988 Prof. J. A. Fleming on the Measurement 
and opposite bridge-arms, and 7 the resistance inserted in 
series with the telephone in the bridge-circuit. 
When 7 was adjusted to produce silence in the telephone, it 
was found that variations to the extent of about 1 per cent. 
either way, and sometimes much less, caused the sound to 
reappear in the telephone, and hence gave the limits within 
which the inductance L could be determined. 
Fig. 1. 

N 
In the experiments here described, the capacity generally 
employed consisted of two leyden-jars, the joint capacity of 
which had been determined carefully with the FJeming- 
Clinton commutator, and found to have the value 0:00272 
microfarad. The tests of this telephone method were made 
by Mr. J. C. Shields on a number of coils of silk-covered 
copper wire, each of which consisted of one layer of the wire 
wound uniformly and in closely adjacent turns upon a wooden 
or glass circular-sectioned rod. One coil, much employed, 
consisted of a wooden rod about two metres in length wound 
over as above described with one layer of no. 32 s.w.é. 
wire in closely adjacent turns. The mean diameter of one 
circular turn of this wire was 4°096 centimetres, and the 
length of the solenoid or spiral wire was 200°3 centimetres, 
and the numher of turns of wire 5000 in all, and hence 
