16 



MAGNETISM. 



no previous reasoning could have led us 

 to anticipate. Each portion, B C, of the 

 fractured magnet is at once converted 

 into a magnet, perfect in itself; that is, 

 each respectively has a north pole at one 

 end, and a south pole at the other. That 

 end of the magnet which, previously to 

 the fracture, was a north pole N, con- 

 tinues to be a north pole, while the 

 other end of that fragment s, that is, the 

 broken end, becomes a south pole. The 

 converse is true of that fragment B 

 which originally contained the south pole 

 of the magnet. It thus appears that the 

 two fractured surfaces n and s, are now 

 converted, the one into a north, and the 

 other into a south pole, although that 

 part had, in the original magnet, been 

 apparently in a neutral state. 



(64.) Similar consequences ensue 

 from the subdivision of one of these 

 fragments into any number of portions, 

 however great; each lesser fragment 

 constituting in itself a complete magnet 

 furnished with its two poles. 



(65.) It is observed by ^pinus, who 

 made many experiments on the effects 

 of the fracture of magnets, and the ob- 

 servation has been confirmed by others, 

 that the neutral point in each fragment 

 of the broken magnet is at first much 

 nearer to the place of their former union 

 than to their other ends. He states that 

 in the space of a quarter of an hour 

 after the separation, the neutral points 

 advance nearer to the middle of each, 

 and continue to do so, by small steps, 

 for some hours, and sometimes days, 

 and finally become stationary at the 

 centre. 



When a magnet is split according to 

 its length, the two portions will have 

 sometimes contrary, and sometimes the 

 same poles as they had when they formed 

 one piece. When one portion is much 

 thinner than the other, the slender frag- 

 ment generally has its poles reversed*. 



CHAPTER II. 



Laws of Magnetic Forces. 



1 . Relation of Intensity to Distance. 



(66.) It would be inconsistent with the 

 elementary views to which we are at 

 present confining ourselves, to engage in 

 the investigation of the mathematical 

 law which regulates the variations of in- 

 tensity of the magnetic forces, both at- 



> Philos P hieal Transactions, vol. xxiv. 



tractive and repulsive, at different dis- 

 tances. We shall here only observe that 

 this law has been made the subject of 

 diligent and careful inquiry by some of 

 the most eminent philosophers of modern 

 times ; and shall content ourselves with 

 merely stating the final result of their 

 labours. It has been ascertained most 

 satisfactorily that the same law of varia- 

 tion obtains in magnetic attractions and 

 repulsions with relation to proximity, as 

 in the electrical : namely, that the in- 

 tensity of the force by which magnetic 

 polarities act upon each other is inverse- 

 ly as the square of their distance. In 

 this respect, therefore, they agree, not 

 only with the electrical forces, but also 

 with that of gravitation ; and it would 

 appear, indeed, to be a property com- 

 mon to all forces which emanate in every 

 direction from a central agent. 



(67.) The variations of the intensities 

 of magnetic attractions and repulsions 

 exerted between any two poles depend 

 solely upon the distances at which they 

 are placed ; and are in no degree affected 

 or interfered with by the interposition of 

 other bodies which are not themselves 

 magnetic. Numerous experiments have 

 been made with a view of discovering 

 whether there exists any substance 

 which can modify or intercept the action 

 of magnets when placed between them 

 and the body acted upon ; but the result 

 has been uniformly the same : namely, 

 that the intervening bodies, of whatever 

 kind they were, provided they were not 

 susceptible of magnetism, occasioned no 

 difference in the observed effects. 



This subject, however, involves a ques- 

 tion, hereafter to be discussed, as to the 

 magnetic susceptibilities of substances 

 which are not of a ferruginous nature. 



2. Mutual Action of Two Magnets. 



(68.) The general law of magnetic 

 force with relation to distance being once 

 established, it becomes interesting to 

 follow its consequences and applications 

 under a variety of circumstances. These 

 consequences are always, even in the 

 simplest cases, more complicated than 

 electrical arrangements ; becatfse in 

 magnets the two polarities are "'ways 

 conjoined, and their influence is never 

 perfectly isolated. In studying the mu- 

 tual actions between two magnets, or 

 even between one magnet and the small- 

 est conceivable piece of iron, we have 

 always four polarities in activity, the two 

 residing in one body, and the two re- 

 siding in the other ; these polarities are 



