Electromotive Forces in the Voltaic Cell. 357 
after a long time, result. Thermopiles show a curious secular 
deterioration with use, and it may be that some alloying action 
goes on, though I have never heard of its being noticed. 
But if no such alloying goes on during the passage of a 
current, then I should say that, in whatever ways chemical 
affinity between two metals is able to show itself, it does not 
show itself as an H.M.F. 
Observe, I do not for a moment question the existence of a 
few hundred microvolts of H.M.F. at a zinc-copper junction. 
I only ask, is this chemical, or is it physical, or is it a mixture 
of the two? Statement No. xxiv. is general enough to take 
into account the possibility of its being a mixture of the two 
at every kind of junction. It is easy to write one of them 
zero, if so it turns out. 
24. We have been led into a pretty wide discussion of contact- 
force in general; and, before digressing again on the question 
of a contact-force-determination of the size of atoms, it may 
be convenient here to quote the remainder of my preliminary 
notes, which aim at summarizing, in a compact form, the 
main argument with respect to the immediate subject of dis- 
cussion, viz. the seat of electromotive force in a voltaic cell, 
and in ordinary Volta-condenser experiments. 
IV. Brief Summary of the Argument. 
xxv. Wherever a current gains or loses energy, there must 
be a seat of H.M.F’. ; and conversely, wherever there is a seat 
of E.M.F. a current must lose or gain energy in passing it*. 
xxvi. A current gains no appreciable energy in crossing 
from copper to zinc, hence there is no appreciable E.M.F. 
there. 
* Note added January 1885,—My attention has just been called to an 
article by Mr. Oliver Heaviside in the ‘ Electrician’ of 2nd February, 
1884, in which he states views very like those contained in these state- 
ments. Had I known of this paper earlier I should of course have 
mentioned it, but I did not know of it. 
Mr. Sprague also, in his book on ‘ Electricity, its Theory, Sources, and 
Applications,’ on page 331 expresses his belief in much the same sort of 
way. Although Mr. Sprague is rather too much occupied in tilting 
against what he considers the absurdities of orthodox and ‘mathematical ” 
views to work out his own ideas in a very thorough and clear form, there 
can be no doubt that he has in his own way arrived at very many of the 
same conclusions as Clerk-Maxwell, though of course in a far less pro- 
found and satisfactory manner. He has scarcely received the credit due 
to him for this, and a cutting review of his bock in ‘Nature’ some years 
back, though justified im parts by Mr Sprague’s supercilious tore, yet 
falls. into more important errors regarding electrical facts than Mr. Sprague 
himself falls into. See ‘Nature,’ June 24, 1875, vol. xii. p 144, 
