48 MEASUREMENT OF HIGH TEMPERATURES. 
Thermo-electrics. — Tbe use of the thermo-couple for high temperature 
pyrometry was suggested and carried to a high state of perfection in 
the great research of Pouillet. 1 He used iron and platinum for his 
couple. Subsequent observers suggested a wide range of substances for 
the purpose, and improved the methods of electrical measurement and 
thermal comparison, the best of them, however, following very closely 
in the footsteps of Pouillet's research. Solly 2 proposed an iron-copper 
couple, without, however, attempting to calibrate it. Eegnault 3 tested 
an iron-platinum element but failed to obtain satisfactory results. This 
unfavorable dictum of the great experimentalist is much to be regretted, 
for it was probably the main reason which threw the subject of thermo- 
electric pyrometry into undeserved disrepute. Fortunately Becquerel 4 
resuscitated tbe method, and in his hands it led to the new results cited 
above. Becquerel's elements were of platinum and palladium, of two 
different kinds of platinum, and of platinum and iron, among which he 
preferred the former. After Becquerel, Schinz 5 began thermo-elec- 
tric pyrometry with great vigor and success, and it is indeed curious 
that Schinz's work is so little known. His iron air thermometer, 
adapted specially for calibration work, has been already described. 
Its chief merit is this, that an iron tube closed within, projects from the 
base of the cylindrical bulb into the interior. This tube, being co-axial 
with the stem of the bulb and the bulb itself, serves for the introduc- 
tion of the thermo-couple, the junction of which may thus be exposed 
at the center of figure of the bulb. The re-entrant form of bulb, to which 
I myself was led in my experiments quite independently of the almost 
unknown paper of Schinz, I regard essential to accurate and expeditious 
calibration work. Deville and Troost 6 condemned bulbs of any other 
than spherical form, though, it seems to me, quite unjustly and without 
sufficient evidence against them. In thermo-electric comparisons the 
chief end in view is to secure identical conditions of exposure for the 
junction of the couple and the bulb of the thermometer; for the errors 
which result if this identity does not obtain, are apt to be much more 
serious than such as are due to small irregularities of contraction of the 
bulb. It does not seem proven, moreover, that a bulb will not contract 
regularly if its form is not spherical. Schinz's bulb is a large iron box, 
with which fine measurements can not possibly be made. The appa- 
ratus, moreover, is not at all adapted to the comparison of results ob- 
tained with different bulbs, a step which L regard as essential. My bulbs 
are of porcelain; they may be easily handled and exchanged one for an- 
other, and the whole method of exposure is such as to secure as much 
1 Pouillet: C. R., vol. 3, 1836, p. 782; Dingler's Jour., vol. 63, 1837, p. 221. 
2 Solly : Philos. Mag., London, 3d series, vol. 19, 1841, p. 391. 
3 Regnault : Relation des Experiences, vol. 1, Paris, 1847, p. 246 (1845). 
4 Becquerel: Ann. ch. et pliys., 3d series, vol. 68, 1863, p. 49. 
5 Schinz: Dingler's Jour., vol.175, 1865, p. 87; Ibid., vol. 179, 1866, p. 436. 
6 Deville et Troost: C. R., vol. 57, 1863, p. 897. 
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