190 REPORTS ON THE STATE OF SCIENCE.—1914, 
wire, BW, can be adjusted to 1 millivolt. This is tested by the 
electromotive force of a cadmium cell, C, which can be opposed to the 
battery electromotive force by means of the upper key, K,, an allowance 
for the known temperature variation of the electromotive force of the 
standard cell used being made by an adjustable contact-maker, D. 
The thermo-electric couple, H, has one lead connected to the lower 
key, K2, and the other set to a set of resistances, S, in the main circuit, 
each of which gives a difference of potential of 1 millivolt when the 
Lo Contact-Haker 
Fie. 11.—Thermo-Electric Bridge. 
adjustments are correct. During an observation, therefore, the battery 
electromotive force opposes that of the couple and the readings of 
the bridge-wire and step resistance taken together measure the electro- 
motive force of the couple when the galvanometer, G, shows a 
balance. The scale of the bridge-wire is graduated to read to 10 micro- 
volts, and single microvolts may be read by estimation. The majority 
of the observations were taken when using a D’Arsonyal galvanometer, 
giving, on a scale distant 110 centimetres, a deflection of 560 milli- 
