THE PRINCIPLE OF THE MOTOR 



333 



d. If a wire is passed through a card and a strong current 

 is sent through the wire, iron filings will, when sprinkled 

 upon the card, arrange themselves in definite directions 

 (Fig. 227). A wire carrying a current is surrounded by a 

 magnetic field of force. 



A magnetic needle held under a current-bearing wire turns 

 on its pivot and finally comes to rest at an angle with the 

 current. The fact that the needle is deflected by the wire 

 shows that the magnetic power of the wire extends into the 

 surrounding medium. 



The magnetic properties of current electricity were discov- 

 ered by Oersted of Denmark less than a hundred years ago ; 

 but since that time practically all important electrical ma- 

 chinery has been based upon one or more of the magnetic 

 properties of electricity. The motors which drive our electric 

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

 tirely to the magnetic action of current 

 electricity. 



309. The Principle of the Motor. If a 

 close coil of wire is suspended between the 

 poles of a strong horseshoe magnet, it will 

 not assume any characteristic position but 

 will remain wherever 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, FIG. 228. The coil 

 is equivalent to a magnet with a north and 

 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 opposite the north pole of the magnet. If, 

 when the coil is at rest in this position, the current is reversed, 

 so that the north pole of the coil becomes a south pole and 



To lattery 



turns in such a way 

 that its north pole is 

 opposite the south 

 pole of the magnet. 



