The Telephone as an Instrument of Precision. 275 
T being included in a closed circuit, one pole of a small battery 
was joined to the interruptor at @, and the other slid along the 
naked copper wire at b, a difference of resistance between the 
interruptor and the telephone (the former being the greater) 
caused the least motion of the wire at b to be revealed. Thus the 
difference of resistance between a quarter and half an inch of 
No. 20 copper wire was readily detected, by the difference in the 
loudness of the sounds heard when the wire was at 6 or c. 
Thermo-electric inversion—A copper wire drawn over a file 
yields audible sounds in the telephone, when the difference of 
temperature simply results from the heat of the hand holding the 
wire. In this way a thermo-electric series of metals can be 
quickly found and arranged in order of electromotive force using, 
say, the steel of the file as the standard of reference. 
The phenomenon of so-called thermo-electric inversion may 
readily be perceived by means of the telephone and interruptor. 
A piece of thin iron wire was twisted round a piece of copper bell 
Scien. Proc., R.D.9. Vou. 11., Pr. 1v. U 
