ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 523 



but perhaps it is no more true. The greater part of a contact force of this 

 kind i3 probably due to a physical difference between the metals, such as 

 difference in atomic velocity, and has no close relation to their cbemical 

 affinities for each other. It is, bowever, just possible that part of a 

 metallic junction-force is due to chemical tendency between the two metals 

 in contact. For instance, take the case of zinc and copper. There is, 

 I suppose, an undoubted affinity between them, as shown by the formation 

 of brass under proper conditions. [If chemists assume the right to demur 

 to this on the ground that the two metals mix equally well in any pro- 

 portions, one can choose any other pair of metals — say, perhaps, copper 

 and tin — for which the statement does not hold.] Now does this affinity 

 result in any E.M.F. between them on making contact ? This question, 

 I apprehend, is to be answered by passing a current for a long time across 

 a coppei'-zinc junction and seeing if any brass does, 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 

 E.M.F. 



Observe, I do not for a moment question the existence of a few 

 hundred microvolts of E.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-foree- 

 cletermination of the size of atoms, it may be convenient here to quote the 

 remainder of my preliminary notes, which aim at summarising, in a com- 

 pact form, the main argument with respect to the immediate subject of 

 discussion, viz., the seat of electromotive force in a voltaic cell, and in 

 ordinary Volta condenser experiments. 



IV. — Brief summary op the argument. 



xxv. Wherever a current gains or loses energy there must be a seat of 

 E.M.F. ; and conversely, wherever there is a seat of E.M.F. a current must 

 lose or gain energy in passing it. 1 



xxvi. A current gains no appreciable enei'gy in crossing from copper to 

 zinc, hence there is no appreciable E.M.F. there. 



xxvii. When a current flows from zinc to acid the energy of the com- 

 bination which occurs is by no means accounted for by the heat there 

 generated, and the balance is gained by the current; hence at a zinc acid 

 junction there must be a considerable E.M.F. (say at a maximum 23 volts). 



xxviii. A piece of zinc immersed in acid is therefore at a lower potential 

 than the acid, though how much lower it is impossible precisely to say, 

 because no actual chemical action occurs. [If chemical action does occur 

 it is due to impurities, or at any rate to local currents, and is of the nature 

 of a disturbance.] 



1 Nate added January 1885. — ~Slj attention has just been called to an article by 

 Mr. 0. Heaviside, in the Electrician of February 2, 188i, in which he states views 

 very like those contained in these statements. Had I known of this paper earlier I 

 should of course have mentioned it, but 1 did not know of it. 



