THE CHARACTERISTICS OF INDUCED CURRENTS 7 
very short duration, since the time required to establish 
the field of force on the one hand, and for its disappear- 
ance on the other, is measured in thousandths of a 
second, and, as we have seen, only during these periods 
do induced currents flow. The current induced in the 
secondary by the make of the primary circuit is usually 
spoken of in physiology as the make shock; that indj*ced 
by the break of the primary is the break shock. 
A feature of induction shocks which commends them 
particularly to the physiologist is the ease with which 
their intensity may be varied. For securing this varia- 
tion advantage is taken of the dependence of the induced 
current upon the number of lines of force which cut the 
secondary coil. There are two ways of varying this 
number : One is by changing the intensity of the primary 
current; the other, by shifting the position of the sec- 
ondary coil with reference to the primary. This latter 
method is the one used in the du Bois-Reymond induc- 
torium, and it is a very satisfactory method, since by 
means of it the strength of the stimulus can be varied 
several hundredfold, from the maximum for the appa- 
ratus to a value negligibly small, by simple shifting of 
the secondary from one end of its slide to the other. 
Many inductoria are so constructed that the secondary 
coil can be rotated about an axis midway of its length. 
In this way the intensity of the induced current can be 
cut down to zero, since when the secondary is at right 
