Pyrometry, the measurement of temperatures beyond the compass of the mercurial Thermometer (q.v.). The leading methods are ocular, calorimetric, and pyrometric. The eye alone is often sufficiently accurate, and can distinguish dull red, 525° C. (say 975° F.); cherry red, 800° C. (say 1450° F.); orange, 1100° C. (2000° F.); white, 1300° C. (2350° F.); dazzling white, 1500° C. (2700° F.). Or we may use cobalt glass as a means of more sharply discriminating the changes of visible colour. Calorimetric: a lump of heated metal is thrown into a known quantity of water; the rise of temperature is measured; the temperature of the heated metal is next calculated from its weight, its specific heat, and the rise of temperature and the quantity of the water. This method admits errors from loss of time and radiation; hence only rough results are attained, comparable with one another, but not numerically reliable. Of pyrometric methods may be named expansion of air, hydrogen or nitrogen (only suited for laboratory purposes, for glass melts, metals become permeable, and porcelain is fragile), or of mercury vapour; dilatation of solids—porcelain, platinum, or iron (Professor J. F. Daniell, 1821)—whose expansions are very small and difficult to measure, as they generally take up a new set or form when alternately heated and cooled; the shrinkage of clay (Wedgwood's pyrometer) giving variable results; the actual fusion of definite metals, alloys, or enamels whose melting-points have been previously ascertained; the temperature acquired by water made to flow uniformly through a tube partially exposed to the heat to be explored; the speeds of outflow of air through an aperture at the atmosphere and at the furnace temperature (Barns, American Journal of Science, 1889); Siemens' electric pyrometer, which measures the change in the resistance of platinum wire exposed to the furnace heat; Becquerel's thermo-electric pyrometer, in which a thermo-electric couple (platinum-palladium) is exposed to the heat. When Le Châtelier's thermo-electric couple, consisting of platinum and platinum-plus-ten-per-cent. of rhodium, is used, the readings of a thermo-electric pyrometer may be consistent with one another.
The whole subject of pyrometry was carefully discussed by M. Le Châtelier before the French Société Technique de l'Industrie du Gaz at its annual meeting in 1889; and a summary of his address will be found in the Gas World, March 15, 1890. See also Poggendorff's Annalen, vol. xxix.; and for Ericsson's Solar Pyrometer, see Nature, vol. xxx.