24 
ON A MAGNETIC DETECTOR OF ELECTRICAL WAVES, ETC. 
Therefore 
C ^ 
C' f R2 ■ 
Since R and II' have both been determined, C is known in terms of the standard 
capacity C\ 
Similarly, the value of L, the self-inductance for rapid frequencies, may also be 
found. If an additional standard inductance L' be introduced into the circuit, 
the value of the capacity remaining unaltered. 
Therefore 
yiC 
L + L' _ ^ 
- ^ - p/. • 
The value of L for rapid frequencies is thus determined in terms of L', a known 
inductance. 
In the experiments on the determination of periods, a detector consisting of twenty 
or more fine insulated steel wires, about 1 centim. long, was used, with one or two 
turns of wire round it through which the oscillatory current passed. This small 
detector coil was fixed before a magnetometer, and was so arranged that it could be 
switched either into the resistance or inductance branch of the divided circuit. 
The inductance of the detector coil was too small to appreciably alter the 
distribution of the current in either circuit. The equality of currents in the two 
circuits could thus be readily compared. 
The “ longitudinal” detector may also be used for rough determinations; but it is 
not so sensitive to slight changes of current as the solenoidal detector of fine wire. 
It was found that the inductance of a current when the wire was of iron was 
nearly the same as when replaced by copper of the same diameter. It was difficult 
to determine the variation of inductance accurately in this case, in consequence of 
the oscillations being rapidly damped when iron wires were used. 
Since the capacities of condensers for very rapid alternations may be determined, 
it was interesting to observe whether the values of the specific inductive capacity of 
glass was the same for slow as for very rapidly varying fields. Some observers had 
found that glass had a much lower specific inductive capacity for rapid oscillations 
than for slow, while others again found values about the same in the tv/o cases. The 
value of the specific inductive capacity found for plate-glass was about 6’5 for periods 
of about 3 million per second. This is considerably higher than the value obtained 
for plate-glass by J. J. Thomson and Blondlot, wdio found values of 27 and 2'3 
respectively for periods of about 20 million per second. 
The value of the specific inductive capacity of ebonite, tested by the same method, 
was found to be about the same as for slow alternations. 
These experiments were performed in the Cavendish Laboratory, Cambridge. 
