concerning Volta's Contact Force. 363 



Contact Electricity") Lord Kelvin has proved exactly the 

 same thing to hold for a dielectrically closed circuit of uniform 

 temperature likewise. The argument in this case he considers 

 quite rigorous, because there is now no questionable gradient 

 of temperature along heat-conductors, and therefore no neces- 

 sarily concomitant irreversible effect; and therefore no question 

 of whether such effect be accidental or essential. 



The only loophole I can perceive for a flaw in the reason- 

 ing is the possibility that the heat of chemical combination 

 varies for small temperature changes, i. e. that it is a per- 

 ceptible function of the temperature ; and I have no reason 

 for pressing this, or for hesitating over accepting the above 

 conclusion to the utmost, so long as the temperature is 

 uniform. 



It is unimportant for the present argument, but it may be worth while 

 to notice in passing, that no such relation as 2(n) = 2(TWEAZ£) has ever 

 been proved for a complex circuit in which differences of temperature are 

 superposed upon other sources of electromotive force. And from what 

 has been s.ud in similar small type just above, no such relation can truly 

 be asserted, without considerable care as to the form of the statement. 

 Its most obvious signification, having reference to 1he actual junctions, 

 would not be correct. It is to be observed that in the Kelvin and Helm- 

 holtz expression for a complex circuit at uniform temperature, the 

 meaning of f/E dt is a change of E.M.F. of the circuit per degree caused 

 by the passage of a current — caused, that is to say, by a thermal polari- 

 zation : the dt/T being the efficiency' of a cyclical Carnot engine. Whereas 

 in such an expression in ordinary thermoelectricity as c?E/^=P = n/T, 

 the meaning of dE is the total E.M.F. observed in a circuit of two metals 

 when its pair of junctions are artificially maintained at a difference of 

 temperature dt. 



It may be said that these two meanings are after all the same : and 

 so they are in a simple circuit of two metals, but not in a complex 

 circuit with juuetions at more than two temperatures. 



But what has all this to do with the localisation of E.M.F. 

 at a given junction ? Admitting, and indeed teaching, all 

 this, I still hold that the reversible heat at a specified junc- 

 tion is a measure of the metallic E.M.F. located there. That 

 in fact at a metallic junction g = II, or J II if anyone thinks 

 it worth while in a matter of this sort to trouble about prac- 

 tical laboratory units. 



Those to whom I referred in the opening paragraph of this 

 communication make the mistake of confusing e with E, — con- 

 fusing the E.M. F. at any one junction with the whole E.M.F. 

 in a circuit. If they do this, they naturally beg the question, 

 and of course locate the whole E.M.F. at the particular 

 junction which takes their fancy at the moment : usually an 

 interface of zinc and copper. 



The truth is that E = 2(e) always, but only in a metallic 

 circuit at constant temperature does S(<?) = 2(II) ; just as 



2 2 



