BARUS. ] 
METHODS OF PYROMETRY. 
25 
Classification of pyrometers, — Thermometers which depend essentially 
on the properties of the substance used for thermal measurement are 
called by Thomson l intrinsic thermoscopes. They may be either con- 
tinuous or not. It is with such intrinsic thermoscopes that practical 
pyrometry must be conducted, although the data of the gas thermometer, 
as appears from the recent pyro chemical researches of Langer and 
Meyer, 2 may safely be regarded as non-intrinsic and absolute, particu- 
larly in the region of high temperatures. Almost every thermal phe- 
nomenon has been utilized for temperature measurement, and the de- 
vices employed may be conveniently classified by aid of these phe- 
nomena as follows : 
I. Dilatation of solids. 
1. A single solid. 
2. Two solids acting differen- 
tially. 
II. Dilatation of liquids. 
III. Dilatation of gases. 
1. Expansion measured in vol- 
ume, manometrically. 
2. Expansion measured in press- 
ures, manometrically. 
3. Expansion measured in vol- 
ume, by displacement. 
IV. Vapor tension. 
V. Dissociation. 
VI. Fusion. 
VII. Ebullition. 
VIII. Specific heat. 
IX. Heat conduction. 
X. Heat radiation. 
XL Viscosity. 
1. Of solids. 
2. Of liquids. 
3. Of gases. 
XII. Spectropbotometry and color. Ro- 
tary polarization. 
XIII. Acoustics (wave length). 
XIV. Thermo-electrics. 
XV. Electrical resistance. 
XVI. Magnetic moment. 
XVII. Miscellaneous. 
Dilatation of solids. — Curiously enough the dilatation thermometers 
were not the first to suggest themselves. Newton, in his scala graduum 
caloris, proposes a method of temperature measurement based on his 
law of cooling, almost as early as 1700. However Musschenbroek (1731), 
Ellicot (173G), Bouger (1745), and others availed themselves of single- 
bar expansion devices, and Mortimer made a thermometer on this prin- 
ciple m 1746. 
The most celebrated apparatus of this kind (1782) is Wedgwood's 3 
pyrometer, in which the attempt is made to express temperature in a 
scale based on the shrinkage experienced by a little compressed cylinder 
of clay after exposure to the said temperature. This apparatus came 
into much more general use than its inventor intended. Its indications 
were vigorously discussed by the physicists of the time, especially by 
Guyton-Morveau, 4 who in attempting to convert Wedgwood's arbitrary 
thermal scale into degrees centigrade showed the apparatus to be un- 
1 Encyclopedia Brit., 9th ed., vol. 11, 1880, p. 580. 
2 Langer and Meyer, Pyrochemische Uutersuchungen, Braunschweig, Vieweg u. 
Sohn, 1885; Berl. Ber., vol. 18, 1885, p. 1501. 
3 Wedgwood: Phil. Trans., Roy. Soc, vol. 72, 1784, p. 305; vol. 74, 1782, p. 358; 
Dingler's Jour., vol. 15, 1824, p. 230. 
4 Guyton-Morveau : Annales de chimie, Paris, 1st series, vol. 46, 1803, p. 276; ibid., 
vol. 73, 1810, p. 254; ibid., vol. 74, 1810, pp. 18, 129; ibid., vol. 90, 1814, pp. 113, 225. 
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