Thermometer

Thermometer, instrument used to measure temperature. The most commonly used thermometer is the mercury-in-glass type, which consists of a uniform-diameter glass capillary that opens into a mercury-filled bulb at one end. The assembly is sealed to preserve a partial vacuum in the capillary. If the temperature increases, the mercury expands and rises in the capillary. The temperature may then be read on an adjacent scale. Mercury is widely used for measuring ordinary temperatures; alcohol, ether, and other liquids are also employed for this purpose.

The invention of the thermometer is attributed to Galileo, although the sealed thermometer did not come into existence until about 1650. The modern alcohol and mercury thermometers were invented by the German physicist Gabriel Fahrenheit, who also proposed the first widely adopted temperature scale, named after him, in which 32° F is the freezing point of water and 212° F is its boiling point at standard atmospheric pressure. Various temperature scales have been proposed since his time; in the centigrade, or Celsius, scale, devised by the Swedish astronomer Anders Celsius and used in most of the world, the freezing point is 0°, the boiling point is 100°.

Vacuum Cleaner

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Vacuum Cleaner, electrical appliance in common use for cleaning furniture, floors, rugs, and carpets by suction. Generally, vacuum cleaners are of two types: the vertical type, which is light and moves over the surfaces to be cleaned, and the canister, or tank, type, which trails a long hose with a nozzle that can be moved over the area to be cleaned. An electric motor inside the appliance turns a fan that creates a partial vacuum and causes outside air to rush into the evacuated space. This forces any dirt or dust near the nozzle into a bag inside the machine or attached to the outside.

Most vacuum cleaners have a variety of attachments that can be used to clean different types of surfaces, such as window sills and thick rugs and carpets; some are also equipped to polish floors and shampoo rugs. A reversible motor on many appliances is used to blow dirt out of difficult and inaccessible surfaces and also to spray paint.

Vacuum Technology

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Vacuum Technology, in physics and engineering, processes and equipment based on the principle that, when the quantity of oppressive gas such as air in a closed vessel is removed, the remaining molecules, atoms, or any electrically charged particles that are derived from them, such as ions and electrons, can move about more freely (see Vacuum). This freedom is proportional to the reduction in the gas pressure. See Atom; Electron; Ion; Molecule; Pressure.

Low and medium vacuums have been commonly used in such household equipment as vacuum bottles and vacuum cleaners since the late 19th century. The distillation of lubricating oils from petroleum residues and the removal of atmospheric oxygen from electric light bulbs also employ vacuum technology. Before World War II, however, high-vacuum techniques, achieving near-complete vacuum conditions, were mainly used in research laboratories, the one exception being radio-tube production (see Vacuum Tubes). During the war, techniques for coating optical lenses with extremely thin films of magnesium fluoride, using high vacuum, became established. This process improved the optical quality of the lenses by reducing light reflection (see Lens; Optics). High-vacuum techniques are also employed in the molecular distillation of fish oils to produce vitamin A concentrate, and in the electromagnetic separation of uranium-235 from nonradioactive uranium with which it is associated in nature. See Isotope; Radioactivity.

One of the more important recent applications of vacuum technology is in large-scale industrial refrigeration. The rate of evaporation of water is accelerated in vacuum conditions and the process is used for freeze-drying foods. The water in the food is removed by sublimation into ice that, at the same time, freezes the food. Metal evaporation in high vacuum is used to coat plastics and other objects to give them a high, metallic luster. This process was an outgrowth of the lens-coating process. Television-tube production rate was greatly accelerated by the introduction of high-speed, high-vacuum pumps. High-vacuum treatment of melted, cast, or sintered metals improves their physical properties by removing gases and other impurities. Single-metal crystals used in transistors and similar electronic devices are “grown,” or prepared, in high-vacuum furnaces (see Crystal; Furnace; Transistor). Electrical transformers and high-voltage cables are vacuum impregnated with high dielectric material to improve the insulation. To obtain maximum insulation from heat for flasks and pipes that store and transport liquid oxygen, nitrogen, and helium, the container walls are maintained at high vacuum. Substrates, or bases, used in making electronic microcircuits are prepared by sputter-coating them with refractory materials such as tantalum and tungsten under high-vacuum conditions.

Vacuum is very important in scientific and technological research. Atomic particle accelerators depend on high and very high vacuum to provide a relatively gas-free unobstructed path for the ionized particles to travel (see Ionization). Large chambers of up to thousands of cubic-meters capacity, and requiring great pumping speeds for gas removal, are used to test aerospace equipment in simulated outer-space conditions. In certain types of analyses, if the material to be analyzed must be in a gaseous state or in the form of electrically charged ions, then vacuum must be used to produce these requirements. The mass spectroscope, electron microscope, and vacuum-fusion and nuclear magnetic resonance analyzers are a few such instruments (see Chemical Analysis; Microscope; Spectroscopy). New uses for the unique capabilities of vacuum operation are continually being discovered.

Vacuum Tubes

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Vacuum Tubes, electronic devices, consisting of a glass or steel vacuum envelope and two or more electrodes between which electrons can move freely. The vacuum-tube diode was first developed by the English physicist Sir John Ambrose Fleming. It contains two electrodes: the cathode, a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the anode, or plate, which is the electron-collecting element. In diodes, the electrons emitted by the cathode are attracted to the plate only when the latter is positive with respect to the cathode. When the plate is negatively charged, no current flows through the tube. If an alternating potential is applied to the plate, the tube passes current only during the positive halves of the cycle and thus acts as a rectifier. Diodes are used extensively in the rectification of alternating current (see Electronics; Rectification).

The introduction of a third electrode, called a grid, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for amplifying current. (The triode was invented in 1906 by the American engineer Lee De Forest.) The function of the grid is to control the current flow. At a certain negative potential, the grid, because it repels electrons, can impede the flow of electrons between the cathode and the anode. At lower negative potentials, the electron flow depends on the grid potential. The grid usually consists of a network of fine wire surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the voltage between the grid and the cathode causing large changes in the number of electrons reaching the anode.

Through the years more complex tubes with additional grids have been developed to provide greater amplification and to perform specialized functions. Tetrodes have an additional grid, closer to the anode, that forms an electrostatic shield between the anode and the grid to prevent feedback to the grid in high-frequency applications. The pentode has three grids between the cathode and the anode; the third grid, close to the anode, reflects electrons that are emitted by the anode as it is heated by electron impact when the electron current in the tube is high. Tubes with even more grids, called hexodes, heptodes, and octodes, find applications as frequency converters and mixers in radio receivers (see Radio).

Vacuum tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable (see Transistor). Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and in military equipment that must resist the voltage pulse (which destroys transistors) induced by an atmospheric nuclear explosion.

Particle Detectors

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Particle Detectors, instruments used to detect and study fundamental nuclear particles (see Atom; Nuclear Energy). These detectors range in complexity from the well-known portable Geiger counter to room-sized spark and bubble chambers.

A. IONIZATION CHAMBER

One of the first detectors to be used in nuclear physics was the ionization chamber, which consists essentially of a closed vessel containing a gas and equipped with two electrodes at different electrical potentials. The electrodes, depending on the type of instrument, may consist of parallel plates or coaxial cylinders, or the walls of the chamber may act as one electrode and a wire or rod inside the chamber act as the other. When ionizing particles of radiation enter the chamber they ionize the gas between the electrodes. The ions that are thus produced migrate to the electrodes of opposite sign (negatively charged ions move toward the positive electrode, and vice versa), creating a current that may be amplified and measured directly with an electrometer—an electroscope equipped with a scale—or amplified and recorded by means of electronic circuits.

Ionization chambers adapted to detect individual ionizing particles of radiation are called counters. The Geiger-Müller counter is one of the most versatile and widely used instruments of this type. It was developed by the German physicist Hans Geiger from an instrument first devised by Geiger and the British physicist Ernest Rutherford; it was improved in 1928 by Geiger and by the German American physicist Walther Müller. The counting tube is filled with a gas or a mixture of gases at low pressure, the electrodes being the thin metal wall of the tube and a fine wire, usually made of tungsten, stretched lengthwise along the axis of the tube. A strong electric field maintained between the electrodes accelerates the ions; these then collide with atoms of the gas, detaching electrons and thus producing more ions. When the voltage is raised sufficiently, the rapidly increasing current produced by a single particle sets off a discharge throughout the counter. The pulse caused by each particle is amplified electronically and then actuates a loudspeaker or a mechanical or electronic counting device.

B. TRACK DETECTORS

C. OTHER TYPES OF DETECTORS