66 PHYSIOLOGICAL PHYSICS. [Chap. vn. 



will be made still more complete if a key be inter 

 posed in short-circuit in the circuit of the secon- 

 dary wires ; that is to say, connect the secondary 

 wires one to each side of the friction key, and from 

 each side of the key carry wires to the muscle. In 

 order, then, to stimulate the muscle the secondary key 

 must be opened before the primary circuit is estab- 

 lished or interrupted. The advantage of this is that, 

 e.g. when the shock due to establishing the primary 

 circuit has been given to the muscle, the shock of 

 interruption may be spared it, if desired, by closing 

 the secondary key, and so short-circuiting the induced 

 current before the primary circuit is broken. 



Thus it is apparent that stimulation is caused 

 by sudden changes in the strength of a current. 

 Tlie change may be effected by interruption of a 

 continuous stream, but induced currents excel by the 

 abruptness of their attack. For the same reason, in 

 an ordinary induction apparatus the induced shock of 

 closure is less irritating, because, owing to the acti >n 

 of the extra current, its maximum is gradually 

 acquired, while the induced shock of opening is more 

 stimulating because there is nothing to diminish its 

 suddenness. 



By a modification of the arrangements a fourth 

 mode of stimulating muscle is obtained. Attach the 

 positive pole of the element to the pillar K of the 

 inductorium (Fig. 23), and the negative pole to a 

 simple key, the second wire from the key going to t/ie 

 binding screw z. Let the other arrangements be as 

 before. This throws into action the Wagner hammer, 

 or interrupter, and as soon as the primary key has 

 been closed, the screw s' being properly adjusted, the 

 primary circuit is rapidly opened and closed by the 

 movements of the hammer. This produces a rapid 

 series of induced shocks, which, on opening the 

 secondary key, go to the muscle, and irritate it so 



