MAGNETISM. 



11 



and as equally 'distributed throughout 

 the whole mass. But this state of equi- 

 librium in a bar is disturbed by the in- 

 fluence of a magnetic pole in the vici- 

 nity, which exerts an inductive influ- 

 ence. This new force which comes into 

 play, tends to transfer one kind of pola- 

 rity to one end of the bar, and the op- 

 posite polarity to the other end. In iron, 

 these changes are readily effected, on 

 account of the facility which its peculiar 

 texture affords for the transmission of 

 these agencies in both directions. No 

 sooner is the cause which has produced 

 these changes, and maintains the sepa- 

 ration of the polarities, removed, than 

 all the effects cease ; for there exists no 

 obstacle to the return of the two polari- 

 ties to their original situations: they 

 revert, therefore, to their former state of 

 equal distribution, and the condition of 

 neutrality is restored in every part. But 

 it is not so with steel ; the constitution of 

 which is such as to interpose impedi- 

 ments to the transfer of the polarities 

 from one part to another. It requires a 

 certain time before the obstruction, what- 

 ever be its nature, can be overcome ; and 

 before the new state of distribution, 

 which induction tends to establish, can 

 be completed ; nor can the changes 

 themselves ever be accomplished to the 

 same extent as they are in bodies which 

 present no such obstacles. It is also a 

 necessary consequence of the resistance 

 which the texture of steel presents to 

 any changes taking place in its mag- 

 netic condition, that these conditions, 

 when once induced, tend to remain in 

 a great degree fixed. Hence we see 

 the reason why steel bars admit of being 

 rendered permanently magnetic, while 

 the magnetism induced upon iron is 

 only temporary. A steel bar, which has 

 as great a degree of magnetic power as 

 it is capable of retaining, is said to be 

 saturated with magnetism. 



(43.) In order to obtain an exact 

 knowledge of the progress of magnetic 

 induction in steel, we should place a bar 

 of this material very near to the pole of a 

 strong magnet, and in the same line with 



Fig. 23. 



it, and provide ourselves with a very small 

 and delicate compass needle, poised 

 on its centre, as in fig. 2, by which the 

 polarity of each part of the bar may be 

 examined in succession. It will be 



evident that an inductive 'action com- 

 mences immediately on the magnet's 

 being presented to the bar ; for the latter 

 is attracted and adheres very strongly 

 to the magnet from the very first. 



The end next the magnet has, there- 

 fore, acquired a polarity of a contrary 

 nature to that of the magnet. For the 

 sake of greater clearness of illustration, 

 we shall suppose the actual pole of the 

 latter to be a north pole. The near end 

 of the steel bar is at once converted into 

 a south pole ; but if we examine the re- 

 mote end we do not find it so immedi- 

 ately converted into a north pole. A 

 sensible time is required for effecting 

 the change in the latter ; and it will be 

 found, upon a more careful investiga- 

 tion, that the different parts of the bar 

 from south to north, acquire, in suc- 

 cession, this northern polarity, which 

 at last settles in the extremity. If 

 the bar be of considerable length, it 



Fig. 24. 



often happens that the northern pola- 

 rity never reaches thus far, but stops at 

 a nearer point ; and in that case, we 

 generally find a weaker south pole ap- 

 pearing at some greater distance ; and 

 this pole also travels slowly onwards 

 till it attains its furthest limit. This 

 is often succeeded by another north 

 pole, and even a greater number of 

 alternations will sometimes take place ; 

 each successive pole, however, be- 

 coming weaker and more diffused in 

 proportion as they are more nume- 

 rous and more distant. The points 

 where the polarities thus change from 

 the one kind to the other have been 

 called consecutive points. It is evident 

 that alternations of this kind must very 

 much disturb the regularity of the mag- 

 netic actions of bars in which they 

 exist, and complicate the resulting 

 phenomena. 



It would appear, from a variety of 

 facts hereafter to be detailed, that a 

 certain time is in all cases required for 

 the complete operation of magnetic in- 

 duction. 



(44.) There are certain circum- 

 stances and modes of treatment which 

 tend to quicken the progress of this in- 

 duction. The first of these is concus- 

 sion. Whatever excites a tremulous or 

 vibratory motion among the particles of 

 the steel, promotes thejtrahsmission of 



