360 Dr. 0. J. Lodge on the Dimensions of a Magnetic 



radians per second in order to produce the same magnetic 

 effects as the electromagnetic unit of current flowing in the 

 same ring. 



Or, conversely, the electrostatic unit magnetic pole would 

 be that which would experience unit force if placed at the 

 centre of a circle of unit radius in which the electrostatic unit 

 of electricity was moving with unit velocity. 



This definition I believe to hold equally well in any homo- 

 geneous medium; for it is pointed out below that the electro- 

 magnetic effect of a current is independent of //.: while as 

 regards K, a quantity which we might perhaps think would 

 be likely to affect the result, we must remember that electric 

 displacement is totally independent of any such circumstance. 

 So, corresponding to the common electrokinetic equation, 



Force =m\ — — j 



we shall have, for 



whence 





-r, mev 



i orce = — — 



[w]=[ML 2 r'] (2) 



If statements like these are in the main correct, and after 

 the experiments of Rowland we are bound, I suppose, to 

 believe in the truth of something of the kind, they ought to 

 remove Dr. Everett's objection (Phil. Mag. June 1882, p. 431) 

 as to the introduction of electrostatic units into magnetism ; 

 unless indeed he maintains the thesis — no doubt a tenable one 

 — that directly you begin to carry a charged body about, the 

 discussion of its performances no longer belongs to electro- 

 statics.) 



Returning from this digression, we have now to ask whether 

 the statements above made are really definite and independent 

 of the magnetic properties of the medium surrounding the 

 circuits, or must we introduce a factor to express the influ- 

 ence of this medium when it is other than air? 



Mr. J. J. Thomson has instructively raised this question 

 ( Phil. Mag. for June); and he and others at Cambridge consider 

 it a matter to be settled by experiment: and they further con- 

 sider that, in order to agree with Maxwell's view, experiment 

 ought to make the magnetic effect of a solenoid and its air- 

 equivalent magnetic shell differ, as soon as they are both intro- 

 duced into some medium for which jm is not unity. Now, 

 though agreeing with this as far as it goes, I venture with 

 diffidence to think that Maxwell Avould have drawn a distinc- 



