ELECTRICITY AND MAGNETISM 



135 



145. Electricity and Magnetism. The construction of 

 telegraph and telephone instruments depends on the fact 

 that an electric current can produce magnetism and that 

 magnetism can produce an electric current. When an elec- 

 tric current is passed around a bar of iron, the bar becomes 

 a magnet, but it re- 

 mains one only as long 

 as the current is passing. 

 A temporary magnet is 

 the important part of a 

 telegraph instrument. 



If a current passes 

 around a bar of steel, 

 the steel becomes a 

 magnet, and does not 

 lose its magnetism after 

 the current ceases. A 

 permanent magnet is a 

 part of every Bell tele- 

 phone receiver. 



If a magnet is put 

 into a coil of wire, a 

 momentary current of 

 electricity passes 



FIG. 69. THE ELECTRIC BELL 



In the figure, m is an electro-magnet, 

 consisting of pieces of iron around which 

 insulated wire is wound. When b is pressed, 

 through the wire. When a current can pass from the battery through 

 , i the coils of wire and back to the battery. 



the magnet IS removed, L Wha tisthe effect on the pieces of iron? 

 a Current Starts in the 2. At a there is another piece of iron on a 

 , -, spring connected with the hammer of the 



Opposite direction. beU What is the effect of m upon a when 

 These facts are made the button is pressed? 3. Why does not 

 j this effect continue when pressure is re- 



use of in producing moved from 6f 



electricity by machines 



called dynamos. They furnish powerful electric currents 



for electric lights, electric furnaces, and for motors which 



are used in running trolley cars, elevators, and all kinds of 



machines in factories. 



