CHAPTER V 
THE DETERMINATIONS OF MUTUAL INDUCTION 
BETWEEN PRIMARY AND SECONDARY COILS 
IN determining the mutual induction for the various 
secondary positions advantage is taken of the fact that 
this factor appears in the expression for the integral 
effect of the induced current. This integral effect is 
represented in the diagram (Fig. 10), by the entire area 
A BCD', its expression is -=- in which M and / have 
JK. 
the same meanings as hitherto, and R equals the resist- 
ance in the secondary circuit. The integral effect can 
be measured by means of the ballistic galvanometer. 
For this purpose the secondary of the induction coil 
under examination is connected in series with a good 
ballistic galvanometer and with the secondary of a 
standard induction coil, the latter apparatus being so 
constructed that the mutual induction between its pri- 
mary and secondary coils can be computed from the con- 
struction of the apparatus and the current through the 
primary. The special features of its construction are 
found in the primary, which is a solenoid of one-layer 
thickness, very evenly wound, and several times longer 
