Pyrometer
a term originally applied to an instrument in the form of a single metallic bar, employed by Muschenbroek about 1730, to indicate temperatures above the boiling point of mercury, 660° F. It is now applied to any instrument used for such purpose.
Tremeschini's pyrometer is founded on the expansion of a thin plate of platinum, heated by a mass of metal previously raised to the temperature of the medium. The Trampler pyrometer is based on the difference in the coefficients of dilatation for iron and graphite; the Gauntlet pyrometer on the difference of those of iron and fire-clay. The Ducomet pyrometer consists of a series of rings made of alloys which have slightly different melting points. In pyrometers on the Watertype principle, the temperature is determined by noting the amount of heat communicated to a current of water of known temperature which is kept circulating in the medium to be observed.