected, and then resumed on a more conservative schedule. NASA was forced to reemphasize expendable rockets to reduce the cost of placing payloads in space.

A second disaster struck the shuttle program on Feb. 1, 2003, when the Columbia broke up during reentry, killing the seven astronauts on board. NASA again halted shuttle launches, and a special commission was appointed to investigate the accident. It is believed that damage to the left wing, which could have been caused by insulation that separated from the external fuel tank during launch, ultimately permitted superheated gas to flow into the wing, weaken it, and cause its failure. Modifications were made to external fuel tank and other parts of the shuttle, and shuttle flights resumed in July, 2005. Further problems with fuel tank insulation that developed during that launch led to the suspension of additional flights for a year while the problems were corrected.

Missions of the space shuttle have included the transport of the Spacelab scientific workshop (see space exploration) and the insertion into orbit of the Hubble Space Telescope (1990), the Galileo space probe (1989), the Chandra X-Ray Observatory (1999), and a wide variety of communications, weather, scientific, and defense-related satellites. Other notable achievements of the shuttle program include the rescue and repair of disabled satellites (including the Hubble Space Telescope in 1993 and 1999) and the first three-person spacewalk (1992). In 1995 the Endeavour's mission of Mar. 2–18 set the record for the longest shuttle flight. It was also in 1995 that the crew of Atlantis accomplished the first of nine shuttle-Mir (Russian space station) docking maneuvers and crew transfers, which were designed to pave the way for the assembly of the International Space Station (ISS). The crew of Discovery made the ninth and final docking in 1998, five months before the Russians orbited Zarya, the first ISS module. A month later the astronauts aboard Endeavour initiated the first assembly sequence of the ISS, linking the Unity module, a passageway that will connect living and work areas of the station, to Zarya. In 1999 the Discovery crew accomplished the first docking of a shuttle with the ISS during a mission to supply the two modules with tools and cranes.

Development of Rockets

2011-02-18T15:34:00.000-08:00

The invention of the rocket is generally ascribed to the Chinese, who as early as A.D. 1000 stuffed gunpowder into sections of bamboo tubing to make military weapons of considerable effectiveness. The 13th-century English monk Roger Bacon introduced to Europe an improved form of gunpowder, which enabled rockets to become incendiary projectiles with a relatively long range. Rockets subsequently became a common if unreliable weapon. Major progress in design resulted from the work of William Congreve, an English artillery expert, who built a 20-lb (9-kg) rocket capable of traveling up to 2 mi (3 km). In the late 19th cent., the Austrian physicist Ernst Mach gave serious theoretical consideration to supersonic speeds and predicted the shock wave that causes sonic boom.

The astronautical use of rockets was cogently argued in the beginning of the 20th cent. by the Russian Konstantin E. Tsiolkovsky, who is sometimes called the "father of astronautics." He pointed out that a rocket can operate in a vacuum and suggested that multistage liquid-fuel rockets could escape the earth's gravitation. The greatest name in American rocketry is Robert H. Goddard, whose pamphlet A Method for Reaching Extreme Altitudes anticipated nearly all modern developments. Goddard launched the first liquid-fuel rocket in 1926 and demonstrated that rockets could be used to carry scientific apparatus into the upper atmosphere. His work found its most receptive audience in Germany. During World War II, a German team under Wernher von Braun developed the V-2 rocket, which was the first long-range guided missile. The V-2 had a range greater than 200 mi (322 km) and reached velocities of 3,500 mi (5,600 km) per hr.

After the war, rocket research in the United States and the Soviet Union intensified, leading to the development first of intercontinental ballistic missiles and then of modern spacecraft. Important U.S. rockets have included the Redstone, Jupiter, Atlas, Titan, Agena, Centaur, and Saturn carriers. Saturn V, the largest rocket ever assembled, developed 7.5 million lb (3.4 million kg) of thrust. A three-stage rocket, it stood 300 ft (91 m) high exclusive of payload and with the Apollo delivered a payload of 44 tons to the moon. Rockets presently being used to launch manned and unmanned missions into space include the U.S. Athena 1 and 2, Taurus, Titan 2 and 4B, Delta 2, 3, and 4, Atlas 2 ,3, and 5, and STS or space shuttle; the Chinese Long March 2C, 2E, and 2F; the Russian Soyuz and Proton K and M; the Japanese H-2A; the European Space Agency's Ariane 5 series; the Indian PSLV (Polar Satellite Launch Vehicle); the Israeli Shavit 2; the Brazilian VSV-30; and the multinational, private Sea Launch Zenit-3SL, which uses a converted oil platform located some 1,400 mi (2,250 km) southeast of Hawaii.

rocket

2011-02-18T15:29:00.000-08:00

Rocket, any vehicle propelled by ejection of the gases produced by combustion of self-contained propellants. Rockets are used in fireworks, as military weapons, and in scientific applications such as space exploration.

Rocket Propulsion

The force acting on a rocket, called its thrust, is equal to the mass ejected per second times the velocity of the expelled gases. This force can be understood in terms of Newton's third law of motion, which states that for every action there is an equal and opposite reaction. In the case of a rocket, the action is the backward-streaming flow of gas and the reaction is the forward motion of the rocket. Another way of understanding rocket propulsion is to realize that tremendous pressure is exerted on the walls of the combustion chamber except where the gas exits at the rear; the resulting unbalanced force on the front interior wall of the chamber pushes the rocket forward. A common misconception, before space exploration pointed up its obvious fallacy, holds that a rocket accelerates by pushing on the atmosphere behind it. Actually, a rocket operates more efficiently in outer space, since there is no atmospheric friction to impede its motion.

Rocket Propellants

The most vital component of any rocket is the propellant, which accounts for 90% to 95% of the rocket's total weight. A propellant consists of two elements, a fuel and an oxidant; engines that are based on the action-reaction principle and that use air instead of carrying their own oxidant are properly called jets. Propellants in use today include both liquefied gases, which are more powerful, and solid explosives, which are more reliable; the space shuttle's main engines use liquid propellant, while its boosters are solid-fuel rockets. The chemical energy of the propellants is released in the form of heat in the combustion chamber.

A typical liquid engine uses hydrogen as fuel and oxygen as oxidant; a typical solid propellant is nitroglycerine. In the liquid engine, the fuel and oxidant are stored separately at extremely low temperatures; in the solid engine, the fuel and oxidant are intimately mixed and loaded directly into the combustion chamber. A solid engine requires an ignition system, as does a liquid engine if the propellants do not ignite spontaneously on contact.

The efficiency of a rocket engine is defined as the percentage of the propellant's chemical energy that is converted into kinetic energy of the vehicle. During the first few seconds after liftoff, a rocket is extremely inefficient, for at least two unavoidable reasons: High power consumption is required to overcome the inertia of the nearly motionless mass of the fully fueled rocket; and in the lower atmosphere, power is wasted overcoming air resistance. Once the rocket gains altitude, however, it becomes more efficient. as the trajectory, at first vertical, curves into a suborbital arc or into the desired orbit.

see: Development of Rockets

Photochemistry

2011-02-17T17:39:00.000-08:00

Photochemistry, study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible light or ultraviolet radiation. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photon, and go to a higher-energy state, or excited state (see quantum theory). Such a molecule is then much more reactive than a ground-state molecule and can undergo entirely different reactions than the more stable molecule, following several different reaction pathways. One possibility is that it can simply emit the absorbed light and fall back to the ground state. This process, called chemiluminescence, is illustrated by various glow-in-the-dark objects. Another possibility is for the molecule to take part in a photo-induced chemical reaction; it may break apart (photodissociate), rearrange, isomerize, dimerize, eliminate or add small molecules, or even transfer its energy to another molecule. Photochromic compounds—compounds that change color reversibly in going from the dark to the light—are generally compounds that are capable of reversible isomerization, or rearrangement. In the absence of light, the compound exists in its most stable form, which exhibits a particular color; in the presence of light, the compound goes to a less stable form, which exhibits a different color. After removal of the light, the compound will revert back to its original state. The best-known and most important photochemical reaction is photosynthesis, the complex, chlorophyll-catalyzed synthesis of sugars from carbon dioxide and water in the presence of light. Other extremely important and complex photochemical reactions take place in the eye. Photochemistry is indispensible to industries involved with dyes, photography, television, and many other applications of light and color.

Aerosol dispenser

2011-02-17T17:36:00.000-08:00

Aerosol dispenser, device designed to produce a fine spray of liquid or solid particles that can be suspended in a gas such as the atmosphere. The dispenser commonly consists of a container that holds under pressure the substance to be dispersed (e.g., paints, insecticides, medications, and hair sprays) and a liquefied gas propellant. When a valve is released, the propellant forces the substance through an atomizer and out of the dispenser in the form of a fine spray. These devices are more properly termed spray dispensers rather than aerosol dispensers because the particles of the dispersed substance are usually larger than the particles of a true aerosol (see colloid), such as a fog or a smoke. Freon was the most common aerosol propellant, but its use has been banned because it is believed to contribute to destruction of the ozone layer of the stratosphere; common propellants now include propane, butane, and other hydrocarbons.

chronometer

2011-02-17T15:49:00.000-08:00

Chronometer, mechanical timekeeping device of great accuracy, particularly one used for determining longitude (see latitude and longitude) at sea. Early weight- and pendulum-driven clocks were inaccurate because of friction and temperature changes and could not be used at sea because of the ship's motion. In 1735 John Harrison invented and constructed the first of four practical marine timekeepers. The modern marine chronometer is suspended to remain horizontal whatever the inclination of the ship and differs in parts of its mechanism from the ordinary watch. A chronometer may provide timekeeping accurate to within 0.1 second per day. See also Ferdinand Berthoud.

Electrocardiography

2011-02-16T05:33:00.000-08:00

Electrocardiography (ĭlĕk'trōkärdēŏg`rəfē), science of recording and interpreting the electrical activity that precedes and is a measure of the action of heart muscles. Since 1887, when Augustus Waller demonstrated the possibility of measuring such action, physicians and physiologists have recorded it in order to study the heart's normal behavior and to provide a method for diagnosing abnormalities. Electrical current associated with contraction of the heart muscles passes through the various tissues and reaches the surface of the body. What is actually recorded is the change in electrical potential on the body surface. The first practical device for recording the activity of the heart was the string galvanometer developed by William Einthoven in 1903. In this device a fine quartz string is suspended vertically between the poles of a magnet. The string is deflected in response to changes in electrical potential and its movement can be optically enlarged and photographed, or, if an immediately visible record is desired, the string's movement can be recorded on a sheet of paper. A more sophisticated form of the electrocardiograph employs a vacuum-tube amplifier. The greatly amplified current from the body deflects a mirror galvanometer that causes a beam of light to move across a light-sensitive film. When an electrocardiograph is taken, electrodes (leads) are attached to the extremities and to the left chest. The recordings obtained in this manner are called electrocardiograms, or more simply EKG's or ECG's. A normal EKG shows a sequence of three waves arbitrarily labeled P, QRS, and T. The P wave is a small, low-amplitude wave produced by the excitation of the atria of the heart. It is followed by a resting interval that marks the passage of electrical impulses into the ventricles. Following this interval comes the QRS wave, a rapid, high-amplitude wave marking ventricular excitation, and then a slow-building T wave denoting ventricular recovery. Abnormalities may be noted from deviation in wave form, height, direction, or duration. The type of abnormal wave may sometimes indicate the type of heart disorder. Usually the physician must associate the EKG with other clinical observations to determine the cause of the abnormality.

nutation

2011-02-16T00:26:00.000-08:00

Nutation, in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.6 years, the same as that of the precession of the moon's nodes, as opposed to the nearly 26,000-year period of the precession of the equinoxes. Nutation was discovered by the English astronomer James Bradley in 1728 but was not explained until 20 years later.

The Elements through the Ages

2011-01-23T06:48:00.000-08:00

Some elements have been known since antiquity. Gold ornaments from the Neolithic period have been discovered. Gold, iron, copper, lead, silver, and tin were used in Egypt and Mesopotamia before 3000 B.C. However, recognition of these metals as chemical elements did not occur until modern times.

Greek Concept of the Elements

The Greek philosophers proposed that there are basic substances from which all things are made. Empedocles proposed four basic "roots," earth, air, fire, and water, and two forces, harmony and discord, joining and separating them. Plato called the roots stoicheia (elements). He thought that they assume geometric forms and are made up of some more basic but undefined matter. A different theory, that of Leucippus and his followers, held that all matter is made up of tiny indivisible particles (atomos).

This theory was rejected by Aristotle, who expanded on Plato's theory. Aristotle believed that different forms (eidos) were assumed by a basic material, which he called hulé. The hulé had four basic properties, hotness, coldness, dryness, and moistness. The four elements differ in their embodiment of these properties; fire is hot and dry, earth cold and dry, water cold and moist, and air hot and moist. Although Aristotle proposed that an element is "one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others," he thought the metals to be made of water, and called mercury "silver water" (chutos arguros). His idea that matter was a single basic substance that assumed different forms led to attempts by the alchemists to transmute other metals into gold.

Evolution of Modern Concepts

Although much early work was done in chemistry, especially with metals, and many recipes were recorded, there were few developments in the conception of the elements. In the 16th cent. Paracelsus proposed salt, mercury, and sulfur as three "principles" of which bodies were made, although he apparently also believed in the four "elements." Van Helmont (c.1600) rejected the four elements and three principles, substituting two elements, air and water.

Robert Boyle rejected these early theories and proposed a definition of chemical elements that led to the currently accepted definition. His definition is strikingly similar to Aristotle's earlier definition. In The Sceptical Chymist (1661) Boyle wrote, "I now mean by elements … certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixed bodies [chemical compounds] are immediately compounded, and into which they are ultimately resolved."

Whereas Aristotle and other early philosophers tried to determine the identity of the elements solely by reason, Boyle and later scientists used the results of numerous experiments to identify the elements. In 1789 Antoine Lavoisier published a list of chemical elements based on Boyle's definition; this encouraged adoption of standard names for the elements. Although some of his elements are now known to be compounds, such as metallic oxides and salts, they were at the time accepted as elements since they could not be decomposed by any method then known.

In 1803 John Dalton proposed (as part of his atomic theory) that all atoms of an element have identical properties (including mass), that these atoms are unchanged by chemical action, and that atoms of different elements react with one another in simple proportions. Although symbols for some of the elements already existed, they were by no means universally accepted, and each compound also had a unique symbol that was unrelated to its chemical composition. Dalton devised a new set of circular symbols for the elements and used a combination of elemental symbols to represent a compound. For example, his symbol for oxygen was ○, and for hydrogen ȯ. Since he thought water contained one atom of hydrogen for every atom of oxygen, he formed the symbol for water by writing the symbols for hydrogen and oxygen touching one another, ȯ&nosp;○. J. J. Berzelius was the first to use the modern method, letting one or two letters of the element's name serve as its symbol. He also published an early table of atomic weights of 24 elements with most values very close to those now in use.

Discovery of the Elements

As noted above, some of the elements were discovered in prehistoric times but were not recognized as elements. Arsenic was discovered around 1250 by Albertus Magnus, and phosphorus was discovered about 1674 by Hennig Brand, an alchemist, who prepared it by distilling human urine. Only 12 elements were known before 1700, and only about twice that many by 1800, but by 1900 over 80 elements had been identified. In 1919 Ernest Rutherford found that hydrogen was given off when nitrogen was bombarded with alpha particles. This first transmutation encouraged further study of nuclear reactions, and eventually led to the discovery in 1937 of technetium, the first synthetic element. Neptunium (atomic number 93) was the first transuranium element to be synthesized (1940). Its discovery prompted the search that led to the discovery of other transuranium elements.

spectroscope

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Spectroscope, optical instrument for producing spectral lines and measuring their wavelengths and intensities, used in spectral analysis (see spectrum). When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material. In the original spectroscope design in the early 19th cent., light entered a slit and a collimating lens transformed the light into a thin beam of parallel rays. A prism then separated the beam into its spectrum. The observer then viewed the spectrum through a tube with a scale that was transposed up the spectrum image, enabling its direct measurement. With the development of photographic film, the more accurate spectrograph was developed. It was based on the same principle as the spectroscope, but it had a camera in place of the telescope. In recent years the electronic circuits built around the photomultiplier tube have replaced the camera, allowing real-time spectrographic analysis of far greater accuracy. Such spectrum analysis, or spectroscopy, has become an important scientific tool for analyzing the composition of unknown material. It has found applications in fields as disparate as astronomy and forensic chemistry.

Modern Agriculture

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In the N and W United States the era of mechanized agriculture began with the invention of such farm machines as the reaper, the cultivator, the thresher, and the combine. Other revolutionary innovations, e.g., the tractor, continued to appear over the years, leading to a new type of large-scale agriculture. Modern science has also revolutionized food processing; refrigeration, for example, has made possible the large meatpacking plants and shipment and packaging of perishable foods. Urbanization has fostered the specialties of market gardening and truck farming. Harvesting operations (see harvester) have been mechanized for almost every plant product grown. Breeding programs have developed highly specialized animal, plant, and poultry varieties, thus increasing production efficiency. The development of genetic engineering has given rise to genetically modified transgenic crops and, to a lesser degree, livestock that possess a gene from an unrelated species that confers a desired quality. Such modification allows livestock to be used as "factories" for the production of growth hormone and other substances (see pharming). In the United States and other leading food-producing nations agricultural colleges and government agencies attempt to increase output by disseminating knowledge of improved agricultural practices, by the release of new plant and animal types, and by continuous intensive research into basic and applied scientific principles relating to agricultural production and economics.

These changes have, of course, given new aspects to agricultural policies. In the United States and other developed nations, the family farm is disappearing, as industrialized farms, which are organized according to industrial management techniques, can more efficiently and economically adapt to new and ever-improving technology, specialization of crops, and the volatility of farm prices in a global economy. Niche farming, in which specialized crops are raised for a specialized market, e.g., heirloom tomatoes or exotic herbs sold to gourmet food shops and restaurants, revived or encouraged some smaller farms in the latter 20th and early 21st cents., but did little to stop the overall decrease in family farms. In Third World countries, where small farms, using rudimentary techniques, still predominate, the international market has had less effect on the internal economy and the supply of food.

Most of the governments of the world face their own type of farm problem, and the attempted solutions vary as much as does agriculture itself. The modern world includes areas where specialization and conservation have been highly refined, such as Denmark, as well as areas such as N Brazil and parts of Africa, where forest peoples still employ "slash-and-burn" agriculture—cutting down and burning trees, exhausting the ash-enriched soil, and then moving to a new area. In other regions, notably SE Asia, dense population and very small holdings necessitate intensive cultivation, using people and animals but few machines; here the yield is low in relation to energy expenditure. In many countries extensive government programs control the planning, financing, and regulation of agriculture. Agriculture is still the occupation of almost 50% of the world's population, but the numbers vary from less than 3% in industrialized countries to over 60% in Third World countries.

Bibliography:
See R. Jager, The Fate of Family Farming (2004).

The Rise of Commercial Agriculture

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As the Middle Ages waned, increasing communications, the commercial revolution, and the rise of cities in Western Europe tended to turn agriculture away from subsistence farming toward the growing of crops for sale outside the community (commercial agriculture). In Britain the practice of inclosure allowed landlords to set aside plots of land, formerly subject to common rights, for intensive cropping or fenced pasturage, leading to efficient production of single crops.

In the 16th and 17th cent. horticulture was greatly developed and contributed to the so-called agricultural revolution. Exploration and intercontinental trade, as well as scientific investigation, led to the development of horticultural knowledge of various crops and the exchange of farming methods and products, such as the potato, which was introduced from America along with beans and corn (maize) and became almost as common in N Europe as rice is in SE Asia.

The appearance of mechanical devices such as the sugar mill and Eli Whitney's cotton gin helped to support the system of large plantations based on a single crop. The Industrial Revolution after the late 18th cent. swelled the population of towns and cities and increasingly forced agriculture into greater integration with general economic and financial patterns. In the American colonies the independent, more or less self-sufficient family farm became the norm in the North, while the plantation, using slave labor, was dominant (although not universal) in the South. The free farm pushed westward with the frontier.

Bibliography:
See R. Jager, The Fate of Family Farming (2004).

Agriculture

2010-11-30T19:03:00.000-08:00

Agriculture, science and practice of producing crops and livestock from the natural resources of the earth. The primary aim of agriculture is to cause the land to produce more abundantly and at the same time to protect it from deterioration and misuse. The diverse branches of modern agriculture include agronomy, horticulture, economic entomology, animal husbandry, dairying, agricultural engineering, soil chemistry, and agricultural economics.

Early Agriculture

Early people depended for their survival on hunting, fishing, and food gathering. To this day, some groups still pursue this simple way of life, and others have continued as roving herders (see nomad). However, as various groups of people undertook deliberate cultivation of wild plants and domestication of wild animals, agriculture came into being. Cultivation of crops—notably grains such as wheat, rice, corn, rye, barley, and millet—encouraged settlement of stable farm communities, some of which grew to be towns and city-states in various parts of the world. Early agricultural implements—the digging stick, the hoe, the scythe, and the plow—developed slowly over the centuries, each innovation (e.g., the introduction of iron) causing profound changes in human life. From early times, too, people created ingenious systems of irrigation to control water supply, especially in semiarid areas and regions of periodic rainfall, e.g., the Middle East, the American Southwest and Mexico, the Nile Valley, and S Asia.

Farming was often intimately associated with landholding (see tenure) and therefore with political organization. Growth of large estates involved the use of slaves (see slavery) and bound or semifree labor. In the Western Middle Ages the manorial system was the typical organization of more or less isolated units and determined the nature of the agricultural village. In Asia large holdings by the nobles, partly arising from feudalism (especially in China and Japan), produced a similar pattern.

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Bibliography:

See R. Jager, The Fate of Family Farming (2004).

Meteorology

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Meteorology, branch of science that deals with the atmosphere of a planet, particularly that of the earth, the most important application of which is the analysis and prediction of weather. Individual studies within meteorology include aeronomy, the study of the physics of the upper atmosphere; aerology, the study of free air not adjacent to the earth's surface; applied meteorology, the application of weather data for specific practical problems; dynamic meteorology, the study of atmospheric motions (which also includes the meteorology of other planets and satellites in the solar system); and physical meteorology, which focuses on the physical properties of the atmosphere.

Aristotle's Meteorologica (c.340 B.C.) is the oldest comprehensive treatise on meteorological subjects. Although most of the discussion is inaccurate in the light of modern understanding, Aristotle's work was respected as the authority in meteorology for some 2,000 years. In addition to further commentary on the Meteorologica, this period also saw attempts to forecast the weather according to astrological events, using techniques introduced by Ptolemy.

As speculation gave way to experimentation following the scientific revolution, advances in the physical sciences made contributions to meteorology, most notably through the invention of instruments for measuring atmospheric conditions, e.g., Leonardo da Vinci's wind vane (1500), Galileo's thermometer (c.1593), and Torricelli's mercury barometer (1643). Further developments included Halley's account of the trade winds and monsoons (1686) and Ferrel's theory of the general circulation of the atmosphere (1856). The invention of the telegraph made possible the rapid collection of nearly simultaneous weather observations for large continental and marine regions, thus providing a view of the large-scale pressure and circulation patterns that determine the weather.

Mendel, Gregor Johann

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Mendel, Gregor Johann (1822-1884), Austrian monk, known as the father of modern genetics. He developed the principles of heredity by studying the variation and heredity of seven pairs of inherited characteristics in pea plants. Although the significance of his work was not recognized during his lifetime, it became the basis for the present day field of genetics.

Mendel was born on July 22, 1822, to a peasant family in Heinzendorf (now Hynčice, Czech Republic). He entered the Augustinian monastery at Brünn (now Brno, Czech Republic), which was known as a center of learning and scientific endeavor. He later became a substitute teacher at the technical school in Brünn. There Mendel became actively engaged in investigating variation, heredity, and evolution in plants at the monastery's experimental garden. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants, carefully analyzing seven pairs of seed and plant characteristics. His tedious experiments resulted in the enunciation of two generalizations that later became known as the laws of heredity (see Mendel's Laws). His observations also led him to coin two terms still used in present-day genetics: dominance, for a trait that shows up in an offspring; and recessiveness, for a trait masked by a dominant gene.

Mendel published his important work on heredity in 1866. Despite, or perhaps because of, its descriptions of large numbers of experimental plants, which allowed him to express his results numerically and subject them to statistical analysis, this work made virtually no impression for the next 34 years. Only in 1900 was his work recognized more or less independently by three investigators, one of whom was the Dutch botanist Hugo Marie de Vries, and not until the late 1920s and the early '30s was its full significance realized, particularly in relation to evolutionary theory. As a result of years of research in population genetics, investigators were able to demonstrate that Darwinian evolution can be described in terms of the change in gene frequency of Mendelian pairs of characteristics in a population over successive generations.

Mendel's later experiments with the hawkweed Hieracium proved inconclusive, and because of the pressure of other duties he ceased his experiments on heredity by the 1870s. He died in Brünn on January 6, 1884.

Astrophysics

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Astrophysics, the branch of astronomy that seeks to understand the birth, evolution, and end states of celestial objects and systems in terms of the physical laws that govern them. For each object or system under study, astrophysicists observe radiations emitted over the entire electromagnetic spectrum and variations of these emissions over time (see Electromagnetic Radiation; Spectroscopy; Spectrum). This information is then interpreted with the aid of theoretical models. It is the task of such a model to explain the mechanisms by which radiation is generated within or near the object, and how the radiation then escapes. Radiation measurements can be used to estimate the distribution and energy states of the atoms, as well as the kinds of atoms, making up the object. The temperatures and pressures in the object may then be estimated using the laws of thermodynamics.

Steady-State Theory

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Steady-State Theory, theory of cosmology, or the study of the universe and its origins, that was once a rival to the big bang theory, which proposes that the universe was created in a giant explosion. The steady-state theory holds that the universe looks, on the whole, the same at all times and places. The Austrian-British astronomer Hermann Bondi and the Austrian-American astronomer Thomas Gold formulated the theory in 1948. The British astronomer Fred Hoyle soon published a different version of the theory based on his mathematical understanding of the problem. Most astronomers believe that astronomical observations contradict the predictions of the steady-state theory and uphold the big bang theory.

Gold, Thomas

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Gold, Thomas (1920- ), Austrian-American astronomer, born in Vienna and educated at the University of Cambridge. He is best known as the developer—with Austrian-British mathematician Hermann Bondi and British astronomer Fred Hoyle—of the steady-state theory of the universe. The theory, proposed in 1948, holds that the universe is homogeneous and that matter is continuously being created as the universe expands. Most scientists currently endorse the big-bang theory instead. Gold also developed the accepted explanation of pulsars as being spinning neutron stars.

Spectroscopy

2008-10-27T21:57:00.003-07:00

Spectroscopy, in physics and physical chemistry, the study of spectra (see Spectrum). The basis of spectroscopy is that each chemical element has its own characteristic spectrum. This fact was recognized in 1859 by German scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen. They developed the prism spectroscope in its modern form and applied it to chemical analysis. One of two principal spectroscope types, this instrument consists of a slit for admitting light from an external source, a group of lenses, a prism, and an eyepiece. Light that is to be analyzed passes through a collimating lens, which makes the light rays parallel, and the prism; then the image of the slit is focused at the eyepiece. One actually sees a series of images of the slit, each a different color, because the light has been separated into its component colors by the prism. The German scientists were the first to recognize that characteristic colors of light, or the spectra, are emitted and absorbed by particular elements.

Refrigeration

2008-10-27T21:57:00.001-07:00

Refrigeration, process of lowering the temperature and maintaining it in a given space for the purpose of chilling foods, preserving certain substances, or providing an atmosphere conducive to bodily comfort. Storing perishable foods, furs, pharmaceuticals, or other items under refrigeration is commonly known as cold storage. Such refrigeration checks both bacterial growth and adverse chemical reactions that occur in the normal atmosphere.

The use of natural or manufactured ice for refrigeration was widespread until shortly before World War I, when mechanical or electric refrigerators became available. Ice owes its effectiveness as a cooling agent to the fact that it has a constant fusion temperature of 0° C (32° F). In order to melt, ice must absorb heat amounting to 333.1 kJ/kg (143.3 Btu/lb). Melting ice in the presence of a dissolving salt lowers its melting point by several degrees. Foodstuffs maintained at this temperature or slightly above have an increased storage life. Solid carbon dioxide, known as dry ice, is used also as a refrigerant. Having no liquid phase at normal atmospheric pressure, it sublimes directly from the solid to vapor phase at a temperature of -78.5° C (-109.3° F). Dry ice is effective for maintaining products at low temperatures during the period of sublimation.

In mechanical refrigeration, constant cooling is achieved by the circulation of a refrigerant in a closed system, in which it evaporates to a gas and then condenses back again to a liquid in a continuous cycle. If no leakage occurs, the refrigerant lasts indefinitely throughout the entire life of the system. All that is required to maintain cooling is a constant supply of energy, or power, and a method of dissipating waste heat. The two main types of mechanical refrigeration systems used are the compression system, used in domestic units for large cold-storage applications and for most air conditioning; and the absorption system, now employed largely for heat-operated air-conditioning units but formerly also used for heat-operated domestic units.

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Compression Systems

2008-10-27T21:56:00.008-07:00

Compression systems employ four elements in the refrigeration cycle: compressor, condenser, expansion valve, and evaporator. In the evaporator the refrigerant is vaporized and heat is absorbed from the material contents or the space being cooled. The vapor next is drawn into a motor-driven compressor and elevated to high pressure, which raises its temperature. The resulting superheated, high-pressure gas is then condensed to liquid in an air- or water-cooled condenser. From the condenser the liquid flows through an expansion valve, in which its pressure and temperature are reduced to the conditions that are maintained in the evaporator.

Refrigerants

2008-10-27T21:56:00.007-07:00

For every refrigerant there is a specific boiling, or vaporization, temperature associated with each pressure, so that it is only necessary to control the pressure in the evaporator to obtain a desired temperature. A similar pressure-temperature relationship holds in the condenser. One of the most widely used refrigerants for many years has been dichlorodifluoromethane, known popularly as Refrigerant-12. This synthetic chlorofluorocarbon (CFC) when used as a refrigerant would, for example, vaporize at -6.7° C (20° F) in its evaporator under a pressure of 246.2 kPa (35.7 psi), and after compression to 909.2 kPa (131.9 psi) would condense at 37.8° C (100° F) in the condenser. The resulting condensed liquid would then enter the expansion valve to drop to evaporator pressure and repeat the cycle of absorbing heat at low temperature and low pressure and dissipating heat at the much higher condenser pressure and temperature. In small domestic refrigerators used for food storage, the condenser heat is dissipated into the kitchen or other room housing the refrigerator. With air-conditioning units the condenser heat must be dissipated out of doors or directly into cooling water.

In a domestic refrigeration system the evaporator, called the freezer, is always placed in an insulated space. In some cases this space constitutes the whole refrigerator cabinet. The compressor is usually oversized, so that if it ran continuously it would produce progressively lower temperatures. In order to maintain the interior of the box within the desired temperature range, the motor driving the compressor is controlled by a thermostatic switch.

A frozen-food refrigerator resembles the household refrigerator except that its compressor and motor must be of sufficient size to handle the larger gas volume of the refrigerant at its lower evaporator pressure. For example, to maintain a temperature of -23.3° C (-10° F) an evaporator pressure of 132.3 kPa (19.2 psi) is required with Refrigerant-12.

Absorption System

2008-10-27T21:56:00.005-07:00

A few household units, called gas refrigerators, operate on the absorption principle. In such gas refrigerators a strong solution of ammonia in water is heated by a gas flame in a container called a generator, and the ammonia is driven off as a vapor, which passes into a condenser. Changed to a liquid state in the condenser, the ammonia flows to the evaporator as in the compression system. Instead of the gas being inducted into a compressor on exit from the evaporator, however, the ammonia gas is reabsorbed in the partially cooled, weak solution returning from the generator, to form the strong ammonia solution. This process of reabsorption occurs in a container called the absorber, from which the enriched liquid flows back to the generator to complete the cycle.

Increasing use of absorption refrigeration now occurs in refrigeration units for comfort space cooling, for which purpose refrigerant temperatures of 45° to 50° F (7.2° to 10° C) are suitable. In this temperature range, water can be used as a refrigerant with an aqueous salt solution, usually lithium bromide, as the absorbent material. The very cold boiling water from the evaporator is absorbed in concentrated salt solution. This solution is then pumped into the generator, where, at elevated temperature, the surplus water is boiled off to increase the salt concentration of the solution; this solution, after cooling, recirculates back to the absorber to complete the cycle. The system operates at high vacuum at an evaporator pressure of about 1.0 kPa (0.145 psi); the generator and condenser operate at about 10.0 kPa (1.45 psi). The units are usually direct-fired or use steam generated in a boiler.

Refrigerants And The Environment

2008-10-27T21:56:00.003-07:00

Refrigerant-12 and related CFCs, Refrigerant-11 and Refrigerant-22, are currently the major compounds used in the cooling and insulation systems of home refrigeration units. It has been found, however, that CFCs are posing a major threat to the global environment through their role in the destruction of the ozone layer. A search has therefore begun for replacements, and some manufacturers of CFCs have already pledged to phase out these products by the end of the century.

Mendel’s Laws

2008-10-27T21:56:00.001-07:00

Mendel’s Laws, principles of hereditary transmission of physical characteristics. They were formulated in 1865 by the Augustinian monk Gregor Johann Mendel. Experimenting with seven contrasting characteristics of pure-breeding garden peas, Mendel discovered that by crossing tall and dwarf parents, for example, he got hybrid offspring that resembled the tall parent rather than being a medium-height blend. To explain this he conceived of hereditary units, now called genes, which often expressed dominant or recessive characteristics. Formulating his first principle (the law of segregation), Mendel stated that genes normally occur in pairs in the ordinary body cells, but segregate in the formation of sex cells (eggs or sperm), each member of the pair becoming part of the separate sex cell. When egg and sperm unite, forming a gene pair, the dominant gene (tallness) masks the recessive gene (shortness).

To corroborate the existence of such hereditary units, Mendel went on to interbreed the first generation of hybrid tall peas and found that the second generation turned out in a ratio of three tall to each short offspring. He then correctly conceived that the genes paired into AA, Aa, and aa (“A” representing dominant and “a” representing recessive). Continuing the breeding experiments, he found that the self-pollinated AA bred true to produce pure tall plants, that the aa plant produced pure dwarf plants, and that the Aa, or hybrid, tall plants produced the same three-to-one ratio of offspring. From this Mendel could see that hereditary units did not blend, as his predecessors believed, but remained unchanged from one generation to another. He thus formulated his second principle (the law of independent assortment), in which the expression of a gene for any single characteristic is usually not influenced by the expression of another characteristic. Mendel's laws became the theoretical basis for modern genetics and heredity.

Fallopio, Gabriello

2008-10-27T21:55:00.011-07:00

Fallopio, Gabriello (1523?-1562), also known as Gabriello Fallopio and Gabriel Fallopius, Italian anatomist, physician, botanist, and surgeon. Born in Modena, Fallopio studied medicine at the University of Ferrara, and after receiving his degree he worked and studied at various European medical schools. Fallopio became professor of anatomy at Ferrara in 1548 and professor of surgery and anatomy at the University of Pisa about a year later. In 1551 Cosimo I dè Medici, grand duke of Tuscany, called him to a similar post at Pisa to succeed Andreas Vesalius, the Belgian anatomist. There he also held the chair of botany and materia medica and was superintendent of the botanical gardens.

Fallopio's work dealt primarily with cranial anatomy , and he added considerably to the knowledge of the ear. He was the first to use the ear speculum instrument to diagnose diseases of the ear and the first to show the connection between the mastoid, a part of the skull that houses the ear, and the middle ear.

His discoveries included the sphenoidal sinuses; the chorda tympani; the canal through which the facial nerve passes after it leaves the auditory, called the Fallopian aqueduct; and the ducts leading from the ovaries to the uterus known as the fallopian tubes. He also named the hard palate, the soft palate, the placenta, and the vagina.

In addition to his work as a surgeon and educator, Fallopio was also a distinguished botanist, and he made important contributions to practical medicine. He was a strong opponent and vocal critic of the theories of Galen, the Greek physician who proposed that the liver is the central organ of the vascular system. His writings included treatises on tumors, ulcers, surgery, the composition of drugs, simple purgatives, thermal waters and baths, a commentary on Wounds in the Head by the Greek physician Hippocrates, and a study on syphilis, De morbo gallico (1564). His best known work was Observationes anatomicae (1561), and his complete works appeared for the first time in Venice in 1584.

Vesalius, Andreas

2008-10-27T21:55:00.009-07:00

Vesalius, Andreas (1514-1564), Belgian anatomist and physician, whose dissections of the human body and description of his findings helped to correct misconceptions prevailing since ancient times and to lay the foundations of the modern science of anatomy.

Vesalius was born in Brussels. The son of a celebrated apothecary, he attended the University of Leuven and later the University of Paris, where he studied from 1533 to 1536. At the University of Paris he studied medicine and showed a special interest in anatomy. Through further study at the University of Padua in 1537, Vesalius obtained his medical degree and an appointment as a lecturer on surgery. During his continuing research, Vesalius showed that the anatomical teachings from antiquity of the Greco-Roman physician Galen, then revered in medical schools, were based on dissections of animals, even though they were intended to provide a guide to the structure of the human body.

Vesalius went on to write an elaborate anatomical work, De Humani Corporis Fabrica (On the Structure of the Human Body, 7 volumes, 1543), which was based on his own dissections of human cadavers. The volumes were richly and carefully illustrated, with many of the fine engravings rendered by Jan van Calcar, a pupil of Titian. The most accurate and comprehensive anatomical textbook to that date, it aroused heated dispute but helped lead to Vesalius's appointment as physician in the imperial household of Charles V, Holy Roman emperor. After Charles abdicated, his son, Philip II, appointed Vesalius one of his physicians in 1559. After several years at the imperial court in Madrid, Vesalius made a pilgrimage to the Holy Land. On the voyage home in 1564, he died in a shipwreck off the island of Zacynthus.

Galen

2008-10-27T21:55:00.007-07:00

Galen (129-199?), the most outstanding physician of antiquity after Hippocrates. His anatomical studies on animals and observations of how the human body functions dominated medical theory and practice for 1400 years. Galen was born of Greek parents in Pergamum, Asia Minor, which was then part of the Roman Empire. A shrine to the healing god Asclepius was located in Pergamum, and there young Galen observed how the medical techniques of the time were used to treat the ill or wounded. He received his formal medical training in nearby Smyrna and then traveled widely, gaining more medical knowledge. About 161 he settled in Rome, where he became renowned for his skill as a physician, his animal dissections, and his public lectures.

Galen dissected many animals, particularly goats, pigs, and monkeys, to demonstrate how different muscles are controlled at different levels of the spinal cord. He noted the functions of the kidney and bladder and identified seven pairs of cranial nerves. He also showed that the brain controls the voice. Galen showed that arteries carry blood, disproving the 400-year-old belief that arteries carry air. Galen also described the valves of the heart and noted the structural differences between arteries and veins, but fell short of conceiving that the blood circulates. Instead, he held the erroneous belief that the liver is the central organ of the vascular system, and that blood moves from the liver to the periphery of the body to form flesh.

Galen was also highly praised in his time as a philosopher. In his treatise ‘On the Uses of the Parts of the Body of Man’ he closely followed the view of the Greek philosopher Aristotle that nothing in nature is superfluous. Galen's principal contribution to philosophic thought was the concept that God's purposes can be understood by examining nature.

Galen's observations in anatomy remained his most enduring contribution. His medical writings were translated by 9th-century Arab thinkers and became highly esteemed by medical humanists of Renaissance Europe. Galen produced about 500 tracts on medicine, philosophy, and ethics, many of which have survived in translated form.

Ultrasonics

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Ultrasonics, branch of physics dealing with high-frequency sound waves, usually in the range above 20,000 hertz (Hz), that is, above the audible range. It is to be distinguished from supersonics (see Aerodynamics), which deals with phenomena arising when the velocity of a solid body exceeds the speed of sound. Modern ultrasonic generators can produce frequencies up to more than several gigahertz (1 GHz = 1 billion Hz) by transforming alternating electric currents into mechanical oscillations. Detecting and measuring ultrasonic waves are accomplished mainly through the use of a piezoelectric receiver or by optical means (see Crystal), because ultrasonic waves are rendered visible by the diffraction of light.

The science of ultrasonics has many applications in various fields of physics, chemistry, technology, and medicine. Ultrasonic waves have long been used for detection and communication devices called sonar, of great importance in present-day navigation, and especially in submarine warfare. Applications of ultrasonics in physics include the determination of such properties of matter as compressibility, specific heat ratios, and elasticity. Ultrasonics is employed in producing emulsions, such as homogenized milk and photographic film, and for detecting flaws in industrial materials. Strong screen illumination in television is accomplished by using ultrasonic waves modulated by light diffraction. Ultrasound in the gigahertz range can be used to produce an acoustic “microscope,” able to visualize detail down to 1 micrometer. Surface acoustic waves of ultrasonic frequency form an important component of electronic control devices.

Aerodynamics

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Aerodynamics, branch of fluid mechanics that deals with the motion of air and other gaseous fluids, and with the forces acting on bodies in motion relative to such fluids. The motion of an airplane through the air, the wind forces exerted on a structure, and the operation of a windmill are all examples of aerodynamic action such as airplanes.

One of the fundamental forces studied in aerodynamics is lift, or the force that keeps an airplane in the air. Airplanes fly because they push air down. The leading edge of an airplane wing is slightly higher than the trailing edge when the plane is maintaining altitude. As the wing moves through the air, it deflects the air that flows underneath it downward. Air flowing over the top of the wing follows the surface of the wing and is also deflected downward. The third law of motion formulated by English physicist Sir Isaac Newton states that every action causes an equal and opposite reaction (see Newton’s Third Law of Motion). As the wing pushes the air down, the air pushes the wing up. Lift is also often explained using Bernoulli’s principle, which relates an increase in the velocity of a flow of fluid (such as air) to a decrease in pressure and vice versa. The pressure on the upper side of an airplane wing is lower than that on the lower side, and many engineers use equations derived from Bernoulli’s principle to design aircraft.

Fluid Mechanics

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Fluid Mechanics, physical science dealing with the action of fluids at rest or in motion, and with applications and devices in engineering using fluids. Fluid mechanics is basic to such diverse fields as aeronautics, chemical, civil, and mechanical engineering meteorology, naval architecture, and oceanography..

Fluid mechanics can be subdivided into two major areas, fluid statics, which deals with fluids at rest, and fluid dynamics, concerned with fluids in motion. The term hydrodynamics is applied to the flow of liquids or to low-velocity gas flows where the gas can be considered as being essentially incompressible. Aerodynamics is concerned with the theory of flight, and compressible fluid flow or gas dynamics with the behavior of gases under flow conditions, where velocity and pressure changes are sufficiently large to require inclusion of the compressibility effects.

Applications of fluid mechanics involve all kinds of flow machinery, including jet propulsion, hydraulics, turbine, compressors, and pumps. Hydraulics mainly concerns machines and structures such as hydraulic turbines, dams, and hydraulic pressures, using water or other liquids.

Newton’s Third Law of Motion

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Newton’s third law of motion states that an object experiences a force because it is interacting with some other object. The force that object 1 exerts on object 2 must be of the same magnitude but in the opposite direction as the force that object 2 exerts on object 1. If, for example, a large adult gently shoves away a child on a skating rink, in addition to the force the adult imparts on the child, the child imparts an equal but oppositely directed force on the adult. Because the mass of the adult is larger, however, the acceleration of the adult will be smaller.

Newton’s third law also requires the conservation of momentum, or the product of mass and velocity. For an isolated system, with no external forces acting on it, the momentum must remain constant. In the example of the adult and child on the skating rink, their initial velocities are zero, and thus the initial momentum of the system is zero. During the interaction, internal forces are at work between adult and child, but net external forces equal zero. Therefore, the momentum of the system must remain zero. After the adult pushes the child away, the product of the large mass and small velocity of the adult must equal the product of the small mass and large velocity of the child. The momenta are equal in magnitude but opposite in direction, thus adding to zero.

Another conserved quantity of great importance is angular (rotational) momentum. The angular momentum of a rotating object depends on its speed of rotation, its mass, and the distance of the mass from the axis. When a skater standing on a friction-free point spins faster and faster, angular momentum is conserved despite the increasing speed. At the start of the spin, the skater’s arms are outstretched. Part of the mass is therefore at a large radius. As the skater’s arms are lowered, thus decreasing their distance from the axis of rotation, the rotational speed must increase in order to maintain constant angular momentum.

Bernoulli’s Principle

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Bernoulli’s Principle, in physics, the concept that as the speed of a moving fluid (liquid or gas) increases, the pressure within that fluid decreases. Originally formulated in 1738 by Swiss mathematician and physicist Daniel Bernoulli, it states that the total energy in a steadily flowing fluid system is a constant along the flow path. An increase in the fluid’s speed must therefore be matched by a decrease in its pressure.

Bernoulli’s principle applies in nozzles, where flow accelerates and pressure drops as the tube diameter is reduced. It is also the principle behind orifice or Venturi flow meters. These meters measure the pressure difference between a low-speed fluid in an approach pipe and the high-speed fluid at the smaller orifice diameter to determine flow velocities and thus to meter the flow rate. Bernoulli’s principle is sometimes used to explain the net force in a system that includes a moving fluid, such as lift on an airplane wing, thrust of a ship’s propeller, or drifting of a spinning baseball. Although equations derived from the principle can be useful in modeling these systems, the principle technically only applies to systems that do not produce a net force.

THE COPERNICAN SYSTEM AND ITS INFLUENCE

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The major premises of Copernicus's theory are that the earth rotates daily on its axis and revolves yearly around the sun. He argued, furthermore, that the planets also circle the sun, and that the earth precesses on its axis (wobbles like a top) as it rotates. The Copernican theory retained many features of the cosmology it replaced, including the solid, planet-bearing spheres, and the finite outermost sphere bearing the fixed stars. On the other hand, Copernicus's heliocentric theories of planetary motion had the advantage of accounting for the apparent daily and yearly motion of the sun and stars, and it neatly explained the apparent retrograde motion of Mars, Jupiter, and Saturn and the fact that Mercury and Venus never move more than a certain distance from the sun. Copernicus's theory also stated that the sphere of the fixed stars was stationary.

Another important feature of Copernican theory is that it allowed a new ordering of the planets according to their periods of revolution. In Copernicus's universe, unlike Ptolemy's, the greater the radius of a planet's orbit, the greater the time the planet takes to make one circuit around the sun. But the price of accepting the concept of a moving earth was too high for most 16th-century readers who understood Copernicus's claims. In addition, Copernicus's calculations of astronomical positions were neither decisively simpler nor more accurate than those of his predecessors, even though his heliocentric theory made good physical sense, for the first time, of planetary movements. As a result, parts of his theory were adopted, while the radical core was ignored or rejected.

There were but ten Copernicans between 1543 and 1600. Most worked outside the universities in princely, royal, or imperial courts; the most famous were Galileo and the German astronomer Johannes Kepler. These men often differed in their reasons for supporting the Copernican system. In 1588 an important middle position was developed by the Danish astronomer Tycho Brahe in which the earth remained at rest and all the planets revolved around the sun as it revolved around the earth.

After the suppression of Copernican theory occasioned by the ecclesiastical trial of Galileo in 1633, some Jesuit philosophers remained secret followers of Copernicus. Many others adopted the geocentric-heliocentric system of Brahe. By the late 17th century and the rise of the system of celestial mechanics propounded by the English natural philosopher Sir Isaac Newton, most major thinkers in England, France, the Netherlands, and Denmark were Copernicans. Natural philosophers in the other European countries, however, held strong anti-Copernican views for at least another century.

EINSTEIN’S SPECIAL THEORY OF RELATIVITY

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Einstein’s third major paper in 1905, “On the Electrodynamics of Moving Bodies,” contained what became known as the special theory of relativity. Since the time of the English mathematician and physicist Sir Isaac Newton, natural philosophers (as physicists and chemists were known) had been trying to understand the nature of matter and radiation, and how they interacted in some unified world picture. The position that mechanical laws are fundamental has become known as the mechanical world view, and the position that electrical laws are fundamental has become known as the electromagnetic world view. Neither approach, however, is capable of providing a consistent explanation for the way radiation (light, for example) and matter interact when viewed from different inertial frames of reference, that is, an interaction viewed simultaneously by an observer at rest and an observer moving at uniform speed.

In the spring of 1905, after considering these problems for ten years, Einstein realized that the crux of the problem lay not in a theory of matter but in a theory of measurement. At the heart of his special theory of relativity was the realization that all measurements of time and space depend on judgments as to whether two distant events occur simultaneously. This led him to develop a theory based on two postulates: the principle of relativity, that physical laws are the same in all inertial reference systems, and the principle of the invariance of the speed of light, that the speed of light in a vacuum is a universal constant. He was thus able to provide a consistent and correct description of physical events in different inertial frames of reference without making special assumptions about the nature of matter or radiation, or how they interact. Virtually no one understood Einstein’s argument.

Mammogram

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Mammogram, X-ray image of the breast that helps physicians detect and evaluate breast abnormalities. Mammography is typically performed on women who do not have symptoms of breast cancer. The procedure can detect cancer in its early stages, when treatment is most effective. A mammogram can detect a breast abnormality as small as 0.5 cm (0.2 in), a size too small for a woman or her doctor to feel it as a lump.

Magnetic Resonance Imaging

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Magnetic Resonance Imaging (MRI), medical diagnostic technique that creates images of the body using the principles of nuclear magnetic resonance. A versatile, powerful, and sensitive tool, MRI can generate thin-section images of any part of the body—including the heart, arteries, and veins—from any angle and direction, without surgical invasion and in a relatively short period of time. MRI also creates “maps” of biochemical compounds within any cross section of the human body. These maps give basic biomedical and anatomical information that provides new knowledge and may allow early diagnosis of many diseases.

MRI is possible in the human body because the body is filled with small biological “magnets,” the most abundant and responsive of which is the proton, the nucleus of the hydrogen atom. The principles of MRI take advantage of the random distribution of protons, which possess fundamental magnetic properties. Once the patient is placed in the cylindrical magnet, the diagnostic process follows three basic steps. First, MRI creates a steady state within the body by placing the body in a steady magnetic field that is 30,000 times stronger than the earth's magnetic field. Then MRI stimulates the body with radio waves to change the steady-state orientation of protons. It then stops the radio waves and “listens” to the body's electromagnetic transmissions at a selected frequency. The transmitted signal is used to construct internal images of the body using principles similar to those developed for computerized axial tomography, or CAT scanners (see X Ray).

Plastic Surgery

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Plastic Surgery, branch of surgery, dealing with the remodeling of any portion of the human body that has been damaged or deformed. The malformation may have occurred congenitally, that is, at birth, as a child born with a cleft palate or a cleft lip (see Birth Defects). Disfigurement may also be the result of injury or of deforming surgery required in treating such diseases as cancer. The primary objectives of plastic surgery are the correction of defects, the restoration of lost function, and the improvement of appearance.

Gynecology

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Gynecology, study of women’s diseases, with special emphasis on the female reproductive organs. Areas of special concentration for gynecologists include disorders of the uterus, or womb, the organ where an unborn fetus develops; ovaries, the organs that produce ova, or eggs, which are the female sex cells; fallopian tubes, the channels connecting the uterus and ovaries; cervix, the organ that connects the vagina and uterus; vagina, the canal between the cervix and vulva, or external female organs; and breasts. See Reproductive System; Human Sexuality.

Transit Instrument

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Transit Instrument, one of the most important of astronomical instruments, consisting of a telescope fixed to a horizontal axis, so as to revolve in the plane of the meridian. It is employed in the observation of the meridian transits of the heavenly bodies. See Transit.

The meridian is marked by a reticle in the telescope composed of from 5 to 15 vertical wires and 2 horizontal wires. A lamp is used to illuminate the wires so that they can be seen at night through the telescope. The horizontal mounting axis is composed of two metal shafts fixed to the telescope tube and exactly perpendicular to it. The axis shafts rest on two support arms and bearings, which allow the telescope to be pointed from the horizontal through to the vertical along the meridian. These axis shafts must be so precisely machined and positioned in line with each other as to be essentially the same, as if they were portions of a solid shaft resting across the support arms. Four adjustments are necessary before a transit can be observed: The axis must be horizontal; the line of collimation must be at right angles to the axis of motion; the axis of motion must be placed so as to point accurately east and west; and the reticle must be exactly at the center of the line of collimation of the object glass. The accurate notation of the instant of time, by the astronomical clock, at which the object, such as a star, is seen to pass the center of the field of view is the essential part of a transit observation. This is effected by the use of electricity. At a certain point of its swing, a seconds pendulum makes a dot on a uniformly moving slip of paper. The instant of transit is similarly noted when the observer taps an electrical contact key; and the distance of this dot from the previous seconds dot, compared with the distance between two seconds dots, gives the time accurately almost to 0.01 sec.

The transit circle differs mainly from the older transit instrument in the addition, on the axis of rotation, of two large graduated circles that are read off by microscopes fixed on an independent coaxial wheel called an alidade; any variation in the position of the alidade may be detected by a large spirit level attached to it.

Olbers, Heinrich Wilhelm Matthäus

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Olbers, Heinrich Wilhelm Matthäus (1758-1840), German physician and astronomer, born in Abergen (now part of Bremen). He was educated at the University of Göttingen and practiced medicine at Bremen. In 1779 he devised a method, still employed by astronomers, for calculating the orbits of comets. (Olbers discovered several comets, the first, named after him, in 1815.) In 1781 he identified Uranus as a planet rather than as a comet, as had previously been assumed. He discovered the asteroids Pallas in 1802 and Vesta in 1807 and first proposed that all asteroids are fragments of a shattered planet that formerly revolved around the sun. He observed, in 1826, that the night sky should be uniformly illuminated if the universe were infinite and homogeneous, with stars in every direction. This observation, called Olbers’s paradox, was only resolved with the discovery of the Redshift, along with the realization that stars have finite lifetimes.

Astrology

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Astrology, the study of how events on earth correspond to the positions and movements of astronomical bodies, particularly the sun, moon, planets, and stars. Astrologers believe that the position of astronomical bodies at the exact moment of a person’s birth and the subsequent movements of the bodies reflect that person’s character and, therefore, destiny. For many years, scientists have rejected the principles of astrology. However, millions of people continue to believe in or practice it.

Astrologers create charts called horoscopes, which map the position of astronomical bodies at certain times, such as when a person is born. A horoscope is illustrated by a circle, called the ecliptic. The ecliptic is the plane on which the earth orbits around the sun in a year. It is divided into twelve sections, called the signs of the Zodiac, which include Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. Astrologers assign every planet (which in astrology includes the sun and moon) with a particular sign, depending on where that planet appears on the ecliptic at the time for which the horoscope is cast. Each planet represents basic human drives, and each sign represents a set of human characteristics. When astrologers designate a person as a certain sign—a Leo or a Pisces, for example—they are referring to the person’s sun sign— that is, the sign that the sun occupied at the time of the person’s birth.

The horoscope also is divided into twelve houses, which make up the 24-hour period during which the earth rotates once on its axis. Each house deals with certain areas of a person’s life, such as marriage, health, work, travel, and death. Astrologers make predictions by interpreting the position of astronomical bodies within the signs and houses of the horoscope.

Astrology is an ancient practice that different civilizations seemed to develop independently. The Chaldeans, who lived in Babylonia (now Iraq), developed one of the original forms of astrology as early as 3000 bc. The Chinese were practicing astrology by 2000 bc. Other varieties formed in ancient India and among the Maya of Central America. These people may have observed that certain astronomical bodies, particularly the sun, affected the change of seasons and the success of crops. Based on such observations, they may have developed a broader system by which the movements of other bodies such as the planets affected or represented additional aspects of life.

By the 500s bc, astrology had spread to Greece, where such philosophers as Pythagoras and Plato incorporated it into their study of religion and astronomy. Astrology was widely practiced in Europe through the Middle Ages, despite the condemnation of such Christian leaders as Augustine, who became archbishop of Canterbury about ad 600. Many scholars viewed astrology and astronomy as complementary sciences until about the 1500s. At that time, the discoveries made by such astronomers as Nicolaus Copernicus and Galileo Galilei undermined some of the foundations of astrology. Since then, few scientists have accepted astrology as a science.

Hoyle, Sir Fred

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Hoyle, Sir Fred (1915-2001), English astronomer and mathematician, who was one of the first to apply relativity equations and modern physics to cosmology, born in Bingley, Yorkshire. He graduated from the University of Cambridge in 1939 and was elected a fellow of its St. John's College. In the field of astrophysics, Hoyle is noted for his computations of the ages and temperatures of stars, the prediction of the existence of quasi-stellar objects that were later found, and his major contributions to the theory that the heavier elements evolved in succession from hydrogen. He was knighted in 1972. A prolific writer, Hoyle has published many popular books on astronomy and works of science fiction.

Gamow, George

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Gamow, George (1904-68), Russian American theoretical physicist, born in Odesa (Odessa), Ukraine, and educated at the University of Leningrad (now Saint Petersburg). His early work in nuclear physics was done at the universities of Leningrad, Göttingen, Copenhagen, and Cambridge. Gamow became professor of physics at Leningrad in 1931 but left the Soviet Union in 1933. The following year he moved to the United States, and he became a naturalized citizen in 1940. He was professor of theoretical physics at George Washington University (1934-56) and professor of physics at the University of Colorado (1956-68). Gamow made important contributions in a wide variety of fields, including radioactivity and cosmogony, as well as astrophysics and nuclear physics. He was one of the leading exponents of the theory of the evolutionary universe. He wrote many books for the general public, including The Birth and Death of the Sun (1940) and One, Two, Three ... Infinity (1947).

Leavitt, Henrietta Swan

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Leavitt, Henrietta Swan (1868-1921), American astronomer, whose work made possible the first accurate determination of extragalactic distances. While working at the Harvard College Observatory on a survey of Cepheid variable stars (stars the luminosity, or brightness, of which varies in an extremely regular manner) she discovered (1912) that the Cepheids having the greatest average brightness also had the longest periods of variation. When, in 1913, the Danish astronomer Ejnar Hertzsprung accurately estimated the distances of a few Cepheids, the distances of all Cepheids could be calculated from Leavitt's period-luminosity correlation. This method of distance determination greatly increased the scientific knowledge of the physical universe.

Hale, George Ellery

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Hale, George Ellery (1868-1938), American astronomer, born in Chicago, Illinois, and educated at the Massachusetts Institute of Technology. While Hale was still in college, his father built the Kenwood Observatory, a small observatory near Chicago. Hale used the observatory for original research and in 1889 invented the spectroheliograph, a device used to study the surface of the sun. In 1892 Hale was appointed associate professor of astrophysics at the University of Chicago and in 1895 he organized the Yerkes Observatory, in Williams Bay, Wisconsin, of which he served as director until 1904. In 1904 he organized the Mount Wilson Observatory, near Los Angeles, California, which he directed until 1923. In 1908, Hale discovered that sunspots have magnetic fields. Hale conceived and helped design the first giant reflecting telescope. The instrument, a reflector with a 200-in (5.08-m) mirror, was installed at Mount Palomar Observatory near San Diego, California, in 1948. It was named the Hale Telescope in his honor.

Hubble Space Telescope

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Hubble Space Telescope (HST), first general-purpose orbiting observatory. Named after American astronomer Edwin P. Hubble, the Hubble Space Telescope was launched on April 24, 1990. The HST makes observations in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum (see Electromagnetic Radiation). The primary mirror of the HST has a diameter of 94.5 in (240 cm), and the optics of the telescope are designed so that, theoretically, when making a visible-light observation, the telescope can resolve astronomical objects that are at an angular distance of 0.05 arcsecond apart. In comparison, traditional large ground-based telescopes, under very good sky conditions, have an image resolution of about 0.5 arcsecond. Originally, the HST was equipped with five detectors: the Wide-Field Planetary Camera, the Faint Object Camera, the Faint Object Spectrograph, the High-Resolution Spectrograph, and the High Speed Photometer (see Spectroscopy). It also has three fine guidance sensors that can be used for precision astronomy measurements such as determining the distances of stars from Earth.

After the HST was launched, scientists discovered that its primary mirror had a systematic aberration, the result of a manufacturing error. A service mission was carried out in December 1993 using the space shuttle Endeavour. A corrective optical device, called the Corrective Optics Space Telescope Axial Replacement (COSTAR), was inserted in the slot for the High Speed Photometer, which had to be removed to make room for COSTAR. The Wide-Field Planetary Camera, which had a different optical path from the other four instruments, was replaced with a second camera, which has a built-in correction for the aberration in the primary mirror. The service mission, which involved numerous intricate procedures, was successful.

Messier, Charles

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Messier, Charles (1730-1817), French astronomer, born in Badonviller, noted for the valuable catalog of nebulous-appearing celestial objects that he compiled from 1758 to 1784. Messier called these objects nebulae, and the catalog's purpose was to help other astronomers to distinguish such objects from comets. Messier was also noted for his discoveries of comets. Today his catalog is known to consist of galaxies and star clusters as well as true nebulas. The catalog numbers are still used in designating the objects that he listed.

Hubble, Edwin

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Hubble, Edwin Powell (1889–1953), American astronomer, who made important contributions to the study of galaxies, the expansion of the universe, and the size of the universe. Hubble was the first to discover that fuzzy patches of light in the sky called spiral nebula were actually galaxies like Earth’s galaxy, the Milky Way. Hubble also found the first evidence for the expansion of the universe, and his work led to a much better understanding of the universe’s size.

Hubble was born in Marshfield, Missouri. He attended high school in Chicago, Illinois, and received his bachelor’s degree in mathematics and astronomy in 1910. He was awarded a Rhodes Scholarship to study at the University of Oxford in England, where he earned a law degree in 1912.

In 1919 Hubble finally accepted the offer from Mount Wilson Observatory, where the 100-in (2.5-m) Hooker telescope was located. The Hooker telescope was the largest telescope in the world until 1948.

While Hubble was working at the Yerkes Observatory, he made a careful study of cloudy patches in the sky called nebulas. Now, astronomers apply the term nebula to clouds of dust and gas within galaxies. At the time that Hubble began studying nebulas, astronomers had not been able to differentiate between nebulas and distant galaxies, which also appear as cloudy patches in the sky.

In 1923 he discovered a Cepheid star in the Andromeda nebula, now known as the Great Andromeda Spiral Galaxy. He also discovered many other nebulas that contained stars and were located outside of the Milky Way. He found that they contained objects similar to those within the Milky Way Galaxy. These objects included round, compact groups of stars called globular clusters and stars called novas that flare suddenly in brightness.

Hubble was an active researcher until his death. He was involved in building the 200-in (508-cm) Hale telescope at the Mount Palomar Observatory, also in southern California. The Hale telescope was the largest telescope in the world from when it went into operation in 1948 until the Keck telescope at the Mauna Kea Observatory in Hawaii was completed in 1990. The Hubble Space Telescope (HST), a powerful telescope launched in 1990 and carried aboard a satellite in orbit around Earth, was named after Hubble and has helped scientists make many important observations.

Big Bang Theory

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Big Bang Theory, currently accepted explanation of the beginning of the universe. The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion years ago, and the universe has since been expanding and cooling.

The theory is based on the mathematical equations, known as the field equations, of the general theory of relativity set forth in 1915 by Albert Einstein. In 1922 Russian physicist Alexander Friedmann provided a set of solutions to the field equations. These solutions have served as the framework for much of the current theoretical work on the big bang theory. American astronomer Edwin Hubble provided some of the greatest supporting evidence for the theory with his 1929 discovery that the light of distant galaxies was universally shifted toward the red end of the spectrum (see Redshift). Once “tired light” theories—that light slowly loses energy naturally, becoming more red over time—were dismissed, this shift proved that the galaxies were moving away from each other. Hubble found that galaxies farther away were moving away proportionally faster, showing that the universe is expanding uniformly. However, the universe’s initial state was still unknown.

In the 1940s Russian-American physicist George Gamow worked out a theory that fit with Friedmann’s solutions in which the universe expanded from a hot, dense state. In 1950 British astronomer Fred Hoyle, in support of his own opposing steady-state theory, referred to Gamow’s theory as a mere “big bang,” but the name stuck. Indeed, a contest in the 1990s by Sky & Telescope magazine to find a better (perhaps more dignified) name did not produce one.

Telescope

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Telescope, device that permits distant and faint objects to be viewed as if they were much brighter and closer to the observer. Telescopes are typically used to observe the skies.

For hundreds of years, telescopes were the only instruments available for studying the planets and stars. Even today, space probes can reach only our closest neighbors in the heavens, and scientists continue to rely on telescopes to learn about distant stars, nebulas, and galaxies. Telescopes are the fundamental research instruments that enable astronomers to tackle scientific questions about the birth of the universe (see Big Bang Theory; Cosmology); the emergence of structure in the early universe; the formation and evolution of stars, galaxies, and planetary systems; and the conditions for the emergence of life itself.

Most telescopes work by collecting and magnifying visible light that is given off by stars or reflected from the surface of planets. Such instruments are called optical telescopes. Conventional optical telescopes use a curved lens or mirror to collect light and bring it to a focus, a point in space where all the light rays converge. A small magnifying lens, called an eyepiece, placed at the focus allows the image to be viewed. In astronomical research, cameras or other instruments placed near the focus make a precise recording of the light gathered by a telescope. The visible light collected by a telescope is divided into component wavelengths, or colors, through a process called spectroscopy. This powerful technique, which uses a prism or diffraction grating, essentially “decodes” starlight to yield information about an object’s temperature, motion and other dynamics, chemical composition, and the presence of magnetic fields.

Brahe, Tycho

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Brahe, Tycho (1546-1601), Danish astronomer, who made precise, comprehensive astronomical measurements of the solar system and more than 700 stars. The data Brahe accumulated was superior to all other astronomical measurements made until the invention of the telescope in the early 17th century.

Brahe was born in Knudstrup in southern Sweden (then part of Denmark). He studied law and philosophy at the universities of Copenhagen and Leipzig; at night, however, Brahe busied himself with observing the stars. With no instruments other than a globe and a pair of compasses, he succeeded in detecting grave errors in the standard astronomical tables, and set about correcting them. In 1572 he discovered a supernova in the constellation Cassiopeia. After Brahe had spent some time traveling and lecturing, Frederick II, king of Denmark and Norway, offered to provide Brahe with funds to construct and equip an astronomical observatory on the island of Hven (now Ven). Brahe accepted the proposal, and in 1576 construction began on the castle of Uranienborg (“fortress of the heavens”), where for 20 years the astronomer pursued his observations.

Brahe never fully accepted the Copernican theory of the universe and sought a compromise by combining it with the old Ptolemaic system (see Copernican System; Ptolemaic System). In Brahe's system, the five known planets were supposed to revolve around the sun, which, with the planets, circled the earth each year. The sphere of the stars revolved around the immobile earth once a day.

Although Brahe's theory of planetary motion was flawed, the data he accumulated during his life played a crucial role in developing the correct description of planetary motion. Johannes Kepler, who was Brahe's assistant from 1600 until Brahe's death in 1601, used Brahe's data to help him formulate his three laws of planetary motion (see Kepler's Laws).

Copernican System

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Copernican System, systematic explanation of the movement of the planets around the sun; advanced in 1543 by the Polish astronomer Nicolaus Copernicus. The Copernican system advanced the theories that the earth and the planets are all revolving in orbits around the sun, and that the earth is spinning on its north-south axis from west to east at the rate of one rotation per day. These two hypotheses superseded the Ptolemaic system, which had been the basis of astronomical theory until that time. The Copernican system first described the precession of the equinoxes (see Ecliptic) but did not explain it.

Publication of the Copernican system stimulated the study of astronomy and mathematics and laid the basis for the discoveries of the German astronomer Johannes Kepler and the British astronomer Sir Isaac Newton.

Ptolemaic System

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Ptolemaic System, in astronomy, theory of the order and action of the heavenly bodies. It was advanced in the 2nd century ad by the Alexandrian astronomer Ptolemy. The Ptolemaic theory held that Earth is stationary and at the center of the universe; closest to Earth is the Moon, and beyond it, extending outward, are Mercury, Venus, and the Sun in a straight line, followed successively by Mars, Jupiter, Saturn, and the so-called fixed stars. Later, astronomers supplemented this system with a ninth sphere, the motion of which supposedly produced the precession of equinoxes (see Ecliptic). A tenth sphere or primum mobile, which was thought to motivate the other heavenly bodies, was also added. To explain the various observed motions of the planets, the Ptolemaic system described them as having small circular orbits called epicycles; the centers of the epicycles, on circular orbits around Earth, were called deferents. The motion of all spheres is from west to east. After the decline of classical Greek culture, Arabian astronomers attempted to perfect the system by adding new epicycles to explain unpredicted variations in the motions and positions of the planets. These efforts failed, however, to resolve the many inconsistencies in the Ptolemaic system, which was finally superseded in the 16th century by the Copernican system.

Kepler’s Laws

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Kepler’s Laws, three laws concerning the motions of planets formulated by the German astronomer Johannes Kepler early in the 17th century. See Orbit; Planet; Solar System.

Kepler based his laws on planetary data collected by the Danish astronomer Tycho Brahe, to whom he was an assistant. The proposals broke with a centuries-old belief based on the Ptolemaic system advanced by the Alexandrian astronomer Ptolemy, in the 2nd century ad, and the Copernican system, put forward by the Polish astronomer Nicolaus Copernicus, in the 16th century, that the planets moved in circular orbits. According to Kepler's first law, the planets orbit the sun in elliptical paths, with the sun at one focus of the ellipse. The second law states that the areas described in a planetary orbit by the straight line joining the center of the planet and the center of the sun are equal for equal time intervals; that is, the closer a planet comes to the sun, the more rapidly it moves. Kepler's third law states that the ratio of the cube of a planet's mean distance, d, from the sun to the square of its orbital period, t, is a constant—that is, is the same for all planets.

These laws played an important part in the work of the 17th-century English astronomer, mathematician, and physicist Sir Isaac Newton, and are important for the understanding of the orbital paths of the moon, the natural satellite of the earth, and the paths of the artificial satellites launched from the earth..

Kepler, Johannes

2008-10-21T22:33:00.001-07:00

Kepler, Johannes (1571-1630), German astronomer and natural philosopher, noted for formulating and verifying the three laws of planetary motion. These laws are now known as Kepler's laws.

Kepler was born on December 27, 1571, in Weil der Stadt in Württemberg and studied theology and classics at the University of Tübingen. There he was influenced by a mathematics professor, Michael Maestlin, an adherent of the heliocentric theory of planetary motion first developed by the Polish astronomer Nicolaus Copernicus. Kepler accepted Copernican theory (see Copernican System) immediately, believing that the simplicity of Copernican planetary ordering must have been God's plan. In 1594, when Kepler left Tübingen for Graz, Austria, he worked out a complex geometric hypothesis to account for distances between the planetary orbits—orbits that he mistakenly assumed were circular. (Kepler later deduced that planetary orbits are elliptic; nevertheless, these preliminary calculations agreed with observations to within 5 percent.) Kepler then proposed that the sun emits a force that diminishes inversely with distance and pushes the planets around in their orbits. Kepler published his account in a treatise entitled Mysterium Cosmographicum (Cosmographic Mystery) in 1596. This work is significant because it presented the first comprehensive and cogent account of the geometrical advantages of Copernican theory.

Kepler held the chair of astronomy and mathematics at Graz University from 1594 until 1600, when he became assistant to the Danish astronomer Tycho Brahe in the latter's observatory near Prague. On the death of Brahe in 1601, Kepler assumed his position as imperial mathematician and court astronomer to Rudolf II, Holy Roman emperor. One of his major works during this period was Astronomia Nova (New Astronomy, 1609), the great culmination of his painstaking efforts to calculate the orbit of Mars. This treatise contains statements of two of Kepler's so-called laws of planetary motion. The first is that the planets move in elliptic orbits with the sun at one focus; the second, or “area rule,” states that a hypothetical line from the sun to a planet sweeps out equal areas of an ellipse during equal intervals of time; in other words, the closer a planet comes to the sun, the more rapidly it moves.

In 1612 Kepler became mathematician to the states of Oberösterreich (Upper Austria). While living in Linz, he published his Harmonice Mundi (Harmony of the World, 1619), the final section of which contained another discovery about planetary motion: The ratio of the cube of a planet's distance from the sun and the square of the planet's orbital period is a constant and is the same for all planets. See Orbit.

At about the same time he began publishing a book that took three years to appear, the Epitome Astronomiae Copernicanae (Epitome of Copernican Astronomy, 1618-1621), which brought all of Kepler's discoveries together in a single volume. Equally important, it became the first textbook of astronomy to be based on Copernican principles, and for the next three decades it was a major influence in converting many astronomers to Keplerian Copernicanism.

The last major work to appear in Kepler's lifetime was the Tabulae Rudolfinae (Rudolfine Tables, 1625). Based on Brahe's data, the new tables of planetary motion reduced the mean errors from 5° to within 10′ of the actual position of a planet. The English mathematician Sir Isaac Newton relied heavily on Kepler's theories and observations in formulating his theory of gravitational force.

Kepler died on November 15, 1630, in Regensburg.

Cryogenics

2008-10-19T04:36:00.000-07:00

Cryogenics, study and use of materials at very low temperatures. The upper limit of cryogenic temperatures has not been agreed on, but the National Institute of Standards and Technology has suggested that the term cryogenics be applied to all temperatures below -150° C (-238° F or 123° above absolute zero on the Kelvin scale). Some scientists regard the normal boiling point of oxygen (-183° C or -297° F), as the upper limit (see Absolute Zero). Cryogenic temperatures are achieved either by the rapid evaporation of volatile liquids or by the expansion of gases confined initially at pressures of 150 to 200 atmospheres. The expansion may be simple, that is, through a valve to a region of lower pressure, or it may occur in the cylinder of a reciprocating engine, with the gas driving the piston of the engine. The second method is more efficient but is also more difficult to apply. See Heat.

Dirac, Paul Adrien Maurice

2008-10-12T20:32:00.012-07:00

Dirac, Paul Adrien Maurice (1902-84), British theoretical physicist and Nobel laureate, renowned for his prediction of the existence of the positron, or antielectron, and for his research in quantum theory.

Dirac was born in Bristol, England, and educated at the universities of Bristol and Cambridge. His quantum theory of electron motion led him in 1928 to postulate the existence of a particle identical to the electron in every aspect but charge, the electron having a negative charge and this hypothetical particle a positive one. Dirac's theory was confirmed in 1932 when the American physicist Carl Anderson discovered the positron. In 1933 Dirac shared the Nobel Prize in physics with the Austrian physicist Erwin Schrödinger, and in 1939 he was made a fellow of the Royal Society. He was a professor of mathematics at Cambridge from 1932 to 1968, a professor of physics at Florida State University from 1971 until his death, and a member of the Institute for Advanced Study periodically between 1934 and 1959. Dirac's writings include Principles of Quantum Mechanics (1930).

Copernicus, Nicolaus

2008-10-12T20:32:00.011-07:00

Copernicus, Nicolaus (1473-1543), Polish astronomer, best known for his astronomical theory that the sun is at rest near the center of the universe, and that the earth, spinning on its axis once daily, revolves annually around the sun. This is called the heliocentric, or sun-centered, system. See Astronomy; Solar System.

Copernicus was born on February 19, 1473, in Thorn (now Toruń), Poland, to a family of merchants and municipal officials. Copernicus's maternal uncle, Bishop Łukasz Watzenrode, saw to it that his nephew obtained a solid education at the best universities. Copernicus entered Kraków Academy (now Jagiełłonian University) in 1491, studied the liberal arts for four years without receiving a degree, and then, like many Poles of his social class, went to Italy to study medicine and law. Before he left, his uncle had him appointed a church administrator in Frauenberg (now Frombork); this was a post with financial responsibilities but no priestly duties. In January 1497 Copernicus began to study canon law at the University of Bologna while living in the home of a mathematics professor, Domenico Maria de Novara. Copernicus's geographical and astronomical interests were greatly stimulated by Domenico Maria, an early critic of the accuracy of the Geography of the 2nd-century astronomer Ptolemy. Together, the two men observed the occultation (the eclipse by the moon) of the star Aldebaran on March 9, 1497.

In 1500 Copernicus lectured on astronomy in Rome. The following year he gained permission to study medicine at Padua, the university where Galileo taught nearly a century later. It was not unusual at the time to study a subject at one university and then to receive a degree from another—often less expensive—institution. And so Copernicus, without completing his medical studies, received a doctorate in canon law from Ferrara in 1503 and then returned to Poland to take up his administrative duties.

See: THE COPERNICAN SYSTEM AND ITS INFLUENCE .

Match

2008-10-12T20:32:00.009-07:00

Match, short, thin piece of wood, cardboard, or waxed string, tipped with a mixture of fire-producing substances, and used to produce a flame. One of the first matches produced was the brimstone match, made by dipping thin strips of wood into melted sulfur; the sulfur points ignited when applied to a spark produced by a flint and steel. In 1812 a chemical match was invented. Coated with sulfur and tipped with a mixture of potassium chlorate and sugar, it ignited when touched to sulfuric acid.

Matches made with phosphorus and ignited by friction were invented in 1827 by the British chemist John Walker, and have been used in improved form ever since. In the modern friction match, one end of the bare stick is dipped in a fireproofing agent, so that it will not burn readily, and the other end is coated with paraffin. The head of the match contains an oxidizing agent, such as potassium chlorate; a substance that oxidizes readily, such as sulfur or rosin; a filler of clay; a binding material, such as glue; and dye to give it distinctive color. At the very tip is a small amount of phosphorus trisulfide, which decomposes and burns at a low temperature; this ignites the paraffin, which burns more readily because of the presence of the other chemicals.

Safety matches are so designed that the head can be ignited only by striking on the friction surface provided on the match package. The tip of the safety match contains antimony trisulfide and an oxidizing agent, which are held in place with casein or glue. The striking surface on the package contains powdered glass for friction, red phosphorus, and glue. When the match is struck, the heat of friction converts the red phosphorus to white phosphorus, which ignites and in turn ignites the head of the match.

Nobel, Alfred Bernhard

2008-10-12T20:32:00.007-07:00

Nobel, Alfred Bernhard (1833-96), Swedish chemist, inventor, and philanthropist, born in Stockholm. After receiving an education in Saint Petersburg, Russia, and in the United States, where he studied mechanical engineering, he returned to Saint Petersburg to work under his father, developing mines, torpedoes, and other explosives. In a family-owned factory in Heleneborg, Sweden, he sought to develop a safe way to handle nitroglycerin, after a factory explosion in 1864 killed his younger brother and four other people. In 1867 Nobel achieved his goal; by using an organic packing material to reduce the volatility of the nitroglycerin, he produced what he called dynamite. He later produced ballistite, one of the first smokeless powders. At the time of his death he controlled factories for the manufacture of explosives in many parts of the world. His will provided that the major portion of his $9 million estate be set up as a fund to establish yearly prizes for merit in physics, chemistry, medicine and physiology, literature, and world peace. (A prize in economics has been awarded since 1969.)

Cosmology

2008-10-12T20:32:00.005-07:00

Cosmology, study of the universe as a whole, including its distant past and its future. Cosmologists study the universe observationally—by looking at the universe—and theoretically—by using physical laws and theories to predict how the universe should behave. Cosmology is a branch of astronomy, but the observational and theoretical techniques used by cosmologists involve a wide range of other sciences, such as physics and chemistry. Cosmology is distinguished from cosmogony, which used to mean the study of the origin of the universe but now usually refers only to the study of the origin of the solar system.

Lavoisier, Antoine Laurent

2008-10-12T20:32:00.003-07:00

Lavoisier, Antoine Laurent (1743-1794), French chemist, who is considered the founder of modern chemistry.

Lavoisier was born on August 26, 1743, in Paris and was educated at the Collège Mazarin. He was elected a member of the Academy of Sciences in 1768. He held many public offices, including those of director of the state gunpowder works in 1776, member of a commission to establish a uniform system of weights and measures in 1790, and commissary of the treasury in 1791. He attempted to introduce reforms in the French monetary and taxation system and in farming methods. As one of the farmers-general, he was arrested and tried by the revolutionary tribunal, and guillotined on May 8, 1794.

Lavoisier's experiments were among the first truly quantitative chemical experiments ever performed. He showed that, although matter changes its state in a chemical reaction, the quantity of matter is the same at the end as at the beginning of every chemical reaction. These experiments provided evidence for the law of the conservation of matter. Lavoisier also investigated the composition of water, and he named the components of water oxygen and hydrogen.

Some of Lavoisier's most important experiments examined the nature of combustion, or burning. Through these experiments, he demonstrated that burning is a process that involves the combination of a substance with oxygen. He also demonstrated the role of oxygen in animal and plant respiration. Lavoisier's explanation of combustion replaced the phlogiston theory, which postulates that materials release a substance called phlogiston when they burn.

With the French chemist Claude Louis Berthollet and others, Lavoisier devised a chemical nomenclature, or a system of names, which serves as the basis of the modern system. He described it in Méthode de nomenclature chimique (Method of Chemical Nomenclature, 1787). In Traité élémentaire de chimie (Treatise on Chemical Elements, 1789), Lavoisier clarified the concept of an element as a simple substance that could not be broken down by any known method of chemical analysis, and he devised a theory of the formation of chemical compounds from elements. He also wrote Sur la combustion en general (On Combustion, 1777) and Considerations sur la nature des acides (Considerations on the Nature of Acids, 1778).

Berthollet, Claude Louis

2008-10-12T20:32:00.001-07:00

Berthollet, Claude Louis, Comte (1749-1822), French chemist, who made contributions to several fields of chemistry. Berthollet was born in Talloires, and educated at the University of Turin. In 1785 Berthollet proposed the use of chlorine as a bleaching agent. After years of skepticism, Berthollet was one of the first to support the correct antiphlogistic combustion theories of the French chemist Antoine Lavoisier, although he opposed Lavoisier's erroneous theory that oxygen is the fundamental acidifying principle. With Lavoisier and others, Berthollet helped devise a new system of chemical nomenclature in 1787 that is the basis of the system currently used. He made important contributions to the knowledge of the chemistry of explosives and the metallurgy of iron. Berthollet's significant work, Essai de statique chimique (Essay on the State of Chemistry, 2 volumes, 1803), presented his theories on chemical affinity and the reversibility of reactions.

Aristotle

2008-10-12T20:31:00.011-07:00

Aristotle (384-322 bc), Greek philosopher and scientist, who shares with Plato and Socrates the distinction of being the most famous of ancient philosophers.

Aristotle was born at Stagira, in Macedonia, the son of a physician to the royal court. At the age of 17, he went to Athens to study at Plato's Academy. He remained there for about 20 years, as a student and then as a teacher.

When Plato died in 347 bc, Aristotle moved to Assos, a city in Asia Minor, where a friend of his, Hermias, was ruler. There he counseled Hermias and married his niece and adopted daughter, Pythias. After Hermias was captured and executed by the Persians in 345 bc, Aristotle went to Pella, the Macedonian capital, where he became the tutor of the king's young son Alexander, later known as Alexander the Great. In 335, when Alexander became king, Aristotle returned to Athens and established his own school, the Lyceum. Because much of the discussion in his school took place while teachers and students were walking about the Lyceum grounds, Aristotle's school came to be known as the Peripatetic (“walking” or “strolling”) school. Upon the death of Alexander in 323 bc, strong anti-Macedonian feeling developed in Athens, and Aristotle retired to a family estate in Euboea (Évvoia). He died there the following year.

Cannizzaro, Stanislao

2008-10-12T20:31:00.009-07:00

Cannizzaro, Stanislao (1826-1910), Italian chemist, born in Palermo, Sicily. After participating in the 1848 Sicilian revolution, Cannizzaro worked (1849-51) in a laboratory in Paris. He was appointed professor of chemistry at the institute in Alessandria (1851) and at the universities of Genoa (1855), Pisa (1861), and Rome (1871). At Alessandria he discovered the reaction that bears his name, Cannizzaro's reaction, which proves that aldehydes in the presence of concentrated alkali are reduced to a mixture of their corresponding alcohol and acid; for example, benzaldehyde yields benzyl alcohol and benzoic acid.

Cannizzaro made a great contribution to atomic theory by clarifying (1858) the distinction between atomic weight and molecular weight. He showed how unknown atomic weights of elements in volatile compounds can be arrived at from known molecular weights of the compounds. Cannizzaro also determined that atomic weights of elements in compounds can be determined if specific heats are known even though vapor densities are unknown. His work on atomic theory was based on Avogadro's law, which states that equal volumes of any two gases contain an equal number of molecules when held under identical conditions of temperature and pressure.

Barometer

2008-10-12T20:31:00.007-07:00

Barometer, instrument for measuring atmospheric pressure, that is, the force exerted on a surface of unit area by the weight of the atmosphere. Because this force is transmitted equally in all directions through any fluid, it is most easily measured by observing the height of a column of liquid that, by its weight, exactly balances the weight of the atmosphere. A water barometer is far too large to be used conveniently. Liquid mercury, however, is 13.6 times as heavy as water, and the column of mercury sustained by normal atmospheric pressure is only about 760 mm (about 30 in) high.

Normal, or standard, atmospheric pressure is usually defined at 1013.25 millibars, which is equivalent to 760 mm (29.9213 in) of mercury or 1.03323 kg/sq cm (14.6960 lb/sq in).

An ordinary mercury barometer consists of a glass tube about 840 mm (about 33 in) high, closed at the upper end and open at the lower. When the tube is filled with mercury and the open end placed in a cup full of the same liquid, the level in the tube falls to a height of about 760 mm (about 30 in) above the level in the cup, leaving an almost perfect vacuum at the top of the tube. Variations in atmospheric pressure cause the liquid in the tube to rise or fall by small amounts, rarely below 737 mm (29 in) or above 775 mm (30.5 in) at sea level. When the mercury level is read with a form of gradated scale, known as a vernier attachment, and suitable corrections are made for altitude and latitude (because of the change of gravity), for temperature (because of the expansion or contraction of the mercury), and for the diameter of the tube (because of capillarity), the reading of a mercury barometer is reliable to within 0.1 mm (0.004 in).

A more convenient form of barometer (and one that is almost as accurate) is the aneroid, in which atmospheric pressure bends the elastic top of a partially evacuated drum, actuating a pointer. A suitable aneroid barometer is often used as an altimeter (instrument measuring altitude), because pressure decreases rapidly with increasing altitude (about 25 mm/1 in. of mercury per 305 m/1000 ft at low altitudes).

Berzelius, Jöns Jakob, Baron

2008-10-12T20:31:00.005-07:00

Berzelius, Jöns Jakob, Baron (1779-1848), Swedish chemist, considered one of the founders of modern chemistry.

Berzelius was born near Linköping. While studying medicine at the University of Uppsala, he became interested in chemistry. After practicing medicine and lecturing, he became a professor of botany and pharmacy at Stockholm in 1807. From 1815 to 1832 he was professor of chemistry at the Caroline Medico-Chirurgical Institute in Stockholm. He became a member of the Stockholm Academy of Sciences in 1808 and in 1818 became its permanent secretary. For his contributions to science, Berzelius was made a baron in 1835 by Charles XIV John, king of Sweden and Norway.

Berzelius's research extended into every branch of chemistry and was extraordinary for its scope and accuracy. He discovered three chemical elements—cerium, selenium, and thorium—and was the first to isolate silicon, zirconium, and titanium. He introduced the term catalyst into chemistry and was the first to elaborate on the nature and importance of catalysis. He introduced the present system of chemical notation, in which each element is represented by one or two letters of the alphabet. In addition, Berzelius was primarily responsible for the theory of radicals, which states that a group of atoms, such as the sulfate group, can act as a single unit through a series of chemical reactions. He developed an elaborate electrochemical theory that correctly stated that chemical compounds are made up of negatively and positively charged components. All of his theoretical work was supported by elaborate experimental measurement. His greatest achievement was the measurement of atomic weights.

Bioenergetics

2008-10-12T20:31:00.003-07:00

Bioenergetics, study of the processes by which living cells use, store, and release energy. A central component of bioenergetics is energy transformation, the conversion of energy from one form to another.

All cells transform energy. Plant cells, for example, use sunlight to make carbohydrates (sugars and starches) from simple inorganic chemicals. In this process, called photosynthesis, radiant energy from the sun is converted into stored chemical energy. If these plant carbohydrates are eaten by an animal (see Food Web), they will be broken down and their chemical energy turned into movement (kinetic energy), body heat (radiant energy), or new chemical bonds (see Adenosine Triphosphate; Citric Acid Cycle; Metabolism).

In all such transformations, some energy is lost to the environment. This lost energy, which is no longer available for useful work, is called entropy. The second law of thermodynamics states that any system tends to run down—that is, increase its entropy—over time. The steady influx of solar energy is required for the survival of all the earth's plants and animals.

Schwarzschild, Karl

2008-10-12T20:31:00.001-07:00

Schwarzschild, Karl (1873-1916), German astronomer, mathematician, and physicist, born in Frankfurt, who predicted the existence of black holes. His first two papers on astronomy were published while still a schoolboy. After studying at the universities of Strasbourg and Munich, he was appointed Director of the Göttingen Observatory in 1901, and of the Astrophysical Observatory in Potsdam in 1909. He volunteered for military service at the start of World War I, but was invalided home in 1916 after contracting a rare skin disease, from which he died. His contributions were in the main theoretical, and related to solar physics, relativity, stellar kinematics, photographic magnitudes, the study of rotating fluid masses, and geometrical optics. In 1916 he postulated the Schwarzschild radius, on the basis of the general theory of relativity newly propounded by Albert Einstein. When a massive star explodes as a supernova, it may leave a remnant so compact that it lies wholly within this radius. Nothing, not even light, can escape from its intense gravitational field. Such objects are now known as black holes.

Bubble Chamber

2008-10-12T20:30:00.014-07:00

Bubble Chamber, device that detects and tracks the paths of high-energy subatomic particles released by radioactive substances. Subatomic particles are invisible to the unaided eye. Bubble chambers provide an indirect way for physicists to “see” a particle and learn about its charge, mass, and energy and how it interacts with other subatomic particles. See also Particle Detectors; Elementary Particles.

A bubble chamber contains liquid, often hydrogen or deuterium, heated beyond its boiling point. This liquid does not boil, however, because it is under pressure and all the impurities have been removed. As a charged particle moves through the liquid, it interacts with atoms and molecules in the liquid, making them ions, which are atoms or molecules with a positive or negative charge. The new ions that form along the path of the charged particles act as impurities, causing the liquid next to them to boil. The tiny bubbles formed by the boiling liquid behind the particle form a line that makes the path of the particle visible. This pathway is usually photographed for later analysis. American physicist Donald A. Glaser built the first bubble chamber in 1952.

The bubble chamber overcomes many of the disadvantages of the cloud chamber, a particle detector used before the bubble chamber’s invention. A cloud chamber uses a gas that condenses along the path of the subatomic particle. Because the atoms and molecules in a liquid are closer together than those in a gas, an incoming particle in a bubble chamber has more interactions than an incoming particle in a cloud chamber in the same amount of time. The greater number of interactions in a bubble chamber makes the paths of subatomic particles easier to photograph and track.

Glaser, Donald Arthur

2008-10-12T20:30:00.013-07:00

Glaser, Donald Arthur (1926- ), American physicist and winner of the 1960 Nobel Prize in physics for his invention of the bubble chamber, a device for detecting high-energy particles (see Particle Detector; Elementary Particle). With this device, Glaser contributed greatly to the understanding of atomic function and provided the technology for the discovery of new atomic particles.

Glaser was born in Cleveland, Ohio, the son of Russian immigrants. An accomplished violinist, Glaser became a member of a symphony orchestra at age sixteen and studied composition at the Cleveland Institute of Music. He received his B.S. degree in mathematics and physics from Cleveland's Case Institute of Technology (now Case Western Reserve University) in 1946, and his Ph.D. degree from the California Institute of Technology in 1950. In 1949 Glaser accepted a position as instructor at the University of Michigan, where he began his bubble chamber experiments. In 1959 Glaser became a professor of physics at the University of California, Berkeley.

In creating the bubble chamber, Glaser built on the work of Nobel Prize winners C. T. R. Wilson, the inventor of the cloud chamber (a device that exposes charged particles by producing a trail of water droplets from air saturated with water), and C. F. Powell, who developed photographic techniques to capture images of charged nuclear particles on film. Both the cloud chamber and the emulsion technique were limited to the detection of low-energy nuclear particles. Glaser wanted to build a device that could reveal high-energy particles. He experimented with soda, seltzer, and beer without success. But he found that nuclear particles left a trail of bubbles as they moved through superheated ether (a colorless, flammable liquid). Using the bubble chamber, scientists have discovered new high-energy atomic particles. They have also used it to track neutral atomic particles advancing their understanding of the mass, lifetime, and decay of these particles.

Wilson, Charles Thomson Rees

2008-10-12T20:30:00.011-07:00

Wilson, Charles Thomson Rees (1869-1959), Scottish physicist and Nobel laureate. Wilson invented the cloud chamber, which gave the first pictures of the paths of subatomic particles (see Elementary Particles) and became an essential tool in the fields of atomic and meteorological physics (see Atom; Meteorology). For his discovery of the method of making the paths of electrically charged particles visible by the condensation of water vapor, Wilson shared the 1927 Nobel Prize in physics with American physicist Arthur Holly Compton.

Wilson was born in Glencorse in the former county of Midlothian, Scotland. He received a B.S. degree from Owens College (now the Victoria Institute of Manchester) in England in 1887 and a B.A. degree from the University of Cambridge in 1892. After teaching at Bradford Grammar School in Bradford, England, for four years Wilson returned to Cambridge in 1896 as a researcher and remained there, eventually as a professor, until he retired in 1936. He remained active in research, publishing his last paper at the age of 87.

Wilson first developed the cloud chamber in the late 1890s to study how water vapor and light interact. Physicists at that time thought that water droplets formed only around dust particles. Wilson established that water droplets can form around charged particles, or ions, in the absence of dust. He found that if he exposed the air in a chamber to X rays, many more droplets formed. He concluded the X rays give the air molecules an electrical charge, or ionize them.

As an ion moves through the cloud chamber, drops of water form around it. Because the ion moves very quickly, the string of drops of water in the air looks like a continuous path marking the movement of the ion. The path is especially apparent when a strong light is directed at the cloud chamber. If a magnetic field (see Magnetism) is applied to the cloud chamber, the ions will follow curved paths depending on the strength and nature of the charge, the mass of the ion, and the strength and direction of the magnetic field. The paths can be photographed for later analysis.

Wilson also intensely studied electrical conduction in air and applied his findings to devising ways to protect British airships from lightning and other discharges of electricity during World War I (1914-1918).

Cloud Chamber

2008-10-12T20:30:00.009-07:00

Cloud Chamber, instrument used to track the paths of charged subatomic particles released by radioactive substances. Physicists learn about a particle’s charge, mass, and energy by observing its path as it moves through a cloud chamber.

A cloud chamber is an airtight container, varying in size from a few square centimeters or inches to a room large enough for a person to enter. The container has at least one glass window through which the particle tracks can be observed and photographed. In order to trace the paths of particles, which are invisible to the naked eye, cloud chambers cool a gas saturated with water vapor, methyl alcohol vapor, or ethyl alcohol vapor so the gas becomes supersaturated—so full of vapor that liquid would begin to condense out of the gas under normal conditions. The vapor in the gas will not condense, however, unless there are foreign particles in the gas that the vapor can condense around.

As a charged particle passes through the supersaturated gas in the cloud chamber, the particle interacts with atoms and molecules of the gas and vapor. This interaction turns the gas particles into ions, atoms or molecules with a positive or negative electric charge. The supersaturated vapor condenses around these ions in the gas, forming a visible string of liquid droplets that illuminate the pathway of the particle.

Scottish physicist and Nobel Prize winner Charles T. R. Wilson invented the cloud chamber in 1911. He had experimented with optical phenomena created by light shining through supersaturated water vapor since the 1890s. In 1896 he discovered that supersaturated vapor condensed around the ions created by X rays just as it did around any other foreign body.

The cloud chamber played a key role in the early discoveries about particle physics. It enabled physicists to study the behavior of individual atoms, to photograph the actual paths of ionizing particles, and to analyze the complicated interactions that take place between charged particles and individual atoms.

Anderson, Carl David

2008-10-12T20:30:00.007-07:00

Anderson, Carl David (1905-91), American physicist and Nobel laureate. Anderson was born in New York City and educated at the California Institute of Technology, where he attained full professorial rank in 1939. In 1932 he discovered the positron, or positive electron, one of the fundamental subatomic particles. For this achievement he was awarded, with Victor Franz Hess, the 1936 Nobel Prize in physics. In 1936 Anderson also confirmed experimentally the existence of the elementary nuclear particle called the meson, which had been predicted in 1935 by the Japanese physicist Yukawa Hideki.

Yukawa Hideki

2008-10-12T20:30:00.005-07:00

Yukawa Hideki (1907-81), Japanese physicist and Nobel laureate, noted for his study of nuclear forces.

Yukawa was born in Tokyo and was educated at the universities of Kyōto and Ōsaka. He became a lecturer in physics at Kyōto University in 1932 and was made professor in 1939. Yukawa also taught (1933-36) at Ōsaka University and was assistant professor there until 1939. He was visiting professor at the Institute for Advanced Studies at Princeton, New Jersey, in 1948 and at Columbia University from 1949 to 1953. Yukawa became (1950) professor emeritus at Ōsaka University and was named (1953) director of the Research Institute for Fundamental Physics at Kyōto University. Yukawa did extensive research in quantum mechanics (see Quantum Theory) and the fields of force affecting elementary nuclear particles. In 1935 he theoretically deduced the existence of the meson (see Elementary Particles), for which he was awarded the 1949 Nobel Prize in physics. The existence of the meson was proved in 1936.

Bunsen Burner

2008-10-12T20:30:00.003-07:00

Bunsen Burner, heating device widely used in laboratories because it provides a hot, steady, smokeless flame. It is named for the German chemist Robert Wilhelm Bunsen, who adapted the concept of a gas-air burner in 1855 and popularized its use. The burner is a short, vertical tube of metal connected to a gas source and perforated at the bottom to admit air. The flow of air is controlled by an adjustable collar on the tube.

Black, Joseph

2008-10-12T20:30:00.001-07:00

Black, Joseph (chemist) (1728-1799), British chemist, best known for his detailed account of the isolation and chemical activity of carbon dioxide. Black was born in Bordeaux, France, and educated at the universities of Glasgow and Edinburgh in Scotland. He was professor of chemistry, medicine, and anatomy at the University of Glasgow from 1756 to 1766; thereafter he was professor of chemistry at the University of Edinburgh. In about 1761 Black discovered the phenomenon of latent heat, and three years later he measured the latent heat of steam. His pupil and assistant James Watt later put these discoveries to practical use when he made improvements to the early steam engine. About 1754 Black discovered carbon dioxide, a gas which he called fixed air, and showed its function in the causticization of lime (making lime more alkaline), thus helping to disprove the phlogiston theory of combustion. He also discovered that different substances have different heat capacities.

Watt, James

2008-10-12T20:29:00.011-07:00

Watt, James (1736-1819), Scottish inventor and mechanical engineer, renowned for his improvements of the steam engine.

Watt was born on January 19, 1736, in Greenock, Scotland. He worked as a mathematical-instrument maker from the age of 19 and soon became interested in improving the steam engines, invented by the English engineers Thomas Savery and Thomas Newcomen, which were used at the time to pump water from mines.

Watt determined the properties of steam, especially the relation of its density to its temperature and pressure, and designed a separate condensing chamber for the steam engine that prevented enormous losses of steam in the cylinder and enhanced the vacuum conditions. Watt's first patent, in 1769, covered this device and other improvements on Newcomen's engine, such as steam-jacketing, oil lubrication, and insulation of the cylinder in order to maintain the high temperatures necessary for maximum efficiency.

At this time, Watt was the partner of the British inventor John Roebuck, who had financed his researches. In 1775, however, Roebuck's interest was taken over by British manufacturer Matthew Boulton, owner of the Soho Engineering Works at Birmingham, and he and Watt began the manufacture of steam engines. Watt continued his research and patented several other important inventions, including the rotary engine for driving various types of machinery; the double-action engine, in which steam is admitted alternately into both ends of the cylinder; and the steam indicator, which records the steam pressure in the engine. He retired from the firm in 1800 and thereafter devoted himself entirely to research work.

The misconception that Watt was the actual inventor of the steam engine arose from the fundamental nature of his contributions to its development. The centrifugal or flyball governor, which he invented in 1788, and which automatically regulated the speed of an engine, is of particular interest today. It embodies the feedback principle of a servomechanism, linking output to input, which is the basic concept of automation. The electrical unit, the watt, was named in his honor. Watt was also a renowned civil engineer, making several surveys of canal routes. He invented, in 1767, an attachment that adapted telescopes for use in measurement of distances. Watt died in Heathfield, England, on August 19, 1819.

Scheele, Carl Wilhelm

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Scheele, Carl Wilhelm (1742-1786), Swedish chemist, noted for his discovery of a great number of elements, compounds, and chemical reactions.

Scheele was born in Stralsund, Germany, which at that time was the capital of Swedish Pomerania. He had no formal training in chemistry and studied the elements of science while apprenticed to an apothecary. In 1770 he came under the guidance of Swedish chemist Torbern Bergman. In 1775 Scheele became the proprietor of a pharmacy in Köping, Sweden, where he continued his chemical research. He is credited with the identification of the elements chlorine and barium, but Scheele believed that they were compounds, not elements. British chemist Sir Humphrey Davy recognized chlorine and barium as elements in the early 1800s. Scheele prepared oxygen from various oxides independently of and somewhat before English chemist Joseph Priestley, who is credited with the discovery of the element. He was the first to prepare many compounds, including tartaric acid, arsine, and hydrogen sulfide. He demonstrated that lactic acid was the acid component of sour milk. He also determined the properties and composition of hydrogen cyanide and those of citric, malic, oxalic, and gallic acids. In 1931 the Collected Papers of Carl Wilhelm Scheele was published.

Priestley, Joseph

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Priestley, Joseph (1733-1804), British chemist, who isolated and described several gases, including oxygen, and who is considered one of the founders of modern chemistry because of his contributions to experimentation.

Priestley was born on March 13, 1733, in Fieldhead, Yorkshire, the son of a Calvinist minister. Priestley trained as a minister of the Dissenting church, which comprised various churches that had separated from the Church of England. He was educated at Daventry Academy, where he became interested in physical science. His first ministry was at Needham Market, Suffolk, in 1755, and he was minister at Nantwich from 1758 to 1761. Later he became a tutor at Warrington Academy in Lancashire, where he was noted for his development of practical courses for students planning to enter industry and commerce. He also wrote a text, Rudiments of English Grammar (1761), which differed from older, classical approaches. He was ordained in 1762.

Priestley was encouraged to conduct experiments in the new science of electricity by the American statesman and scientist Benjamin Franklin, whom he met in London in 1766. Priestley wrote The History of Electricity the following year. He also discovered that charcoal can conduct electricity. In 1767 Priestley became minister at Leeds, where he grew interested in research on gases. His innovative experimental work resulted in his election to the French Academy of Sciences in 1772, the same year in which he was employed by William Petty Fitzmaurice, 2nd earl of Shelburne, as librarian and literary companion.

During Priestley's experiments in 1774, he discovered oxygen and described its role in combustion and in respiration. An advocate of the phlogiston theory, however, Priestley called the new gas dephlogisticated air and did not completely understand the future importance of his discovery. The Swedish chemist Carl Wilhelm Scheele may have discovered oxygen before Priestley, but did not make his work known in time to be credited with its discovery. Priestley also isolated and described the properties of several other gases, including ammonia, nitrous oxide, sulfur dioxide, and carbon monoxide. During his career, Priestley remained opposed to the revolutionary theories of the French chemist Antoine Lavoisier, who gave oxygen its name and correctly described its role in combustion.

In 1780 Priestley left his position with Petty because of religious differences. He became a minister in Birmingham. By this time he had turned to Unitarian thinking, and was considered a religious radical. His book, History of the Corruptions of Christianity (1782), was officially burned in 1785. Because of his open support of the French Revolution, his house and effects were burned by a mob in 1791. He went to live in London, and in 1794 he emigrated to the United States, where he pursued his writing for the remainder of his life. Priestley died in Northumberland, Pennsylvania, on February 6, 1804. His posthumously collected Theological and Miscellaneous Works (25 volumes, 1817-1832) and Memoirs and Correspondence (2 volumes, 1831-1832) cover a wide variety of subjects in science, politics, and religion.

Franklin, Benjamin

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Franklin, Benjamin (1706-1790), American printer, author, diplomat, philosopher, and scientist, whose many contributions to the cause of the American Revolution (1775-1783), and the newly formed federal government that followed, rank him among the country’s greatest statesmen.

Franklin was born on January 17, 1706, in Boston. His father, Josiah Franklin, a tallow chandler by trade, had 17 children; Benjamin was the 15th child and the 10th son. His mother, Abiah Folger, was his father’s second wife. The Franklin family was in modest circumstances, like most New Englanders of the time. After his attendance at grammar school from age eight to ten, Benjamin was taken into his father’s business. Finding the work uncongenial, however, he entered the employ of a cutler. At age 13 he was apprenticed to his brother James, who had recently returned from England with a new printing press. Benjamin learned the printing trade, devoting his spare time to the advancement of his education. His reading included Pilgrim’s Progress by the British preacher John Bunyan, Parallel Lives, the work of the Greek essayist and biographer Plutarch, Essay on Projects by the English journalist and novelist Daniel Defoe, and the Essays to Do Good by Cotton Mather, the American Congregational clergyman. When he acquired a copy of the third volume of the Spectator by the British statesmen and essayists Sir Richard Steele and Joseph Addison, he set himself the goal of mastering its prose style.

In 1721 his brother James Franklin established the New England Courant, and Benjamin, at the age of 15, was busily occupied in delivering the newspaper by day and in composing articles for it at night. These articles, published anonymously, won wide notice and acclaim for their pithy observations on the current scene. Because of its liberal bias, the New England Courant frequently incurred the displeasure of the colonial authorities. In 1722, as a consequence of an article considered particularly offensive, James Franklin was imprisoned for a month and forbidden to publish his paper, and for a while it appeared under Benjamin’s name.

CGS System

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CGS System, also centimeter-gram-second system (usually written “cgs system”), a metric system based on the centimeter (c) for length, the gram (g) for mass, and the second (s) for time. It is derived from the meter-kilogram-second (or mks) system but uses certain special designations such as the dyne (for force) and the erg (for energy). It has generally been employed where small quantities are encountered, as in physics and chemistry.

Clocks and Watches

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Clocks and Watches, devices used to measure or indicate the passage of time. A clock, which is larger than a watch, is usually intended to be kept in one place; a watch is designed to be carried or worn. Both types of timepieces require a source of power and a means of transmitting and controlling it, as well as indicators to register the lapse of time units.

In a clock, the source of power may be produced by weight, a mainspring, or an electric current. Except in electric or electronic clocks, periodic adjustments, such as lifting the weight or tightening the spring, are needed. The motive force generated by the power source in a mechanical clock is transmitted by a gear train and regulated by a pendulum or a balance wheel. In such a clock, the time may be reported audibly by the striking of a gong or chime and is registered visually by the rotation of wheels bearing numerals or by the position of hands on a dial. In electric or electronic clocks, time may be shown by a display of numbers.

A mechanical watch uses a coiled spring as its power source. As in spring-powered clocks, the watch conserves energy by means of a gear train, with a balance wheel regulating the motive force. In self-winding watches, the mainspring is tightened automatically by means of a weight on a rotor that responds to the arm movements of the wearer.

Dentistry

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Dentistry, practice of preventing and treating diseases of the teeth, gums, and tissues of the mouth. Unlike other human tissue, such as skin, that continuously grows and self-rejuvenates, dental structures generally cannot repair themselves and require regular care to retain their health and vitality. If not treated, dental health problems can lead to complications in other parts of the body. Thorough and timely dental care is not only important for maintaining healthy teeth and gums, it is essential to overall human health.

Dentists and dental hygienists are health care professionals trained and licensed to provide dental care. General dentistry emphasizes treatments that prevent oral health problems, especially dental caries, commonly called tooth decay. Tooth decay is one of the most prevalent diseases in the United States, second only to the common cold. Dentists help prevent tooth decay by cleaning teeth to remove buildup of calculus, or tartar, which forms when plaque, a sticky film of bacteria, hardens on the teeth. As they feed on sugar and food residue on the teeth, the bacteria produce acids. If not removed regularly, these acids eat away the tooth enamel, leaving decayed holes in the teeth called cavities. To help prevent cavities, dentists apply fluoride (a mineral that strengthens tooth enamel) to teeth. Dental sealants—clear plastic coatings that are brushed onto the chewing surfaces of molars—are also effective in preventing tooth decay.

Huygens, Christiaan

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Huygens, Christiaan (1629-1695), Dutch astronomer, mathematician, and physicist, born in The Hague. His numerous, original scientific discoveries won him wide recognition and honors among scientists of the 17th century. Among his discoveries was the principle (later named after him) that states that every point on the front of an advancing wave is itself a source of new waves. From this principle he developed the wave theory of light. In 1655 he found a new method of grinding and polishing lenses. The sharper definitions obtained enabled him to discover a satellite of Saturn and to give the first accurate description of the rings of Saturn. The need for an exact measure of time for observing the heavens led to his applying the pendulum to regulate the movement of clocks. In 1656 he devised an eyepiece for the telescope that bears his name. In Horologium Oscillatorium (1673) he determined the true relation between the length of a pendulum and the period of oscillation and developed theories on centrifugal force in circular motion that assisted the English mathematician Sir Isaac Newton in formulating the laws of gravity. In 1678 he discovered the polarization of light by double refraction in calcite.

Diode

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Diode, electronic device that allows the passage of current in only one direction. The first such devices were vacuum-tube diodes, consisting of an evacuated glass or steel envelope containing two electrodes—a cathode and an anode. Because electrons can flow in only one direction, from cathode to anode, the vacuum-tube diode could be used as a rectifier (see Rectification). The diodes most commonly used in electronic circuits today are semiconductor diodes. The simplest of these, the germanium point-contact diode, dates from the early days of radio, when the received radio signal was detected by means of a germanium crystal and a fine, pointed wire that rested on it. In modern germanium (or silicon) point-contact diodes, the wire and a tiny crystal plate are mounted inside a small glass tube and connected to two wires that are fused into the ends of the tube. See Electronics; Vacuum Tubes.

Junction-type diodes consist of a junction of two different kinds of semiconductor material. The Zener diode is a special junction-type diode, using silicon, in which the voltage across the junction is independent of the current through the junction. Because of this characteristic, Zener diodes are used as voltage regulators. Another special junction-type diode is used in solar cells; a voltage appears spontaneously when the junction is illuminated. In light-emitting diodes (LEDs), on the other hand, a voltage applied to the semiconductor junction results in the emission of light energy. LEDs are used in numerical displays such as those on electronic digital watches and pocket calculators. See Photoelectric Effect.

Electronics

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Electronics, field of engineering and applied physics dealing with the design and application of devices, usually electronic circuits, the operation of which depends on the flow of electrons for the generation, transmission, reception, and storage of information. The information can consist of voice or music (audio signals) in a radio receiver, a picture on a television screen, or numbers and other data in a computer.

Electronic circuits provide different functions to process this information, including amplification of weak signals to a usable level; generation of radio waves; extraction of information, such as the recovery of an audio signal from a radio wave (demodulation); control, such as the superimposition of an audio signal onto radio waves (modulation); and logic operations, such as the electronic processes taking place in computers.

Topics:

Electronic Components
Power Suplly Circuits
Amplifier Circuits
Oscillators
Switching and Timing Circuits
Recent Developments

Electronics

2008-10-12T20:27:00.009-07:00

Electronics->> ELECTRONIC COMPONENTS

Electronic circuits consist of interconnections of electronic components. Components are classified into two categories—active or passive. Passive elements never supply more energy than they absorb; active elements can supply more energy than they absorb. Passive components include resistors, capacitors, and inductors. Components considered active include batteries, generators, vacuum tubes, and transistors.

A. Vacuum Tubes
B. Transistors
C. Integrated Circuits
D. Resistors
E. Capacitors
F. Inductors
G. Sensing Devices and Transducers

Electronics

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Electronics->> Electronic Components->> VACUUM TUBES

A vacuum tube consists of an air-evacuated glass envelope that contains several metal electrodes. A simple, two-element tube (diode) consists of a cathode and an anode that is connected to the positive terminal of a power supply. The cathode—a small metal tube heated by a filament—frees electrons , which migrate to the anode—a metal cylinder around the cathode (also called the plate). If an alternating voltage is applied to the anode, electrons will only flow to the anode during the positive half-cycle; during the negative cycle of the alternating voltage, the anode repels the electrons, and no current passes through the tube. Diodes connected in such a way that only the positive half-cycles of an alternating current (AC) are permitted to pass are called rectifier tubes; these are used in the conversion of alternating current to direct current (DC) (see Electricity; Rectification). By inserting a grid, consisting of a spiral of metal wire, between the cathode and the anode and applying a negative voltage to the grid, the flow of electrons can be controlled. When the grid is negative, it repels electrons, and only a fraction of the electrons emitted by the cathode can reach the anode. Such a tube, called a triode, can be used as an amplifier. Small variations in voltage at the grid, such as can be produced by a radio or audio signal, will cause large variations in the flow of electrons from the cathode to the anode and, hence, in the circuitry connected to the anode.

Electronics

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Electronics->> Electronic Components->> TRANSISTORS

Transistors are made from semiconductors. These are materials, such as silicon or germanium, that are “doped” (have minute amounts of foreign elements added) so that either an abundance or a lack of free electrons exists. In the former case, the semiconductor is called n-type, and in the latter case, p-type. By combining n-type and p-type materials, a diode can be produced. When this diode is connected to a battery so that the p-type material is positive and the n-type negative, electrons are repelled from the negative battery terminal and pass unimpeded to the p-region, which lacks electrons. With battery reversed, the electrons arriving in the p-material can pass only with difficulty to the n-material, which is already filled with free electrons, and the current is almost zero.

The bipolar transistor was invented in 1948 as a replacement for the triode vacuum tube. It consists of three layers of doped material, forming two p-n (bipolar) junctions with configurations of p-n-p or n-p-n. One junction is connected to a battery so as to allow current flow (forward bias), and the other junction has a battery connected in the opposite direction (reverse bias). If the current in the forward-biased junction is varied by the addition of a signal, the current in the reverse-biased junction of the transistor will vary accordingly. The principle can be used to construct amplifiers in which a small signal applied to the forward-biased junction causes a large change in current in the reverse-biased junction.

Another type of transistor is the field-effect transistor (FET). Such a transistor operates on the principle of repulsion or attraction of charges due to a superimposed electric field. Amplification of current is accomplished in a manner similar to the grid control of a vacuum tube. Field-effect transistors operate more efficiently than bipolar types, because a large signal can be controlled by a very small amount of energy.

Electronics

2008-10-12T20:27:00.003-07:00

Electronics->> Electronic Components->> INTEGRATED CIRCUITS, RESISTORS AND CAPACITORS

Integrated Circuits

Most integrated circuits are small pieces, or “chips,” of silicon, perhaps 2 to 4 sq mm (0.08 to 0.15 sq in) long, in which transistors are fabricated. Photolithography enables the designer to create tens of thousands of transistors on a single chip by proper placement of the many n-type and p-type regions. These are interconnected with very small conducting paths during fabrication to produce complex special-purpose circuits. Such integrated circuits are called monolithic because they are fabricated on a single crystal of silicon. Chips require much less space and power and are cheaper to manufacture than an equivalent circuit built by employing individual transistors.

Resistors

If a battery is connected across a conducting material, a certain amount of current will flow through the material (see Resistance). This current is dependent on the voltage of the battery, on the dimensions of the sample, and on the conductivity of the material itself. Resistors with known resistance are used for current control in electronic circuits. The resistors are made from carbon mixtures, metal films, or resistance wire and have two connecting wires attached. Variable resistors, with an adjustable sliding contact arm, are often used to control volume on radios and television sets.

Capacitors

Capacitors consist of two metal plates that are separated by an insulating material (see Capacitor). If a battery is connected to both plates, an electric charge will flow for a short time and accumulate on each plate. If the battery is disconnected, the capacitor retains the charge and the voltage associated with it. Rapidly changing voltages, such as caused by an audio or radio signal, produce larger current flows to and from the plates; the capacitor then functions as a conductor for the changing current. This effect can be used, for example, to separate an audio or radio signal from a direct current in order to connect the output of one amplifier stage to the input of the next amplifier stage.

Electronics

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Electronics->> Electronic Components->> INDUCTORS, SENSING DEVICES AND TRANSDUCERS

Inductors

Inductors consist of a conducting wire wound into the form of a coil. When a current passes through the coil, a magnetic field is set up around it that tends to oppose rapid changes in current intensity (see Induction). As a capacitor, an inductor can be used to distinguish between rapidly and slowly changing signals. When an inductor is used in conjunction with a capacitor, the voltage in the inductor reaches a maximal value for a specific frequency. This principle is used in a radio receiver, where a specific frequency is selected by a variable capacitor.

Sensing Devices and Transducers

Measurements of mechanical, thermal, electrical, and chemical quantities are made by devices called sensors and transducers. The sensor is responsive to changes in the quantity to be measured, for example, temperature, position, or chemical concentration. The transducer converts such measurements into electrical signals, which, usually amplified, can be fed to instruments for the readout, recording, or control of the measured quantities. Sensors and transducers can operate at locations remote from the observer and in environments unsuitable or impractical for humans.

Some devices act as both sensor and transducer. A thermocouple has two junctions of wires of different metals; these generate a small electric voltage that depends on the temperature difference between the two junctions. A thermistor is a special resistor, the resistance of which varies with temperature. A variable resistor can convert mechanical movement into an electrical signal. Specially designed capacitors are used to measure distance, and photocells are used to detect light (see Photoelectric Cell). Other devices are used to measure velocity, acceleration, or fluid flow. In most instances, the electric signal is weak and must be amplified by an electronic circuit.

Electronics

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Electronics->> POWER SUPPLY CIRCUITS

Most electronic equipment requires DC voltages for its operation. These can be provided by batteries (see Battery) or by internal power supplies that convert alternating current as available at the home electric outlet, into regulated DC voltages. The first element in an internal DC power supply is a transformer, which steps up or steps down the input voltage to a level suitable for the operation of the equipment. A secondary function of the transformer is to provide electrical ground insulation of the device from the power line to reduce potential shock hazards. The transformer is then followed by a rectifier, normally a diode. In the past, vacuum diodes and a wide variety of different materials such as germanium crystals or cadmium sulfide were employed in the low-power rectifiers used in electronic equipment. Today silicon rectifiers are used almost exclusively because of their low cost and their high reliability.

Fluctuations and ripples superimposed on the rectified DC voltage (noticeable as a hum in a malfunctioning audio amplifier) can be filtered out by a capacitor; the larger the capacitor, the smaller is the amount of ripple in the voltage. More precise control over voltage levels and ripples can be achieved by a voltage regulator, which also makes the internal voltages independent of fluctuations that may be encountered at an outlet. A simple, often-used voltage regulator is the zener diode. It consists of a solid-state p-n-junction diode, which acts as an insulator up to a predetermined voltage; above that voltage it becomes a conductor that bypasses excess voltages. More sophisticated voltage regulators are usually constructed as integrated circuits.

Electronics

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Electronics->> AMPLIFIER CIRCUITS

Electronic amplifiers are used mainly to increase the voltage, current, or power of a signal. A linear amplifier provides signal amplification with little or no distortion, so that the output is proportional to the input. A nonlinear amplifier may produce a considerable change in the waveform of the signal. Linear amplifiers are used for audio and video signals, whereas nonlinear amplifiers find use in oscillators, power electronics, modulators, mixers, logic circuits, and other applications where an amplitude cutoff is desired. Although vacuum tubes played a major role in amplifiers in the past, today either discrete transistor circuits or integrated circuits are mostly used.

Audio Amplifiers

Audio amplifiers, such as are found in radios, television sets, citizens band (CB) radios, and cassette recorders, are generally operated at frequencies below 20 kilohertz (1 kHz = 1000 cycles/sec). They amplify the electrical signal, which then is converted to sound in a loudspeaker. Operational amplifiers (op-amps), built with integrated circuits and consisting of DC-coupled, multistage, linear amplifiers are popular for audio amplifiers.

Video Amplifiers

Video amplifiers are used mainly for signals with a frequency spectrum range up to 6 megahertz (1 MHz = 1 million cycles/sec). The signal handled by the amplifier becomes the visual information presented on the television screen, with the signal amplitude regulating the brightness of the spot forming the image on the screen. To achieve its function, a video amplifier must operate over a wide band and amplify all frequencies equally and with low distortion. See Video Recording.

Radio Frequency Amplifiers

These amplifiers boost the signal level of radio or television communication systems. Their frequencies generally range from 100 kHz to 1 GHz (1 billion cycles/sec = 1 gigahertz) and can extend well into the microwave frequency range.