728 THE ELECTRIC BELL. 



current of course traverses this circuit in an opposite 

 direction. The effect of the passage of the current 

 through the spiral is to cause the armature to be 

 attracted, and when this happens the hammer k strikes 

 the bell ; but at that instant contact ceases between the 

 spring / 2 and the extremity of the screw s, the current 

 is therefore interrupted, and as there is no attraction 

 now between the bobbins and the armature, the bar a 

 is brought back by the spring to its original position, 

 and contact is again made with s. As soon as the 

 circuit is completed again, the same action is repeated, 

 and in this way the keeper is kept in a state of rapid 

 oscillation, and the hammer k keeps on constantly strik- 

 ing the bell. 



If h and i were permanently connected with the 

 battery the bell would ring constantly. A key or push- 

 button is therefore inserted in 

 some part of the circuit which 

 allows of conveniently closing 

 the circuit only when required 

 Fig. 367 A shows a section of 

 such a key, while B represents 

 the internal arrangement. A 



FIG. 367 (B. an. pry.; A and B . i 



i real size). disc of wood, A, is fixed to a 



plug in the wall by a screw which passes through the disc. 

 Two pieces of sheet brass are screwed to the disc ; one 

 of them, &, lies flat with its whole surface in contact 

 with the wood ; the other piece 6, forms a spring, and 

 is screwed to the wood and in contact with it at one 

 end, while the remainder stands a little off from the 

 wood. The edge of the disc has a screw cut into it, so 

 that a cover, c c, may be screwed upon it ; the cover 



