CH. x.] THE INDUCTION COIL. 115 



copper to the zinc ; if we have a key on the course of this wire 

 the current can be made or broken at will. If in the neighbour- 

 hood of this wire we have a second wire forming a complete 

 circle, nothing whatever occurs in it while the current is flowing 

 through the first wire, but at the instant of making or breaking 

 the current in the first or primary ivire, a momentary electrical 

 current occurs in the secondary wire, which is called an induced 

 current ; and if the secondary wire is not a complete circle, but 

 its two ends are connected by a nerve, this induction shock tra- 



Fig. 137. Du Bois Raymond's induction coil. 



verses the nerve and stimulates it; this causes a nervous impulse 

 to travel to the muscle, which in consequence contracts. 



If the firsthand second wires are coiled many times, the effect 

 is increased, because each turn of the primary coil acts induc- 

 tively on each turn of the secondary coil. 



The direction of the current induced in the secondary coil is 

 the same as that of the current in the primary coil at the break ; 

 in the opposite direction at the make. The nearer the secondary 

 coil is to the primary the stronger are the currents induced in 

 the former. 



Fig. 137 represents the Du Bois Ileymond coil, the one gene- 

 rally employed in physiological experiments, c is the primary 

 coil, and d and d' its two ends, which are attached to the battery, 

 a key being interposed for making and breaking ; g is the secon- 

 dary coil, the two terminals of which are at its far end ; to these 

 the electrodes to the nerve are attached ; the distance between 



i 2 



