BARU8.] 
CALIBRATION OF ELECTRICAL PYROMETERS. 
97 
the crucibles (Figs. 13, 14, 15), but they become electrically conducting 
and are objectionable. Insulators of fire clay are therefore preferable, 
inasmuch as they are at least as refractory as the crucibles, and Messrs. 
Hall & Sons, of Buffalo, to whom I sent the above machine, succeeded 
excellently in making them. Work with fire-clay calls for great expe- 
rience and skilled manipulation, and it is therefore best to put this work 
into trained hands. The Buffalo insulators are 25 cm to 30 cm long. I 
received two sizes, the larger of which is O.G5 cm thick and the smaller 
only 0.45 cm thick. The capillary canals in both cases are slightly less 
than 0.1 cm in diameter. The rods are hard and firm and withstand con- 
siderable usage. 
METHOD OF MEASUREMENT. 
Thermo element. — A zero method of measurement has been used 
throughout. All results for thermo-electric force are referred to the 
Latimer-Clarke, and other standards. This has the advantage of not 
requiring an iron-free observatory for magnetic work, a desideratum 
with which I, for instance, had to dispense. The thermo electric use of 
the zero method was first introduced by Kohlrausch and Ammaun. 1 
Dr. Strouhal and I introduced certain improvements by which this 
method can be made to yield results of exceptionally great accuracy. 
These improvements have been described in Bulletin No. 14, U. S. Geol. 
Survey, p. 34, but in the interest of completeness it is desirable to re- 
capitulate the chief features of this method here, also to make compari- 
sons of the electromotive force of the hydroelectric standards. 
Fig. 18. Disposition of thermo electric apparatus. 
Fig. 19. Double key. 
In Fig. 18 a diagram of the connections as actually made with some 
view to the practical disposition of apparatus is given. In this figure 
B R l are rheostats with 1 to 50,000 ohms available, r a bridge rheo- 
stat with pairs of units 0.1, 1, 10, 100, 1,000, and 10,000 ohms available. 
A and B are commutators, IT and C keys. The terminals of the thermo- 
couple communicate respectively with the points Pi and P,. This is 
1 Kohlrausch u. Aminaim : Pogg. Aim., vol. 21, 1870, p. 450. 
Bull. 54 7 (751) 
