ELEMENTARY MAGNETISM. 41 1 



I dwell thus long on elementary principles, because 

 they are of the first importance, and it is the temptation 

 of this age of unhealthy cramming to neglect them. 

 Now follow me a little farther. In examining the 

 distribution of magnetism in your strip of steel you 

 raised the needle slowly from bottom to top, and found 

 what we called a neutral point at the centre. Now 

 does the magnet really exert no influence on the pole 

 presented to its centre ? Let us see. 



Let s N, fig. 13, be our magnet, and let n represent 

 a particle of north magnetism placed exactly opposite 

 the middle of the magnet. Of course this is an ima- 

 ginary case, as you can never in reality thus detach 

 your north magnetism from its neighbour. But sup- 

 posing us to have done so, what would be the action of 

 the two poles of the magnet on n ? Your reply will of 

 course be that the pole s attracts n while the pole N 

 repels it. Let the magnitude and direction of the 

 attraction be expressed by the line n m, and the mag- 



N 



Fid. 13. 



nitude and direction of the repulsion by the line n o. 

 Now, the particle n being equally distant from s and N, 

 the line n o, expressing the repulsion, will be equal to 

 m ?i, which expresses the attraction. Acted upon by 

 two such forces, the particle n must evidently move in 

 the direction n p, exactly midway between m n and n o. 

 Hence you see that, although there is no tendency of 



