CHAPTER XL 



PERMANENT MAGNETISM. 



PHYSICAL PHENOMENA. 



398. IT is found that certain bodies, known as magnets, will attract or 

 repel one another, while a magnet will also exert forces on pieces of iron or 

 steel which are not themselves magnets, these forces being invariably attrac- 

 tive. The most familiar fact of magnetism, namely the tendency of a 

 magnetic needle to point north and south, is simply a particular instance of 

 the first of the sets of phenomena just mentioned, it being found that the 

 earth itself may be regarded as a vast aggregation of magnets. 



The simplest piece of apparatus used for the experimental study of 

 magnetism is that known as a bar-magnet. This consists of a bar of steel 

 which shews the property of attracting to itself small pieces of steel or iron. 

 Usually it is found that the magnetic properties of a bar-magnet reside 

 largely or entirely at its two ends. For instance, if the whole bar is dipped 

 into a collection of iron filings, it is found that the filings are attracted in 

 great numbers to its two ends, while there is hardly any attraction to the 

 middle parts, so that on lifting the bar out from the collection of filings, we 

 shall find that filings continue to cluster round the ends of the bar, while the 

 middle regions will be comparatively free. 



Poles of a Magnet. 



399. The two ends of a magnet or, more strictly, the two regions in 

 which the magnetic properties are concentrated are spoken of as the 

 " poles " of the magnet. If the magnet is freely suspended, it will turn so 

 that the line joining the two poles points approximately north and south. The 

 pole which places itself so as to point towards the north is called the " north- 

 seeking pole/' while the other pole, pointing to the south, is called the " south- 

 seeking pole." 



By experimenting with two or more magnets, it is found to be a general 

 law that similar poles repel one another, while dissimilar poles attract one 

 another. 



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