THE ELECTRIC BURGLAR-ALARM. 



59 



The bell used is that common with different forms of electrical 

 instruments. It consists of a gong and a clapper vibrated by the com- 

 bined action of an electro-magnet and a spring. The magnet, when 

 the current passes, draws 'the clapper to itself and in doing so opens 

 the circuit ; this destroys its magnetism and allows the spring to carry 

 the clapper back. This " make " and " break " action, rapidly repeat- 

 ed as long as the current is passing, produces a continuous ringing 

 of the bell. Reference to Fig. 2 * will make this movement clear. 



Fig. 2. 



One end of the wire of the coils of the magnet M M is secured to the 

 binding-post B, and the other to the post C. The arm of the clapper 

 k is a rather stiff spring, which in its normal position holds the arma- 

 ture e carried by it from the poles of the magnet. It then presses 

 against the spring r, attached to the post D. The posts A and E 

 holding the wires from the battery are respectively connected with B 

 and D by metallic sti'ips. The current enters at A, traverses the 

 coils of the magnet M M, passes through the armature e, and out 

 by way of spring r and posts D and E. In doing so, the soft-iron 

 cores of the magnet are magnetized and attract the armature e. 

 This in moving breaks its contact with the spring r, and interrupts 

 the cm-rent. The clapper then springs back into position. In the 

 bell now generally used the ringing continues not only while the 

 door or window is open, but until the indicating parts of the annun- 

 ciator are restored to position. 



* Figs. 2 and 3 are reproduced, through the kindness of Mr. George B. Prescott, from 

 his works on " The Electric Light," etc., and " Electricity," etc. 



