158 
Mr. A. Campbell on the Use of 
done, it will be found that no one of the sections differs from 
its neighbours by more than 1 part in 1000, and each is 
O'l millihenry. The placing of the movable secondary coil 
midway between the planes of the two primary ones ensures 
that small axial displacements shall have very little effect on 
the mutual inductance. 
4. Adjustment and Calibration of Variable Mutual 
Inductances. 
The equality of any pair of sections can be tested by con- 
necting them in series with their windings in opposition in 
circuit with a ballistic galvanometer and reversing the current 
in the primary. It should be noticed here that, if a primary 
coil have any number of secondary circuits, the mutual 
inductance to all the secondaries in series is equal to the 
algebraic sum of their separate mutual inductances (+ or — 
according to the direction of the winding). Owing to this 
very important property we can build up and step down in the 
values as easily as if we were dealing with resistances, and 
there is the further simplification that we can subtract as 
well as add the values. The marking of the scale and the 
setting of the coil F are done by comparison with a fixed 
standard mutual inductance such as I have described else- 
where*. The comparison may be made by Maxwell's method f, 
using a sensitive ballistic galvanometer or a vibration galva- 
nometer as detector. When a vibration galvanometer is 
used as in fig. 3, it should be remembered that, for a 
Fig. 3. 
Tl, R, 
to — 
■4M/V— > 
£, a 
R 
<[> 
c, 
'VWw 
balance, two conditions must be satisfied, viz., 
Mj/Mo^iyRo 
and Li/L 2 =R 1 /R 2 , 
* Phys. Soc. May 1907 ; Phil. Mag. [6] vol. xiv. p. 494, Oct. 1907. Also 
see Proc. Roy. Soc. A. vol. lxxix: p. 428, June 5, 1907. 
f Maxwell, Elect, and Mag. 2nd edition, ii. § 755. 
