334 



MAGNETS AND CURRENTS 



the former south pole becomes a north pole, the result is that 

 like poles of coil and magnet face each other. But since 

 like poles repel each other, the coil will move, and will rotate 

 until its new north pole is opposite to the south pole of the 

 magnet and its new south pole is opposite the north pole. 

 By sending a strong current through the coil, the helix is 

 made to rotate through a half turn ; by reversing the current 

 when the coil is at the half turn, the helix is made to con- 

 tinue its rotation and to swing through a whole turn. If 

 the current could be repeatedly reversed just as the helix 

 completed its half turn, the motion could be prolonged; 

 periodic current reversal would produce continuous rotation. 

 This is the principle of the motor. 



It is easy to see that long-continued rotation would be im- 

 possible in the arrangement of Figure 228, since the twisting 

 of the suspending wire would interfere with free motion. If 

 the motor is to be used for continuous motion, some device 

 must be employed by means of which the helix is capable of 



continued rotation 

 around its support. 



In practice, the rotat- 

 ing coil of a motor is 

 arranged as shown in 

 Figure 229. Wires from 

 the coil terminate on 

 metal disks and are se- 

 curely soldered there. 

 The coil and disks are 

 supported by the strong 



and well-insulated rod R, which rests upon braces, but which 

 nevertheless rotates freely with disks and coil. The current 

 flows to the coil through the thin metal strips called brushes, 

 which rest lightly upon the disks. 



FIG. 229. Principle of the motor. 



