166 MAGNETISM 



and South when freely suspended horizontally. Such a bar was 

 formerly termed an artificial magnet. 



In describing the fundamental phenomena we shall suppose 

 that we are dealing with steel magnets. Natural magnets are now 

 mere curiosities and have not baen usefully employed since the 

 eighteenth century. The prefix "artificial" has long ceased to be 

 applied to steel magnets. 



Poles. The two ends of a magnetised bar round which the 

 attractive properties are exhibited are termed its poles. The end 

 setting towards the North is termed the North -seek ing pole, the 

 other the South-seeking pole, or more shortly the North and the 

 South poles respectively. They may be designated by NSP and 

 SSP, or more shortly by N and S. The NSP is frequently marked 

 on the magnet either by a cut across it, or by N cut in the 

 metal. 



Mechanical actions of poles on each other. li ire 

 suspend one magnet so that it is free to move round on a pivot, 

 forming in fact a compass needle, and then pre>ent to its poles in 

 succession each of the poles of a second magnet, \\e find that two 

 unlike poles attract each other, while two like poles repel each 

 other, or : 



NSP attracts SSP and repels NSP and 

 SSP NSP SSP. 



The two poles have opposite mechanical action f. \Yhere one at t iaeU 

 the other repels, and the actions of the two poles lend to neutralise 

 each other. We may therefore call them respectively positive and 

 negative, and if we consider a NSP to be positive, \\e musf consider 

 a SSP to be negative. 



Position of poles. In a well-magnetised bar the poles are 

 near the ends. For, on dipping the bar into iron filings, we find 



S N 



=* 1= 



FIG. 108. 



that the filings cluster chiefly about the ends, and hardly adhere 

 at all near the middle. But though we speak of the poles of a 

 magnet we must remember that they are not definite points, but 

 rather regions, in general near the ends of the magnet. 



The two opposite poles always accompany each other 

 on a magnet. If we break a magnetised knitting needle, for 

 instance, we do not isolate the two poles, for two new poles are at once 

 developed at the broken ends, as in Fig. 108, so that each fragment 

 becomes a complete magnet with its opposite poles. All experi- 



