378 Prof. 0. Lodge on the Controversy 



same amount of work whether we carry it through the 

 dielectric, or whether we plunge it into the copper and then 

 pull it out of the zinc. If this statement of fact be not dis- 

 agreed with, it is surely more convenient to say that the zinc 

 and copper in contact are at the same potential, and that the 

 air or space in their immediate but not molecular neighbour- 

 hood differs in potential *. In other words, that there is a 

 sudden step of potential in crossing the frontier from metal 

 to air, a step which I have reckoned as about 1*8 volts up 

 out of zinc, and about *8 volt up out of copper, and so a 

 (roughly speaking) 1 volt gradual descent of potential in the 

 air or space near them. 



I have found in the reported discussion at the Electrical Engineers 

 (Journ. Inst. E. E. ; 1885, vol. xiv. p. 233) the following ingenious and 



: r~ 



* It may be permissible to quote a sentence from Dr. John Hopkin- 

 son's contribution to the discussion on the same subject at the Institution 

 of Electrical Engineers (Journal, 1885, p. 235) : — " Professor Perry defines 

 the difference of potential between a piece of copper and a piece of zinc to be 

 the energy required to transfer a unit of electricity from near the copper to 

 near the zinc through the dielectric. On the other hand, as I understand 

 the matter, Dr. Lodge, and probably Clerk Maxwell, define the difference 

 of potential to be the energy required to transfer a current of electricity from 

 the copper to the zinc through the junction between the copper and the 

 zinc. Now, it appears to me that Dr. Lodge has very clearly shown that 

 his way of looking at the matter is the more convenient ; for, in the first 

 place, it expresses, with the greatest ease, all facts that have been 

 observed. When copper and zinc are in contact with air there is a 

 difference of potential at the three junctions, and the sum of the three 

 differences of potential is not equal to zero ; and we have no difficulty in 

 explaining all the electrostatic effects that we have obtained. Dr. Lodge 

 has so clearly proved the convenience of his way of looking at the matter 

 that it would be only wasting your time if I were to attempt to reproduce 

 his arguments. It seems to me better to take that basis and pursue it. 

 Xow, if we take that basis as our starting-point, it is very easy to set 

 forth all the theory of thermoelectricity, and the formula given at the 

 end of Clerk Maxwell's chapter on that subject can be proved with the 

 greatest ease ; it is also easy to extend these formulae to the case in 

 which we have chemical action going on, and to foreshadow the precise 

 method by which we may be able to ascertain exactly what the difference 

 of potential is between copper or zinc or other metals and electrolytes." 



He then makes some further remarks of considerable interest, which 

 were expanded by him later in the Phil. Mag. for October 1885. All I 

 would add to the above admirable statement is that the emphasis required 

 is upon the word " near " in the 3rd & 4th lines, and that the immediately 

 following specification of path — "through the dielectric" — is not really 

 essential. The potential-difference is independent of path in this case. 

 Replacement of dielectric by electrolyte, unable to stand the potential- 

 gradient statically, may be said to destroy equilibrium and yield a cyclical 

 E.M.F. and a current ; but, strictly speaking, the cyclical potentfal- 

 difference is still zero, when resistance to steady current is taken info 

 account; always- zero, in fact, even within the variable stage," when, 

 acceleration-reaction or self-induction is likewise included." 



