MAGNETISM. 



adhere to it just as if the key had itself 

 become a magnet. They also col- 

 lect and cohere together, as if animated 

 by a common sympathy. When this 

 has taken place, let us suddenly remove 

 the magnet : that moment all these ef- 

 fects cease at once, the key returns to 

 its natural or unmagnetic state ; the 

 bits of iron which had attached them- 

 selves to it immediately fall off, and 

 show no tendency either to cohere among 

 themselves, or to adhere to the key. 



(21.) Secondly, the vicinity of a mag- 

 net to a piece of iron gives it the pro- 

 perty of attracting and repelling the re- 

 spective poles of another magnet, in the 

 same way as a magnet would have done. 

 The truth of this proposition may easily 

 be proved by placing a small compass 

 needle poised as in fig. 2, in various si- 

 tuations relative to the ends of the key or 

 any other piece of iron of a lengthened 

 shape, while in the vicinity of the magnet. 

 It will be seen by this examination that 

 the piece of iron has acquired by induction 

 two poles, the qualities of which will be 

 discovered by their attractions or repul- 

 sions of the poles of the compass needle, 

 as they are respectively presented to 

 each ; and it will be found that these 

 two poles are disposed in the manner 

 specified above. 



(22.) Thirdly, the iron, which has be- 

 come magnetic by induction, has at the 

 same time acquired the power of in- 

 ducing a similar state of magnetism on 

 the iron in its neighbourhood. Thus, 

 while the bar B,y?g-. 5, is rendered mag- 

 netic by the influence of the magnet A, 

 it exerts itself a similar power on an- 

 other bar C, rendering it also magnetic. 

 The bar C, in its turn, will act in like 

 manner upon another bar, D, and so on. 

 In this way the influence of the magnet A 

 may be made to extend along a series of 

 iron bars or pieces of any other shape, 

 each acquiring magnetism by the induc- 

 tive power of the preceding piece ; and in 

 its turn inducing magnetism on the next. 



(23.) But this is not all. The piece 

 of iron which has been rendered magne- 

 tic by the vicinity of a magnet, not only 

 acts upon the other iron that is near it, 

 but also reacts upon the magnet from 

 which its power is derived, and increases 

 the intensity of its magnetism. The 

 power of a magnet is, in fact, augmented 

 by the exertion of its inductive influence 

 on a piece of iron in its neighbourhood. 

 A simple experiment is sufficient to 

 prove this fact. 



Let a piece of iron be suspended from 

 one of the poles of a straight magnet ; 



and let the weight which this magnet 

 will carry be ascertained by attaching to 

 the iron a scale, capable of holding the 

 weights necessary for this trial, and 

 which may be gradually increased till 

 the piece of iron drops off from the 

 magnet. Repeat this experiment, hav- 

 ing previously placed a bar of iron in 

 contact with the other pole of the mag- 

 net, and it will be found that the mag- 

 net will now support a much greater 

 weight ; showing the increase of power 

 it has derived from the presence of the 

 bar of iron which has been applied to 

 the other pole, and the induced magnet- 

 ism of which, although solely derived 

 from the magnet, reacts, by a kind of 

 secondary induction, upon that magnet. 

 We have already had occasion, in the 

 Treatise on Electricity, to notice the 

 same kind of reaction in the case of elec- 

 tric induction. The increased intensity 

 which a magnet acquires by induction 

 often leads to the permanent acquisition 

 of power by the magnet. Hence we may 

 understand the reason why a magnet that 

 is employed for magnetizing a neutral bar 

 of steel, by means of its inductive power, 

 becomes itself stronger by the operation. 



(24.) It is a necessary consequence of 

 the law of magnetic induction that it is 

 accompanied by attraction : for the po- 

 larity of the adjacent end of the piece of 

 iron on which the magnetism is induced, 

 is always of the opposite kind to that 

 of the pole of the piece which induces it : 

 according to the fundamental law of 

 magnetism, therefore, a mutual attrac- 

 tion must take place between them. The 

 remote end of the piece on which the 

 magnetism has been induced is indeed 

 repelled, because its polarity is similar 

 to that of the inducing pole : but it is 

 evident that the attractive action of the 

 adjacent and dissimilar poles will always 

 be stronger than the repulsive action of 

 the more distant poles ; and will there- 

 fore always prevail. 



(25.) This remark leads us" to a very 

 important step in the generalization of 

 the magnetic phenomena. We have 

 hitherto spoken of the attractive power 

 of magnets for iron as one of the 

 primary facts in the science : but we now 

 see that it is merely a necessary result of 

 a more general law, namely, that of in- 

 duction, together with the law of action 

 of the two polarities upon each other : 

 or, in other words, that it is itself com- 

 prehended in these more general facts. 

 A magnet attracts a piece of unmag- 

 netic iron, not from any inherent dispo- 

 sition to attract it in that state, but in 



