108 INDUCTION SHOCKS 
means of distinguishing the poles of the secondary cir- 
cuit is to apply them to a sheet of filter paper moistened 
with a mixture of starch paste and potassium iodide 
solution. If a strong primary current is made and 
broken rapidly, and the secondary makes or breaks are 
short-circuited each time, a blue deposit presently ap- 
pears at the anode, indicating the accumulation there 
of iodin ions which react with the starch. It should be 
remembered that make shocks and break shocks are 
opposite in direction, so that the pole which is revealed 
as the anode for break shocks is the cathode for makes. 
The development of a mathematical expression for the 
influence of secondary resistance on stimulating value 
has shown the fallacy of a method sometimes employed 
for varying stimulating strengths quantitatively by in- 
cluding known resistances in the secondary circuit and 
assuming that the strength of stimulus is reduced in 
exact proportion with the increase of resistance. As the 
equation (p. 76) shows, the strength of stimulus not 
only does not vary in exact proportion with the resist- 
ance, but the relationship actually existing is not apt to 
be the same in two successive experiments, owing to the 
interrelation between secondary resistance and cathode 
surface. 
This same interrelation explains the error of another 
procedure which has sometimes been employed for 
the purpose of overcoming inequalities in stimulation 
