84 



ELECTRO-MAGNETISM. 



to the wire which acts upon it. When 

 the magnet and wire have attained this 

 relative position, it is evident that, since 

 the adjacent currents move in the same 

 direction in both, an attraction will take 

 place between them. All this, as we 

 have seen, is in perfect accordance with 

 the observed phenomena. 



(272.) It is unnecessary to pursue 

 the application of this theory to the 

 endless variety of cases of the mutual 

 actions of magnets and conducting 

 bodies, because, having already fully 

 gone into the details of the explanation 

 which is afforded of these facts by the 

 principle of a tangential force ema- 

 nating from both these agents, it will 

 necessarily follow that they are all 

 equally explicable on the electro-dyna- 

 mic theory, if it be once proved that 

 the basis of the former theory, namely 

 the tangential force, is itself a direct 

 consequence of the latter. Now this 

 has already been established experi- 

 mentally by the phenomena exhibited 

 by the helices and electro- dynamic cy- 

 linders described in a former Chapter, 

 107, and the same has also been de- 

 duced from theory, according to what 

 was stated in 249. It has been shown 

 that the same tangential force results 

 from the heliacal disposition of the cur- 

 rent, whatever be the position of the 

 axis of the helix relatively to the con- 

 ductor on which it acts. We are war- 

 ranted, therefore, in transferring this 

 conclusion to the action of the circular 

 currents assumed as existing in mag- 

 nets, and as being the sole source of 

 their activity. 



(273.) Guided by these principles, 

 we find no difficulty in explaining the 

 phenomena of revolving motions so 

 frequently resulting from the mutual 

 actions of magnets and conducting 

 wires ; and which take place in exactly 

 the same manner when helices or elec- 

 tro-dynamic cylinders are substituted 

 for the magnets. It is instructive, how- 

 ever, to examine the particular cases 

 we have already given in exemplification 

 of the rotatory tendency arising from a 

 tangential force, by applying to them 

 the more general principles of electro- 

 dynamic action. In many instances it 

 will be found that the rotatory motions, 

 although in part produced by the action 

 of the currents in the magnet upon the 

 current in the straight wire, are also in 

 a still greater extentf dependant on the 

 influence of those portions of the cur- 

 rent that traverse the mercury into 



which the conducting wires or the mag- 

 nets are immersed. 



(274.) This is exemplified, in the fol- 

 lowing arrangement represented in Jig. 

 158, where the bent wire, proceeding 



Fig. 158. 



from the positive cup P, terminates in a 

 steel point that is made to dip into the 

 surface of a quantity of mercury con- 

 tained in the vessel A B, in the centre 

 of which a magnet, M, is kept floating 

 in a perpendicular position by being 

 loaded with a weight of platina at the 

 lower end. A ring of copper is placed 

 on the surface of the mercury, from the 

 side of which proceeds a wire, which 

 terminates in the cup N. The electric 

 current, in passing from the steel point 

 to the ring of copper, traverses the mer- 

 cury, radiating from that point as from 

 a centre, and consequently giving a 

 revolving tendency to the currents in 

 the magnet below them. The magnet, 

 under these circumstances, revolves on 

 its axis. A similar effect, but in a con- 

 trary direction, takes place when the 

 course of the electric stream is reversed, 

 and is made to traverse the mercury 

 from the copper ring towards the steel 

 point, producing converging instead of 

 diverging currents. The explanation of 

 these phenomena is obvious, from what 

 has been said in 203, 204. For let M 

 in fig. 159, be one of the currents at the 



upper end of the magnet, and C D one 

 of the diverging currents ; the action of 

 the portion E D will be to produce a 

 revolving motion of the magnet in the 



