concerning Voltas Contact Force. o55 



Excepting in portions relating to f{t) there is, I apprehend, 

 so far no controversy. And yet, if all this be admitted, my 

 contention that, in the equation II = TdE/c#, the E refers to 

 the whole circuit and not to any particular part of it, though 

 the II may refer to a junction, is substantiated. 



Nevertheless this statement has been, and probably still 

 will be, seriously disputed *, but I hope that if it is still dis- 

 puted the actual subject will be dealt with, and not hydraulic 

 illustrations of doubtful applicability given. 



Controversy begins as soon as localization is attempted ; 

 as soon as one says that dTIi is a measure of the E.M.F. 



* For instance, I might quote several physicists who have said (some- 

 what casually, as I think), since a discussion at the Institute of Electrical 

 Engineers (Journ. Inst. E. E. 1885), that the Peltier heat-evolution is no 

 measure of a contact E.M.F. existing at the junction where it occurs, hut 

 is a measure of the rate of variation of this E.M.F. with temperature ; 

 but it will suffice if I quote part of Prof. Ayrton's remark in that same 

 discussion, with an occasional interposition of my own in square brackets, 

 because he expresses himself clearly and strongly — always a desirable 

 thing to do : — 



11 Dr. Lodge's fundamental argument — the argument in fact upon which 

 his whole paper is based — is this : He assumes that in all cases where 

 there is a considerable difference of potential at a [metallic] junction, heat 

 or co]d must be developed when a current flows across that junction." 

 |_I should myself word it rather differently, to avoid misunderstanding, 

 though misunderstanding is hardly possible in what follows ; but apart 

 from possible misunderstanding I have no objection to the statement, 

 except that I do not regard it as an assumption, unless the conservation 

 of energy is an assumption.] " He states what is quite true and well 

 known — that practically no heat is developed when a current flows from 

 zinc to copper across the junction ; therefore he concludes that there can 

 be no contact-potential difference at this junction like three-quarters of a 

 volt. But this reasoning is based on a totally wrong conception of the 

 Peltier effect, which I am astonished to find exists in the mind of a man 

 with the scientific powers of Dr. Lodge. The amount of heat generated 

 when a unit current is sent through a junction for a second is undoubtedly 

 the measure of the coefficient of the Peltier effect ; bid this coefficient is in 

 no sense a measure of the contact-potential difference existing at the 

 junction." [Italics are Prof. Ayrton's.] " What the Peltier effect, at a 

 junction of the two substances at a given temperature, measures, is the 

 product of the absolute temperature into the rate of variation, with 

 temperature, of the contact-potential difference at that temperature ; in 

 other words, if n is the Peltier effect at a junction which is at an abso- 

 lute temperature t, and if V is the contact-potential difference at that 



dV 

 temperature, n = t — . 



dt dV 



"Hence the magnitude of n does not measure V, but merely t -r., 



and the error of Dr. Lodge is of the same sort of order as saying that 

 a train going at a very nearly uniform speed is necessarily moving 

 very slowly because the change of speed per second is very small. This 

 disposes of the objection based on the smallness of the Peltier effect." 

 [No, it is not disposed of quite so easily as that. The difference between 

 velocity and acceleration is somewhat familiar.] 



