254 ScientiJiG Proceedings, Royal Dnhlin Society. 



and then we have— 



R dx 



E, dy 



E, dz 



From these it appears at once that the magnetic effect of the 

 displacement currents is nil. For the components of the magnetic 

 force a, 13, y, are — 



dz dy 



n_dH _ dF 



dx dz 



_dF _ dG- 



dy dx 



and X disappears from this, and there only remains the magnetic 

 action due to the current that I have assumed to represent the 

 moving superficial electricity. The effect of this is obviously the 

 same as if the whole quantity of electricity were moving at its 

 centre, and this is the same as Mr. Thomson's result (I.e. p. 236). 



It is to be observed that Mr. Thomson has intentionally 

 omitted the self-induction of these displacement currents on 

 one another, and it may legitimately be omitted when the motion 

 is comparatively slow, but a complete solution of the question 

 would be most interesting. 



It may be worth while remarking that no effect except light, 

 has ever yet been traced to the displacement currents assumed 

 by Maxwell, in order to be able to assume all currents to flow in 

 closed circuits. It has not, as far as I am aware, been ever 

 actually demonstrated that open circuits, such as Leyden jar 

 discharges, produce exactly the same effects as closed circuits, 

 and until some such effect of displacement currents is observed, 

 the whole theory of them will be open to question. 



