feAKtrs.] METHODS OF PYROMETRY. 51 
suggested by Quincke, was carried out practically by Keissig. 1 This is 
simply a wheatstone-bridge adjustment, not different in any essential 
respect from 0. W. Siemens's 2 pyrometer, except in so far as the latter 
endeavored to calibrate his electrical apparatus by the calorimetric 
method. Siemens's pyrometer is too well known to need special descrip- 
tion. In the final form, currents are measured electrolytically, and to 
give the method greater sensitiveness two identical voltameters to cor- 
respond to the hot and the cold wires are used simultaneously. This 
makes the apparatus to some extent independent of the local and time 
errors of the galvanometer. Siemens's resistance-temperature measure- 
ments are made with platinum, copper, and iron, and the data obtained 
are formulated. 
Siemens's pyrometers were tested by Weinhold (1. c.) and pronounced 
sufficiently in keeping with the air thermometer to be of reliable serv- 
ice to the metallurgist. After this Forster, 3 Williamsen, 4 and Fischer 5 
find that the effect of long-continued exposure of a Siemens pyrometer 
is an incremeut of the resistance of the exposed wire. Eecalibration 
from time to time is therefore essential. An important series of meas- 
urements of the relation between resistance and temperature was made 
for quite a number of metals by Benoit. 6 His temperatures run as high 
as 860° (boiling point), and all the relations are formulated. Iridio- 
platinum wire was tested with regard to its resistance at 15° and at 
white heat by Bucknill. 7 Formulas applying for silver platinum, iron- 
gold, and platinum iridium alloys were computed with great care and 
from many experiments by MacGregor and Knott f but their ranges 
of temperature did not much exceed 150°. A critical comparison of the 
data of the resistance temperature formulas of Siemens (1. c), Benoit 
(1. c), which apply for j)latinum, wa s made by Nichols, 9 and the dis- 
crepancies between these results fully pointed out. Nichols, moreover, 
expressed resistance in terms of his dilatation thermometer. Perhaps 
one of the most careful measurements of resistance as varying with 
temperature, and indeed the only ones which to my knowledge were 
made at high temperatures and by direct comparison with the porcelain 
air thermometer, are due to Schleiermacher. 10 This observer wrapped 
his wires directly around the thermometer bulb or exposed them in sim- 
ilar unexceptionable ways. Recognizing the variable character of ordi- 
1 Reissig : Dingler's Jour., vol. 171, 1864, p. 351. 
2 Siemens: Proc. Royal Soc. London, vol. 19, 1871, p. 443. Dingler's Jour., vol. 198, 
1870, p. 394 ; ibid., vol. 209, 1873, p. 419 ; ibid., vol. 217, 1875, p. 291. 
3 Forster: Chemical News, vol. 30, 1874, p. 138. 
4 Williamsen: Dingler's Jour., vol. 210, 1873, p. 176. 
5 Fischer : Dingler's Jour., vol. 225, 1877, p. 463. 
6Be"uoit : C. R., vol. 76, 1873, p. 342. 
7 Bucknill : Jour. Soc. Tel., Eng., vol. 7, 1878, p. 327. 
8 MacGregor and Knott: Trans. Royal Soc. Edinburgh, vol. 29, 1880, p. 599. 
9 Nichols: Am. Jour. Sci., 3d series, vol. 22, 1881, p. 303. 
10 Schleiermacher: Wied. Auu., vol. 26 ; 1885, p. 287. 
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