160 Mr. A. Campbell on the Use of 
secondary D, the scale can be considerably modified, and can 
be designed to suit special purposes. The most generally 
useful type of scale becomes more and more open as the 
readings approach zero, for this gives good accuracy of 
reading even at very low values. Fortunately in the present 
instance this type of scale can readily be attained. Further 
discussion of this point will be found in an Appendix. 
5. Employment of Variable Mutual Inductances. 
Variable inductances of this type may be used for several 
purposes. Their most obvious application is to the calibration 
of ballistic galvanometers for magnetic testing ; and I have 
found them of the greatest value for quickly adjusting a 
galvanometer circuit to read B directly for any given iron ring. 
I need not dwell on this application at present, but proceed 
to describe several methods to which they are applicable. 
6. Measurement of Mutual Inductance (Method A). 
Any unknown mutual inductance, whose value lies within 
the range of the variable standard, can be at once determined 
by connecting the primaries of the unknown and the variable 
in series to B (fig. 5), a source of alternating or intermittent 
Ficr. 5. 
current, while the secondaries, with their windings in oppo- 
sition, are connected in series to a vibration galvanometer G. 
The variable M is then adjusted to bring the deflexion to 
zero, and the reading gives directly the value of the 
unknown M. This is an extremely simple method as it 
involves no knowledge of any resistances. A ballistic 
galvanometer and a commutated current may be used. This 
method does not apply to mutual inductances higher than 
the maximum value of the variable standard ; for these 
another method will be described below (see § 8). 
7. Measurement of Self Inductance (Method B). 
In fig. 6 let a variable mutual inductance M whose primary 
includes the subdivided coil be connected into a Wheatstone's 
network, as shown, along with a self inductance L 2 . Let 
the resistances of the arms be P, Q, B, and S respectively, 
