THE ELECTRIC CURRENT 229 



c. If a helix is held near a suspended magnet, one end of 

 the helix attracts the north pole of the magnet, while the op- 

 posite end repels the north pole of the magnet. In fact, the 

 helix behaves in every way as a magnet, with a north pole at 

 one end and a south pole at the other. If the current is sent 

 through the helix in the op- 

 posite direction, the north 



and south poles exchange 

 places. 



If the number of turns in 

 the helix is reduced until 

 but a single loop remains, 

 the result is the same; the 

 single loop acts like a flat 



'A~ C -U~ 1 FIG. ioo. A wire through which current flows 



magnet, one side ot the loop is surrounded by a field of magnetic force, 

 always facing north and one 



south, and one face attracting the north pole of the suspended 

 magnet and one repelling it. 



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. ioo). 

 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 



