ElECTRO-MAGNETisM. 



science of electro-magnetism may, as we 

 have seen, be reduced to three classes, 

 or general facts : the first being the evo- 

 lution of a tangential and rotatory force 

 usually exerted between a conducting 

 body and a magnet ; the second, the 

 transverse induction of magnetism by 

 the former in such bodies as are suscep- 

 tible of receiving it ; and the third, the 

 attractive or repulsive force exerted be- 

 tween two electric currents traversing 

 different conductors. In the magnetic 

 theory already discussed, the first of 

 these is considered as being the most 

 general fact, and the other two as being 

 merely its consequences. Ampere, on 

 the contrary, assumes the last of these 

 facts that is, the mutual attractions and 

 repulsions of electric currents, as the 



Enmary or fundamental fact, to which, 

 y the help of a particular hypothesis 

 as to the constitution of magnets, all 

 the other facts, not only of electro-mag- 

 netism, but of magnetism also, are 

 reducible. 



(264.) His supposition is, that all 

 bodies that possess magnetic properties, 

 the globe of the earth being included 

 among the number, derive those proper- 

 ties from currents of electricity continu- 

 ally circulating among the parts of which 

 they are composed, and having, with re- 

 lation to the axes of these bodies, one 

 uniform direction of revolution, in planes 

 perpendicular to those axes. 



(265.) The striking resemblance which 

 exists between the action of magnets, 

 and that of electro-dynamic cylinders 

 already described, and which extends 

 through a wide range of phenomena, 

 very naturally suggested the hypothesis 

 on which this theory is founded ; for 

 since the circular currents in the latter 

 are observed to produce effects similar 

 to magnetic polarity, it is but an exten- 

 sion of the analogy to consider a magnet 

 as deriving its properties from similar 

 currents continually circulating in its 

 substance. 



(266.) In the account we have given 

 of magnetism it will be seen that the 

 phenomena attending the fracture of a 

 magnet oblige us to consider magnet- 

 ized iron as an aggregate of small par- 

 ticles of iron, each of which has the pro- 

 perties of a separate magnet (see MAG- 

 NETISM, 141). In like manner, the 

 hypothesis just stated, relative to the 

 circulation of electric currents in the 

 substance of a magnet, must receive a 

 similar modification to that given to the 

 theory of magnetism t Since the frag- 



ments detached from a magnetic bar are 

 themselves complete magnets, the elec- 

 tric currents, from which it derives its 

 properties, must be conceived as circu- 

 lating round each of these fragments 

 separately, or rather round particles 

 smaller than any that can be obtained 

 by mechanical division. Each particle, 

 or magnetic element, may be regarded 

 as constituting a voltaic circuit, analo- 

 gous to a voltaic pile of which the two 

 ends are united by conductors ; the 

 vitreous and the resinous electric fluids 

 being separated at one point of the cir- 

 cuit, circulating in contrary directions 

 round the particle, until they meet toge- 

 ther, and by their reunion again forming 

 the neutral fluid. The course of the 

 fluids during this circulation is repre- 

 sented in fig. 154 ; V and R denoting 

 respectively the paths of the vitreous 

 and resinous electricities emanating from 

 the point E in the particle of iron P, 

 and flowing in the directions indicated 

 Fig. 154. 

 E 



by the arrows, till they meet and coa- 

 lesce on the opposite side. But as the 

 effects of the one fluid are exactly the 

 reverse of those of the other, the result 

 is equivalent to the continued circula- 

 tion of one of the fluids, the vitreous, 

 for example, in one constant direction, 

 EV PR. 



(267.) A magnet, then, is to be con- 

 sidered as composed of an assemblage of 

 parallel filaments, each of which is con- 

 stituted by a series of particles, round 

 which electric currents are circulating in 

 the manner just described, all of them 

 flowing in the same direction with re- 

 ference to the axis of the filament, and 

 moving in planes perpendicular to that 

 axis. That extremity of the filament in 

 which, when uppermost, the current of 

 positive electricity is moving in a direc- 

 tion similar to that of the hands of a 

 watch (the dial of which is also upper- 

 most), has the properties of a south 

 magnetic pole, and vice versa. If the 

 filament be placed horizontally, its north 

 pole pointing to the north, then the cur- 

 rents on the western side are ascending, 

 pass from west to east in the upper sur- 

 face, descend on the eastern side, and 

 return from east IQ west in the lower 



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