ADVANCED ELECTRICITY AND MAGNETISM. 



When a bar-magnet is suspended in a horizontal position by a 

 fine thread, it places itself approximately north and south like 

 a compass needle. The north pointing end of the magnet is 

 called its north pole, and the south pointing end of the magnet 

 is called its south pole. 



The north poles of two magnets repel each other, the south 

 poles of two magnets repel each other, and the north pole of one 

 magnet attracts the south pole of another magnet; that is to 

 say, like magnetic poles repel each other, and unlike magnetic poles 

 attract each other. 



The mutual force action of two magnets is, in general, resolvable 

 into four parts, namely, the forces with which the respective 

 poles of one magnet attract or repel the respective poles of the 

 other magnet. In the following discussion we consider only the 

 force with which one pole of a magnet acts upon one pole of another 

 magnet, not the forces with which one complete magnet acts on 

 another complete magnet. 



3. Distributed poles and concentrated poles. The poles of a 

 bar magnet are always distributed over considerable portions 

 of the bar. This is especially the case with short thick bars. 

 In the case of a long slim bar magnet, however, the poles are 

 ordinarily approximately concentrated at the ends of the bar. 

 The forces of attraction and repulsion of concentrated magnet 



poles are easily formulated, there- 

 fore the following discussion applies 

 to ideally concentrated poles at the 

 ends of ideally slim bar magnets. 



4. Definition of unit pole. Con- 

 sider a large number of pairs of 

 magnets a, b, c, d, etc., as shown 

 in Fig. I, the two magnets of each 

 From such a set it would be possi- 



* That is, the magnets of each pair are made of identically the same kind of 

 steel, sebjected to the same kind of heat treatment and magnetized by the same 

 means. 



N N N N N N N N 



Fig. 1. 

 Pairs of exactly similar magnets. 



pair being exactly alike.* 



