416 



THE ELECTRIC TELEGRAPH. 



Siemens constructed an instrument, called, in German, a 

 " Wippe," which is, in fact, a self-acting commutator capa- 

 ble of interrupting or reversing the currents with great 

 rapidity. 



If the end c of a cable were connected by two parallel cir- 

 cuits, a and b (Fig. 186), with the two anvils a' and b r , in the 

 reach of two movable contact springs, c and d, c being 



Fig. 187.- 



connected with one pole of a battery, the other pole of which 

 is to earth, and d being connected to earth direct, then, upon 

 pressing the spring c for an instant upon , a charge would 

 flow into the cable from the battery, through a, in the direction 

 represented by the arrow ; and if, c and a' being interrupted, 

 the lower spring d were pressed upon the contact b, a quan- 

 tity of the electricity held in charge in the cable would rush 

 out and pass through the side b as shown by the arrow. 

 Thus, by connecting a' and b f with c and d alternately, so 

 that the one contact is always broken when the other is 

 made, a succession of currents will circulate in the arm a 

 towards the cable, and in the arm b from it. 



By inserting a measuring instrument in either of these 



