62 On the Seat of the Electromotive Forces in a Voltaic Cell. 



by means of mechanism enclosed by the case. Now let there 

 be, outside the case, a secondary circuit, and let the primary 

 enclosed circuit have its current diminished at such a rate that 

 a constant current is maintained for a short length of time in 

 the secondary by induction. We know quite well that such 

 an inductive effect might take place, and that the energy of 

 the secondary circuit must somehow come from the primary ; 

 and it must be remembered that it need not have been by a 

 diminution of the primary current, but an increase of the 

 primary current, that this induction takes place. We cannot 

 see how it is possible that Prof. Poynting's electro-potential 

 surfaces can proceed from the primary so as to pass through 

 the secondary circuit ; we can, however, imagine that, instead 

 of equipotential surfaces, such as we speak of in Electrostatics, 

 if we speak of a series of surfaces cutting the lines of electrical 

 displacement at right angles, then an explanation becomes 

 quite possible. In fact, it seems to us that if Prof. Poynting's 

 views are logically deducible from Maxwell's theory, it is 

 necessary to assume that Maxwell's differences of potential are 

 essentially different from electrostatic difference of potentials. 

 September 3, 1885. 



Note added December 14, 1885. 



Since these notes of our remarks made in the discussion on 

 Dr. Lodge's paper were sent to the Editors at the beginning 

 of September, a paper on Contact Electricity has been read 

 by Dr. Hopkinson before the British Association, and has 

 appeared in the Philosophical Magazine. We are glad to see 

 that Dr. Hopkinson _, who has entered so fully into the energy 

 equations, not only for Sir William Thomson's view, but also 

 for that of Prof. Maxwell and Dr. Lodge, has come to the same 

 conclusions as ourselves. 



We leave our formulas on page 58 of these Notes as they 

 were written in April last, following Prof. Maxwell's lettering ; 

 but for the sake of those who may have studied that part of 

 Dr. Hopkinson's paper which refers to Sir William Thomson's 

 theory, it is well to state that 



he uses/(£) where we use t Y a b, 



» <K0 „ » tk a, 



„ yjr(i) „ „ tk b . 



