ELECTRICAL MACHINERY 473 



631. Electric bell. The electric bell is a simple applica- 

 tion of the electromagnet. The current enters at A 

 (Fig 354), passes through the horseshoe magnet B, over 

 the closed circuit breaker C, and out at D. The instant 

 the circuit is completed through the coils a magnet is 

 formed, which attracts the armature E, and rings the 

 gong F. But as soon as the armature is drawn down 



FIG. 353 ELECTROMAGNET 



FIG. 354 ELECTRIC BELL 



against the poles ct the magnet the circuit is broken at C, 

 hence the current stops flowing and the magnet becomes 

 nil. As soon as the magnet has no strength the force of 

 the spring G draws the armature back and makes contact 

 at C again, and the operation is repeated. 



632. Electromagnetic induction. In a previous para- 

 graph it has been shown that there is a magnetic field sur- 

 rounding all electric currents. If a wire be arranged so 

 as to form a closed circuit and then moved across a mag- 

 netic field a reverse action of that explained above will 

 take place. In other words, if a closed circuit be moved 



