228 MAGNETS AND CURRENTS 



The main facts about magnets may be summed up thus : 



a. A magnet points nearly north and south if it is allowed 

 to swing freely. 



b. A magnet contains two unlike poles, one of which per- 

 sistently points north, and the other of which as persistently 

 points south, if allowed to swing freely. 



c. Poles that are alike repel each other; poles that are unlike 

 attract each other. 



d. A magnet possesses the power of attracting certain sub- 

 stances, like iron, and this power of attraction is not limited to 

 the magnet itself but extends into the region around the magnet. 



Magnetic properties of an electric current If a current- 

 bearing wire is really equivalent in its magnetic powers to a 



magnet, it must possess all of 

 the characteristics mentioned in 

 the preceding section. That a 

 coil through which current flows 

 possesses the characteristics a, 

 b, c, and d of a magnet is shown 

 as follows : 



a, b. When a helix marked 

 at one end with a red string is 

 arranged so that it is free to 

 rotate and a strong current is 



FIG. 99. A helix through which current . . 



flows always points north and south, Sent through It, the helix im- 



if it is free to rotate. mediately turns and faces about 



until it points north and south. If it is disturbed from this 

 position, it swings slowly back and forth until it occupies its 

 characteristic north-and-south position. The end to which the 

 string is attached persistently points either north or south. If 

 the current is sent through the coil in the opposite direction, 

 the two poles exchange positions and the helix turns until the 

 new north pole points north. 



