88 



ELECTRO-MAGNETISM. 



This tendency, indeed, is but feeble; 

 and the first endeavours of Ampere to 

 discover it failed, in consequence of his 

 employing inadequate means ; but, on 

 repeating the experiment with more 

 powerful batteries and magnets, he per- 

 fectly succeeded in rendering the action 

 sensible. 



It were much to be desired that this 

 important experiment, upon the accu- 

 racy of which so much is made to 

 depend in accounting for magnetic 

 induction in Ampere's theory, were 

 carefully repeated, and with every pos- 

 sible variation in its circumstances, so 

 as to determine whether the effect which 

 he observed is uniformly sustained, is 

 invariably connected with its supposed 

 cause, and is always proportioned to it ; 

 or whether it be not dependent upon 

 some particular conditions in the current 

 with relation to its tension, velocity, or 

 intensity, or upon some temporary va- 

 riation taking place in these conditions. 

 In the particular form in which the ex- 

 periment has been tried, it seems scarcely 

 to warrant the very general conclusion 

 which Ampfere has deduced from it. 



(285.) Even if we admit it to be esta- 

 blished as a general fact, that electric 

 currents, circulating in one body, are 

 attended by similarly directed currents 

 in neighbouring conductors, we are 

 still not in a condition to decide the ques- 

 tion, whether, in imparting magnetism 

 to metals, there is an, actual production 

 of electric currents, or simply a change 

 effected in the directions of currents 

 previously existing in their particles. 

 There is, however, no inconsistency in 

 the supposition that the effect may be 

 due to both these causes ; for the action 

 of an electric current may consist in 

 giving a common direction to pre-exist- 

 ing currents, while it, at the same time, 

 augments their intensity. 



(286.) It is unnecessary to enter into 

 long details as to the modes in which, 

 according to the theory we are consider- 

 ing, an electric current, passing through 

 conductors of different forms, whether 

 straight, or bent into spirals, or helices, 

 or a magnet, in which currents are sup- 

 posed to circulate, induce magnetic po- 

 larity in the adjacent parts of pieces of 

 iron or steel brought within the sphere 

 of their influence, when we regard that 

 polarity as consisting in the establish- 

 ment of circular currents of the same 

 description as those of the inducing 

 magnet. It will be sufficient to show 

 ihat the fundamental fact, namely, that 



either pole of a magnet tends to induce 

 the opposite polarity in the adjacent end 

 of a magnetizable body in its vicinity, is 

 the direct and necessary consequence of 

 the hypothesis. That this is the case 

 will readily appear from considering that 

 when the elementary magnetic filament 

 A B,/g-. 163, is brought near to a simi- 

 lar elementary filament, C D, in a neutral 



Fig. 163. 



B C D 



state, the currents which circulate in the 

 former will excite in the latter a circu- 

 lation of currents in the same direction, 

 thereby rendering it magnetic. But since, 

 according to the theory ($ 267), the kind 

 of polarity manifested at either end of 

 a magnet depends altogether upon the 

 direction of the currents with respect to 

 the axis at the extremity, it is evident 

 that if the current at the end B revolves, 

 as seen by a spectator looking at that 

 end, in the direction of the hands of a 

 watch on the dial, constituting the 

 southern polarity, the current induced 

 at the end of the other piece, C, revolv- 

 ing in the same direction in space, will 

 appear to a spectator looking at that 

 end to move in the contrary direction ; 

 it will therefore have a northern polarity, 

 that is, one contrary to that of the ad- 

 jacent end B, but similar to that of the 

 remote end A. In like manner the 

 polarity of D, if the inductive influence 

 extend to that distance, will be the same 

 as that of B ; for the circumstances at- 

 tending the revolution of its current are 

 precisely the same in both. 



(287.) When, on the other hand, the 

 neutral bar is placed near and parallel 

 to the inducing magnet, the action of 

 the currents on the adjacent side of the 

 latter will prevail over that of the cur- 

 rents on the remote side, on account of 

 their greater proximity to the bar, and 

 induce in its adjacent side currents 

 running in the same direction ; but these 

 two sets of currents, being situated in 

 different sides of their respective axes, 

 will constitute magnetic currents in con- 

 trary directions, and, therefore, of oppo- 

 site properties. Hence the poles of the 

 induced magnet are reversed with re- 

 lation to those of the inducing magnet. 

 This will readily appear from an inspec- 

 tion of Jig. 164. The same opposition 

 of direction takes place when two paral- 

 lel rollers turn upon one another, in con- 



