CH. X.] BATTERIES AND KEYS 109 



But it will be seen in the Du Bois Eeymond key (fig. 134) that 

 there are four binding screws. This key is used as a bridge or short 

 circuiting key, and for many reasons this is the best way to use it. 

 The next diagram (fig. 136) represents this diagrammatically. The 

 two wires from the battery go one to each side of the key ; the elec- 

 trodes come off one from each side of the key. When the key is open 

 no current can get across it, and therefore all the current has to go to 

 the electrodes with the nerve resting on them ; but when the key is 

 closed, the current is cut off from the nerve, as then practically all of 

 it goes by the metal bridge, or short cut, back to the battery. Theo- 

 retically a small amount of current goes through the nerve ; but the 

 resistance of animal tissues to electrical currents is enormous as com- 

 pared to that of metal, and the amount of electricity that flows through 

 a conductor is inversely proportional to the resistance ; the resistance 

 in the metal bridge is so small that for all practical purposes, all the 

 current passes through it. 



Another form of electrical stimulus is the induced current, pro- 

 duced in an induction coil. 



In a battery of which the metals are connected by a wire, we have 



FIG. 186. 



seen that the current in the wire travels from the 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 neighbourhood 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 wire, 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 

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

 to travel to the muscle, which in consequence contracts. 



If the first and second wires are coiled many times, the effect is 

 increased, because each turn of the primary coil acts inductively 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. 



