Electromotive Forces in a Voltaic Cell. 341 



passed is addition of copper to one plate, loss of copper 

 from the other plate ; what could be inferred ? Imagine a 

 region enclosing the anode, when a current has passed, what 

 changes have occurred within the region ? An equivalent of 

 copper has disappeared from the anode, and that same quan- 

 tity of copper has departed and gone outside the region. 

 But by our supposition, nothing else has happened barring 

 increase of volume for liquid by diminished volume of metallic 

 copper ; there is no more and no less CuS0 4 in the region, 

 the same quantity therefore of S0 4 . All the work done in 

 the region is to tear off a little copper from the surface of the 

 anode and to remove it elsewhere. If the fact were as as- 

 sumed it would follow that the passage of the current did 

 little work in the passage from copper to sulphate of copper, 

 and consequently that the difference of potential between the 

 two is small. But the fact is, other things happen in the cell 

 than increase of the kathode and diminution of the anode. 

 In contact with the anode there is an increase of CuS0 4 , in 

 contact with the kathode CuS0 4 disappears : this is a familiar 

 observation to every one. Reconsider the region round the 

 anode. Assume as another extreme hypothesis that after 

 a current has passed we have in this region the same 

 quantity as before of copper, but more CuS0 4 ; S0 4 has 

 entered the region and has combined with the copper. A 

 large amount of energy is therefore brought into the region, 

 which can only be accounted for by supposing that the elec- 

 tricity has passed from a lower potential in the copper to a 

 higher potential in the electrolyte. The legitimate conclusion 

 is, then, that there is between Cu and CuS0 4 a difference of 

 potential corresponding to the energy of combination ; and the 

 basis of the conclusion is the simple observation that the 

 copper is dissolved off one plate but remains in its neighbour- 

 hood, whilst it is precipitated on the other plate, impoverishing 

 the solution. In other words, it is the S0 4 that travels, not 

 the Cu. 



Now consider the ordinary Daniell's cell. Is there a sub- 

 stantial difference of potential at the junction of CuS0 4 and 

 ZnS0 4 ? Is there, in fact, a difference apart from the 

 Peltier difference ? Imagine a region enclosing the junction 

 in question ; it might have been that the effect of a current 

 passing was to increase the zinc and diminish the copper by an 

 equivalent of the electricity which passed, from which we should 

 have inferred that the seat of the electromotive force in a 

 Daniell's cell was at the junction of the two solutions. But 

 it is more nearly the fact that no change whatever occurs in 

 the region in question when a current passes, and that all that 



