44 INDUCTION SHOCKS 
for this effect in computing the values of MI must 
be made, whenever / is large, by introducing a correc- 
tion factor. This factor can be obtained without diffi- 
culty by the use of the ballistic galvanometer, since the 
deflections of that instrument are affected by core mag- 
netization. Inspection of the formula MI = M'S (p. 40) 
shows that so long as M and M ' remain constant, /, the 
current through the primary of the coil under examina- 
tion, must vary directly as S, the current through the 
primary of the standard coil. This relationship is found 
by experiment to hold in ordinary induction coils for 
values of / up to o.i ampere, but above that point the 
value of S is always larger than the equation calls for. 
In other words, when core magnetization is present the 
primary current produces a greater deflection than it 
does in the absence of this effect. The variation due to 
the magnetization of the core is not very difficult to 
correct, because, as repeated experiment has shown, the 
ratio between the actual values of / and those computed 
from the values of S depend upon an easily determined 
factor which is constant for any given iron core. 
To determine this factor some position of the second- 
ary coil must be selected at which primary currents up 
to i ampere give galvanometer deflections not greater 
than the entire scale. With the secondary in this posi- 
tion primary currents of increasing intensity, beginning 
at about o.oi ampere, are broken, and the deflections 
