162 
Mr. A. Campbell on the Use of 
Exactly the same equations hold when the positions o£ the 
source and the galvanometer are interchanged. 
The most useful case is when the noninductive arms are 
made equal, i. e. S = R; then (1) and (2) become 
and 
P=Q, 
L 2 -L, = 2M. 
(3) 
This case gives an extremely convenient way of measuring- 
small self inductances, which is done as follows. 
B 
The arrangement is shown in fig. 7. The noninductive 
arms are equal (R, R). In the arm AB there is the secondary 
coil a of self inductance L in series with a practically non- 
inductive rheostat r. In the arm AC is placed a " balancing " 
coil b also of self inductance L and of resistance equal to or 
slightly greater than that of b. By adjusting r the bridge 
will balance when M = 0. The small self inductance N to be 
measured is now inserted in series with coil b in arm AC, 
and a balance obtained by altering r and M. Then, by (3), 
N = 2M. Thus N is found directly from the reading of M, 
and the range of values that can be measured runs from up 
to twice the highest reading of the variable mutual inductance. 
[For values of N above this range the more general case 
(equations (1) and (2)) may be used.] The L's of the coils 
a and b should be adjusted to equality once for all by putting 
M at zero and setting one of the coils. An exact setting is 
convenient, but not necessary, for if L a and L b differ slightly, 
they can be balanced (without N) by a small reading M . If 
