MAGNETISM. 



35 



cal state, the redundant fluid in the 

 former, exerting a repulsive influence on 

 the fluid at the nearest end of the latter, 

 will give it a tendency to move towards 

 the remote end. It will obey this ten- 

 dency, provided the texture of the iron 

 offers no obstruction to its transmis- 

 sion ; and a certain portion of the fluid 

 will accordingly be transferred from the 

 near to the remote end. The bar will 

 now exhibit magnetic properties; its 

 near end being undercharged, will pos- 

 sess a polarity of an opposite nature to 

 that of the magnetic pole presented to 

 it ; the remote end, being overcharged, 

 will have a polarity of the same kind as 

 the pole of the magnet. 



(143.) A series of changes exactly the 

 converse of these will take place when, 

 instead of the overcharged pole, we 

 present the undercharged pole of the 

 magnet to the bar. The redundant iron 

 now attracts the magnetic fluid of the 

 bar, and draws it towards the adjacent 

 end, converting it into an overcharged 

 pole, while the other end, from which 

 the fluid has been drawn, becomes the 

 undercharged pole. 



(144.) The effects, however, do not 

 end here. The bar, thus rendered mag- 

 netic, reacts upon^ the magnet from 

 which it had derived its power, and tends 

 to increase the magnetism it originally 

 possessed. This increased magnetism, 

 in its turn, tends to produce an augmen- 

 tation of the induced magnetism of the 

 bar ; and these alternate actions and re- 

 actions proceed till all action is balanced 

 and every thing remains quiescent. In 

 soft iron this is accomplished almost in 

 a moment : but in steel the process is 

 somewhat different ; for its texture 

 presenting great impediment to the mo- 

 tion of the magnetic fluid, the changes 

 of distribution take place much more 

 slowiy, and to a less extent than they do 

 in iron. The adjacent end of a steel bar 

 soon acquires a degree of polarity oppo- 

 site to that of the end of the magnet 

 presented to it ; but the polarity of the 

 same kind travels slowly onwards, and 

 does not reach the other extremity of 

 the bar till after a considerable time ; 

 and if the bar be very long, may possibly 

 never reach it. 



In this last case, we have a curious 

 phenomenon produced from the influ- 

 ence of a secondary induction ; namely, 

 the appearance of a second set of poles, 

 at a certain distance from the first. Thus, 

 if a north pole has been presented, the 

 adjacent end of the bar will be a south 



pole ; at a little distance from this we 

 shall have a north pole; beyond this 

 again will appear another south pole, 

 and perhaps at the furthest end a se- 

 cond north pole. Sometimes, indeed, 

 there will be only three poles, the middle 

 one being of an opposite character to 

 those at the two ends, which are similar 

 to one another. 



(145.) Let us now remove the mag- 

 net; what will happen to the iron bar? 

 The cause which maintained the mag- 

 netic fluid in the forced state of excess 

 at one end, and of deficiency at the 

 other, no longer operating, the fluid will 

 now tend to resume its original state of 

 uniform distribution over the whole mass 

 of iron ; and if no obstacle exist in the 

 structure of the iron to impede its mo- 

 tion, it will immediately revert to that 

 state. But if the bar be of steel, which 

 presents obstacles to the passage of the 

 fluid, which the force derived from its 

 tendency to equable diffusion is insuffi- 

 cient to overcome, the fluid which had 

 passed will remain stationary, and the 

 induced magnetism will continue as at 

 first that is to say, the bar will have 

 been converted into a permanent magnet. 



(146.) On the same principle may be 

 explained the effect of hammering, or 

 any other kind of mechanical concus 

 sion, in impairing the magnetism of a 

 steel bar; for the tremulous motions 

 excited among the particles will open a 

 passage for the fluid, which will thus 

 escape from the situations where it is 

 condensed, and return to those where it 

 is rarefied. 



(147.) Heat, as we have seen, weak- 

 ens and finally destroys magnetic 

 power ; its operation may in like man- 

 ner be understood, by its occasioning 

 the separation of the particles of iron to 

 a greater distance than before. Hence 

 the interstices will be enlarged, and the 

 obstacles to the motion of the fluid will 

 be diminished, or even entirely removed. 

 The magnetic fluid will thus be enabled 

 to regain its natural state of uniform 

 diffusion among the particles. But in- 

 dependently of its mechanical operation, 

 there are yet many other ways in which 

 heat may be conceived to contribute to 

 the destruction of magnetism. It may 

 change the action of the particles of 

 iron on those of the fluid, or of the fluid 

 on each other, and by altering the dis- 

 tribution of the fluid with respect to the 

 particles of iron, may greatly affect the 

 law of action between one magnet and 

 another. 



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