232 



MAGNETS AND CURRENTS 



cars and mills, to drive electric fans, and to set in motion many 

 other forms of machinery. 



\\ Feed l ^~ ~\ V ^=~ 



Trolley wire or Third rail 



Power house 

 Generator 



\ Mrtor 

 FIG. 104. The electric street car. 



The strength of a motor is increased by replacing the singly 

 coiled armature by one closely wound on an iron core ; in some 

 armatures there are thousands of turns of wire. The presence 



of soft iron within the 

 armature causes greater 

 attraction between the 

 armature and the out- 

 side magnet, and hence 

 greater force of motion. 

 The magnetic strength 

 of the field magnet in- 

 fluences greatly the 

 speed of the armature; 

 the stronger the field 

 magnet the greater the 

 motion, so electricians 

 make every effort to 

 strengthen their field 

 FIG. 105. A motor runs the sewing machine. magnets. 



When very powerful motors are necessary, the field mag- 

 net is so arranged that it has four or more poles instead of 

 two; and the armature consists of several portions. But no 



