1891.] on Electro-magnetic Bepulsion. 311 



current, the effect of tlie copper or conductive circuit, which is 

 linked with the iron or magnetic current, is similarly to introduce a 

 hack magnetomotive force into the magnetic circuit by reason of the 

 magnetic field set up by the secondary current generated in that 

 copper or conducting circuit. In other words, the secondary current 

 induced in the copper circuit by any change in the magnetomotive 

 in the iron circuit is in such a direction that it operates to oppose 

 that primary magnetomotive force, chiefly, however, at the spot 

 where the copper circuit passes round the iron. The general result 

 may be stated to be that the action of the interlinked copper circuit 

 is to cause the magnetic induction in the iron circuit to leak across 

 through the air and partly to escape, passing through the secondary 

 circuit. This escape of induction is called magnetic leakage, and 

 the induced current set up in the closed secondary circuit is the 

 cause of this magnetic leakage. There is a symmetry in the relations 

 of magnetomotive force and the magnetic induction and electromotive 

 force and electric current, and we can, as Faraday pointed out long 

 ago, make the symmetry complete, if we suppose the two interlinked 

 magnetic and electric circuits immersed in an imperfectly conducting 

 medium. If, then, we throttle a magnetic circuit, such as a laminated 

 iron bar with copper coils closed upon themselves, and place a 

 magnetising coil at one end, the closed conducting circuits hinder 

 the rise of magnetic induction in the bar ; in other words, they give 

 it what may be called magnetic self-induction. If the source of 

 magnetism is a rapidly-reversed pole, the consequences of this delay 

 or " lag " in the induction is that a series of alternating magnetic 

 poles are always travelling with retarded speed up the bar, and these 

 may be considered to be represented by tufts of lines of magnetic 

 force which spring out from and move laterally up the bar. If the 

 bar is not laminated and not throttled, the eddy currents set up in 

 the mass of the bar itself act in the same way, and operate to resist 

 the rise of induction in the bar and to delay the propagation of 

 magnetism along it. Hence we must think of such a throttled bar, 

 when embraced by a magnetising coil at one end, as surrounded by 

 laterally moving bunches of lines of magnetic force, which move up 

 the bar. Each reversal of current in the magnetising coil calls into 

 existence a fresh magnetic pole at the one end of the bar, which is, 

 as it were, pushed along the bar to make room for the pole of 

 opposite name, which appears the next instant behind it. When an 

 iron disc is held near such a laminated und throttled bar, these 

 laterally moving lines of force induce poles in the disc which travel 

 after the inducing poles, and hence the disc is continually pulled 

 round. If the disc is a copper disc, the laterally moving lines of 

 magnetic force induce eddy currents in the disc, and these, by the 

 principle already explained, create a repulsion between the pole and 

 the part of the disc in which the eddy currents are set up. 



§ 14. The progression of alternate poles along a bar can be 

 investigated by means of an experiment due to Mr. A. Wright. 



