Thermometer

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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°.

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