of Small Inductances and Capacities. 587 
inductances. Since that date, these appliances have been 
extensively used for this purpose in the Pender Electrical 
Laboratory at the University College. 
In the measurement of small inductances lying in value 
between 100,000 and 10,000 centimetres, it is essential to 
use in connexion with the modification of the Anderson 
method *, described in our paper, a very sensitive galvano- 
meter ; and when small inductances of this order are being 
measured we have since found that the stray field from the 
motor employed to drive the commutator produces, by a 
dynamo action, a small electromotive force in the commutator 
which makes itself evident in the galvanometer circuit, and 
so gives rise to an irregularity, vitiating the results. The 
remedy for this, of course, is to employ an enclosed iron-clad 
motor, or else to place the commutator at a greater distance 
from the motor, connecting the two by a long shaft. This 
has already been done and has been found to be effective. 
Meanwhile, in the course of the experiments to overcome 
these difficulties, the attempt was made to use a telephone in 
place of the galvanometer and a simple interrupted current 
in the battery-circuit. In the bridge arrangement described 
by Prof. Anderson (loc. cit.) we substituted an ordinary 
buzzer in the battery-circuit to interrupt the current at the 
rate of about 100 per second, and in the bridge-circuit an 
ordinary Bell telephone for the galvanometer, the commutator 
being abolished. Under these circumstances, it was found 
that an observer with sharp hearing could obtain a very good 
balance when a coil having small inductance was placed in 
one arm of the bridge, and a condenser of suitable capacity 
placed as described by Prof. Anderson (see fig. 1). 
Mr. J. C. Shields, who has been engaged in experiments 
on this matter in the Pender Laboratory, found that with 
this arrangement he could make very quick and fairly 
accurate determinations of small inductances, the accuracy of 
the reading being determined by the limits within which a 
value could be assigned to r in the equation given by Prof. 
Anderson, viz. : 
L=C{r(R +8) + RQ}, 
the value of 7 being that of a resistance inserted in the bridge- 
circuit, which is “varied until no sound is heard in the 
telephone. 
In the above equation J. is the inductance and R the 
resistance of the coil being measured, C the capacity of 
the condenser, and §S and Q the resistances of the adjacent 
* See Phil. Mag. vol. xxxi. p. 329 (1891). 
