166 



ELECTRICITY 



190. Electric Bells. A common call bell is shown 

 in Fig. 122. The hammer or striker is attached to a 

 spring s; m is an electro-magnet. Follow the course of 



the current carefully. When 

 the circuit is closed, m is mag- 

 netized and attracts the ham- 

 mer; this moving quickly 

 toward m strikes the bell once. 

 But in so moving, a is also 

 moved away from c, breaking 

 the circuit at that point. At 

 once m loses its magnetism 

 and the spring s causes the 

 hammer to move back again; 

 but this also brings a again 

 in contact with c. Thus again 

 the circuit is closed, m is mag- 

 netized, the hammer hits the 

 bell, and all is repeated. This happens very rapidly, 

 producing the familiar buzzing sound of electric bells. 



191. Electroplating, Articles covered with a thin 

 layer of metal (gold, nickel, silver, etc.) are said to be . 

 plated. Plating is commonly done by use of the electro- 

 lytic effect of electrical energy ( 154). The articles to 

 be plated are hung in water that contains a salt ( 213) 

 of the metal to be used; a plate of the metal is also 

 hung in the liquid. This plate and the articles are 

 connected by separate wires with a dynamo or battery, 

 in such a way that the current has to pass through 

 the liquid (Fig. 123) from the plate to the articles. The 



FIG. 122 



