PRIMARY AND SECONDARY COILS 39 
than the secondary. The lines of force through the 
secondary, placed at the middle of the primary, are then 
practically straight. The arrangement of the apparatus 
is shown diagrammatically in Fig. 7. 
The secondary of the inductorium whose values of 
M are desired is set successively at points i or 2 cm. 
apart. At each point the galvanometer deflection 
caused by breaking a primary current of known inten- 
sity is determined. Since each galvanometer deflec- 
tion represents a certain integral effect, no matter how 
produced, and since the integral effect affords means 
of computing M, a determination of the intensity of cur- 
rent which has to be broken in the primary of the 
standard coil to produce these same deflections provides 
all the data required for calculating the values sought. 
The formula used for computing M is developed in the 
following manner: The expression for the integral effect 
MI 
is, as stated above, Let this represent the gal- 
R 
vanometer deflection caused by breaking a current of 
intensity / in the primary of the coil whose values of M 
M'S 
are desired. Let the expression represent the same 
R 
galvanometer deflection caused by breaking a current of 
intensity S through the primary of the standard coil. 
Equating these, we have-^ - = - The method of 
R R 
