404 



ELEMENTARY LESSONS ON [CHAP. xn. 



tip of a screw, from which a return wire passes back to 

 the zinc pole of the battery. As soon as the lever is 

 attracted forward the circuit is broken at C by the spring 

 moving away from contact with the screw ; hence the 

 current stops, and the electromagnet ceases to attract the 

 armature. The lever and hammer therefore fall back, 



Fig. 165. 



again establishing contact at C, whereupon the hammer 

 is once more attracted forward, and so on. The push 

 P is shown in section on the right of Fig. 165. It 

 usually consists of a cylindrical knob of ivory or porcelain 

 capable of moving loosely through a hole in a circular 

 support of porcelain or wood, and which, when pressed, 

 forces a platinum-tipped spring against a metal pin, and 

 so makes electrical contact between the two parts of the 

 interrupted circuit. 



433. Electric Clocks. Clocks may be either driven 

 or controlled by electric currents. Bain, Hipp, and 

 others, have devised electric clocks of the first kind, in 

 which the ordinary motive power of a weight or spring is 



