THE MAGNETISM OF IRON. 



or less violently into motion (coming quickly to rest) because of 

 the force which the magnet exerts on the compass needle. In 

 general, any two adjacent magnets exert forces on each other, 

 and this mutual force action is always resolvable into four parts, 

 namely, the force with which each of the poles of one magnet 

 acts upon each of the poles of the other magnet. The north pole 

 of each magnet attracts the south pole of the other magnet, the 

 north poles of both magnets repel each other, and the south 

 poles of both magnets repel each other. Unlike magnetic poles 

 attract each other, like magnetic poles repel each other. 



31. Distributed and concentrated magnet poles. The poles of 

 a magnet, that is, the seats of the attracting and repelling forces 

 above described, are distributed over considerable portions of the 

 bar, generally the end portions. This is especially the case with 

 short thick bars. In the case of long slim magnets, however, the 

 poles are ordinarily more nearly concentrated at the ends of the 

 bar. In the first case we have what are called distributed poles, 

 and in the second case we have what are called concentrated poles. 

 The laws of attraction and repulsion of magnets are quite simple 

 for long slim magnets with concentrated poles, and the ideal slim 

 magnet with concentrated poles will be made use of in the follow- 

 ing development of the fundamental ideas relating to the mag- 

 netism of iron and to the magnetic action of the electric current. 



N N N 



S S 



N N N N 



32. Strength of a magnet pole. The poles of a magnet may 

 attract iron with greater or less force 

 according to the size of the magnet 

 and according to the thoroughness 

 with which the magnet has been 

 magnetized. The poles of a magnet 

 are said to be strong when they at- 

 tract iron or steel with a relatively 

 great force. Consider a pair of long 

 slim magnets a, Fig. 29, another pair b, another pair c, another 

 pair d, and so on, the two magnets of each pair being exactly 



S S S S 



Fig. 29. 



S S 



