METHODS OF ELECTRICAL STIMULATION 67 



Presently the muscle will shorten. (Compare 

 pages 175 and 176.) Observe that this contrac- 

 tion was the result of a break induction current, 

 not a make. 



Cautiously move the secondary coil still nearer 

 the primary, making and breaking the current as 

 before. 



A point will be reached at which the make 

 induction also causes contraction. Obviously, 

 the break current is a stronger stimulus than 

 the make induction current. The cause of the 

 greater intensity of the break induction current 

 lies in the primary coil. The current which en- 

 ters the primary coil induces a current in this 

 coil as well as in the secondary coil. The direc- 

 tion of this "self-induced" current is opposite to 

 that of the primary current, and hence weakens 

 it and delays its development. The stimulating 

 power of electricity increases with both the inten- 

 sity of the current and the quickness with which 

 the intensity alters. Hence the stimulating power 

 of the make induction current is lessened by the 

 self-induction of the primary coil. When, on 

 the other hand, the primary circuit is broken, 

 the current stops, and although self-induction 

 again takes place, it cannot affect the primary 

 current, because the latter no longer exists. The 

 self-induced current at the break of the primary 



