10 



MAGNETISM. 



pole of the same denomination as that 

 of the first magnet to which the key ad- 

 heres, is applied to the lower end of the 

 key. The first action of the lower mag- 

 net, as it approaches the key under 

 these circumstances, is to repel it ; but 

 on being brought still nearer, its induc- 

 tive influence becomes so great as to re- 

 verse the poles of the key, which is now 

 attracted by the pole which before re- 

 pelled it : it then generally drops off and 

 adheres to the lower magnet. 



(39.) The effect of applying similar 

 poles to the two ends of a bar of iron is 

 generally that of inducing the opposite 

 polarity on both ends of the bar, and the 

 same polarity at the middle. Thus,^* 

 21, the north poles N, N, ofthetwomag- 



Fig. 21. 



A 



nets A and C being applied lengthwise to 

 the ends of an intermediate bar B, will 

 render it a magnet with three poles, 

 those at the end being south poles, and 

 the middle being a north pole. In this 

 case the bar will be attracted by both 

 the magnets, though less powerfully than 

 when the acting poles of the latter are 

 of opposite kinds, as in the situation 

 shewn by/g-. 13. In more complicated 

 cases, and more especially when the 

 form of the piece of iron is irregular, it 

 is difficult to predict the exact mode in 

 which the poles will arrange themselves 

 when magnetism is induced upon it by a 

 single magnet, and still more when the 

 operation of two or more magnets, es- 

 pecially if they be of unequal strength, 

 is to be estimated. The following, how- 

 ever, is one of those cases in which the 

 process that takes place is more obvious, 

 and which furnishes an amusing illus- 

 tration of the general principle. 



(40.) Take a piece of iron, C (Jig. 

 22), formed into the shape pj ff 2 2 

 of a fork, or of the letter ' 

 Y, and suspend it by one 

 of the branches of the fork 

 to the north pole of a mag- 

 net A ; its lower end will 

 immediately acquire a nor- 

 thern polarity, and will at- 

 tract another small piece of 

 iron, such as a key, which 

 may therefore easily be 

 supported by it. ^While the 

 key is thus hanging from 

 its lower end, apply to the 

 other branch of the fork the 



south pole of another magnet B, the 

 key will instantly drop off. The reason 

 is that the magnet B tends to induce 

 upon the remote or lower end of the fork, 

 a contrary polarity to that which is in- 

 duced upon it by A, and thus destroys 

 its power of attracting. The fork will 

 have a south pole at a, a north pole at b, 

 while its lower end will be neutral. If, on 

 the contrary, the northpole of the magnet 

 B had also been applied to the branch b 

 of the fork, its influence would have 

 conspired with that of A in inducing a 

 northern polarity at C, and the key 

 would have been more strongly attracted. 



6. Different Qualities of Iron and 

 Steel with regard to Magnetic Sus- 

 ceptibility and Retentiveness. 

 (41.) All the effects we have hitherto 

 described, as attendant on the induction 

 of magnetism on iron, are of a temporary 

 nature, depending altogether on the in- 

 fluence excited by the neighbouring 

 poles of a magnet ; for we find that, the 

 moment the magnet is removed, all these 

 effects cease, and the iron returns to its 

 original state of neutrality, and loses all 

 its magnetic properties. But the case 

 is different when steel is made the sub- 

 ject of experiment. Magnetism, it is 

 true, may be induced on steel ; but the 

 induction proceeds very slowly, and is, 

 at first, much more feeble than it is with 

 iron. On the other hand, steel does not, 

 like iron, lose what it has acquired ; for, 

 on the removal of the magnet which 

 gave it the magnetic properties, it re- 

 tains these properties permanently : it 

 has, in fact, become itself a real magnet. 

 (42.) This remarkable difference ex- 

 isting between iron and steel in their re- 

 spective susceptibility to receive, and 

 capacity to retain magnetism, must, no 

 doubt, arise from some peculiar ar- 

 rangement of their particles, the exact 

 nature of which is at present entirely 

 unknown. 



It is, however, exceedingly analogous 

 to the difference in the qualities of elec- 

 trics and non- electrics with regard to 

 the power of conducting electricity ; and 

 the magnetic phenomena depending 

 upon it admit of being explained on an 

 hypothesis very similar to that by which 

 we are enabled to account for the elec- 

 trical phenomena which correspond to 

 them. The two magnetic polarities 

 may be conceived to reside constantly 

 in all iron or steel, and in the natural 

 or neutral condition of these bodies, may 

 be regarded as in a state of equilibrium, 



