164 
Mr. A. Campbell on the Use of 
method will also give the difference between two unknown 
self inductances introduced into AB and AC. 
The addition of a condenser to Method (B) gives an inter- 
esting corroborative method (Method C). If (fig. 8) the 
condenser K is in shunt with S, 
Fiar. 
it may be shown that, for a balance, 
SP - RQ =p 2 RSK(L 2 - M), . 
and S(L 1 -M)-R(L 2 + M) = QRSK. 
When R = S and 1^ = 1,=!, 
P-Q=^ 2 RK(L-M), . . . 
2M = QRK 
and 
(4> 
(5) 
(6) 
(7) 
Equation (7) gives a check of K against M without having 
to measure the frequency. The adjustment is dependent on p, 
however, and therefore a steady frequency is necessary, which 
is not required in Carey Foster's Method. 
From (6) and (7) we get 
Q(P-Q)=2^M(L-M) 
(8) 
8. Measurement of Mutual Inductance greater than 
Standard (Method D). 
When the unknown M is beyond the range of the variable 
standard the following method is applicable. 
Let the primaries of the variable standard and the unknown 
M be connected up as in fig. 9, adding to the latter if 
