Five different temperature scales are in use today:
(1) the Celsius scale, known also as the Centigrade scale,
(2) the Fahrenheit scale,
(3) the Kelvin scale,
(4) the Rankine scale, and
(5) the international thermodynamic temperature scale.
The Celsius scale, with a freezing point of 0° C and a
boiling point of 100° C, is widely used throughout the world, particularly for
scientific work, although it was superseded officially in 1950 by the
international temperature scale. In the Fahrenheit scale, used in
English-speaking countries for purposes other than scientific work and based on
the mercury thermometer, the freezing point of water is defined as 32° F and
the boiling point as 212° F (see Mercury). In the Kelvin scale, the most
commonly used thermodynamic temperature scale, zero is defined as the absolute
zero of temperature, that is, -273.15° C, or -459.67° F. Another scale
employing absolute zero as its lowest point is the Rankine scale, in which each
degree of temperature is equivalent to one degree on the Fahrenheit scale. The
freezing point of water on the Rankine scale is 492° R, and the boiling point
is 672° R.
In 1933 scientists of 31 nations adopted a new international
temperature scale with additional fixed temperature points, based on the Kelvin
scale and thermodynamic principles. The international scale is based on the
property of electrical resistivity, with platinum wire as the standard for
temperature between -190° and 660° C. Above 660° C, to the melting point of
gold, 1063° C, a standard thermocouple, which is a device that measures
temperature by the amount of voltage produced between two wires of different
metals, is used; beyond this point temperatures are measured by the so-called
optical pyrometer, which uses the intensity of light of a wavelength emitted by
a hot body for the purpose. (Encarta Encyclopedia)
