230 



MAGNETS AND CURRENTS 



To battery 



fans, our mills, and our trolley cars owe their existence en- 

 tirely to the magnetic action of current electricity. 



The principle of the motor. When a coil of wire is sus- 

 pended between the poles of a strong horseshoe magnet, it 

 does not assume any characteristic position but remains where- 

 ever placed. If, however, a current is sent 

 through the wire, the coil faces about and 

 assumes a definite position. This is because 

 a coil, carrying a current, is equivalent to a 

 magnet with a north and a south face ; and, in 

 accordance with the magnetic laws, tends to 

 move until its north face is opposite the south 

 pole of the horseshoe magnet, and its south 

 face is opposite the north pole of the magnet. 

 If, when the coil is at rest in this position, the 

 FIG. loi. The coil current j s reversed, so that the north pole of 



turns m such a way < r 



that its north pole is the coil becomes a south pole and the former 

 ^thfmagnet fh 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 moves, 

 and rotates until its new north pole is opposite 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 cur- 

 rent when the coil is at the half turn, the helix is made to 

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

 the current is repeatedly reversed just as the helix completes its 

 half turn, the motion is continuous. Periodic current rever- 

 sal produces 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 101, since the twisting 

 of the suspending wire would interfere with free motion. In 



