PYROMETER, an instrument for measur ing temperatures higher than the boiling-point of mercury (350° C.). Regnault and other careful experimenters employed the expansion of air and vapors in measuring high tempera tures. These pyrometers can only be used in a laboratory; for use in a manufactory less ac curate methods of measurement have to be em ployed. In Wedgwood's pyronieter (1782) it was assumed that dry clay, when exposed to heat, contracted regularly, and the instrument was constructed on this principle; but it is now known to be untrustworthy. An instrument in which the expansion of a metal bar (generally of platinum) was magnified by means of levers has often been employed to measure the tem perature of a furnace. Morveau, Brongniart and Daniell invented instruments on this prin ciple. The thermoelectric law of Tait may be used to calculate the temperature to which a junction of copper and iron has been sub jetted, the electric .current ,produced.• beieg measured by a galvanometer. • In this way high temperatures have been. measured with consid-• accuracy. By 'Siemens' method tures are measured by observing the increased electric resistance of a platinum wire when heated, two coils of the same kind of wire being prepared so as to have equal resistances at the same temperature, their ends connected by rather long thick copper wires and the current from a constant battery passing through them measured by means of a galvanometer. One of
the coils is kept at a known temperature, the other has the temperature which it is desired to measure. The law discovered by Siemens — where R is the resistance at the absolute tem perature T, and where a, .13 arid are .serrain numbers, enables the temperature to be Wet Pouiliet determined the following tem peratures by means of an air-thermonieterf They correspond to the stages of incandescence, of a metal bar. - , red -beat S2S° Dull red 700° Cherry red Darkcrams 100° White 1 ' Dowding white 1300° In potteries cones and rings of clay' are employed. to measure temperatures. The cones are made to soften at a known heat and the rings indicate rising temperature by' For optical purposes pyrometers are made to indicate by showing metallic colors that are compared with a standard. A simple form of pyrometer measures radiation and st pointer by a bimetallic spring. The thereto-' electric principle is now employed in recording pyrometers that may be located at a distance. from the inmate. 'See THEerstinEray.