PRIMARY AND SECONDARY COILS 41 
with the secondary coil at 12 cm. from the zero position 
the breaking of a primary current of o.i ampere gives a 
deflection on the galvanometer scale of 5, and that to 
secure the same deflection with the standard coil a cur- 
rent of 6.8 amperes must be broken through its primary. 
Substituting these values in the equation we obtain for 
M 5,150,000. By repeating this procedure the values 
of M for every secondary position can be obtained. 
If is a true expression for the physiological effect 
i-j 
of break shocks, evidently with L constant the product 
M I must also be constant so long as it represents a uni- 
form stimulus, no matter how the value of M may be 
varied by shifting the secondary coil. Experiment shows 
that MI does remain constant for a constant stimulus 
over the entire field of the inductorium, except that, 
when an iron core is present, it varies in the part of the 
field directly over the iron core. This is the region in 
which, as stated in a former paragraph, the value of L 
is affected by the presence of such a core.* 
If a constant stimulus gives a constant value of M7, 
however the secondary coil may be shifted, it follows 
that if / is made constant, in other words, if a fixed cur- 
rent is broken through the primary coil, the strengths 
of stimulus at different secondary positions must vary 
* For experimental evidence, see Martin: Amer. Jour, of PhysioL, 
1908, xxii, p. 124. 
