462 Prof. 0. Lodge on the Controversy 



impurity in the metal, enabling galvanic action to occur, is 

 essential to this process, but that is here not a matter of 

 primary importance. 



Consider now a Daniell cell of the simplest theoretical 

 kind : 



Cu | CuS0 4 | ZnS0 4 | Zn. 



As soon as the circuit is completed, zinc goes into solution 

 at one end and copper comes out. The difference between 

 the energy of combination of Zn and Cu with S0 4 gives the 

 whole E.M.F.; since, as is well known, the d'E/dt and therefore 

 the 2(11) happen in this case to be practically zero. 



But what about the location of the E.M.F.? Leaving on 

 one side the extraordinary contention in favour of the zinc- 

 copper junction, there are three other junctions at which the 

 chief portion of the E.M.F. might be located ; and at first 

 sight the liquid/liquid junction at the porous partition seems the 

 most likely. For here it is that the actual exchange of Zn for 

 Cu takes place ; and hence it may be said, either heat must 

 here be locally developed or else the energy must be used 

 in propelling the electric current and developing the same 

 quantity of heat elsewhere. 



But, as Hopkinson virtually pointed out (p. 341, Phil. Mag. 

 Oct. 1885), this involves the tacit assumption that the metallic 

 ions do all the travelling, while the S0 4 is stationary ; — in 

 that case the metallic zinc which goes into solution would at 

 first be merely passed on, and the real energy-production 

 would occur where zinc atoms took the place of copper atoms, 

 viz., at the junction of solutions in the porous partition. 

 After that there is a mere passing on again, though now of 

 copper. 



But the migration ratio of the ions is in reality different 

 from this, and the S0 4 is by no means really stationary. As 

 another extreme assumption, suppose that the S0 4 ion did all 

 the travelling, then the zinc sulphate solution is strengthened 

 and the copper sulphate solution is impoverished, and the 

 whole positive energy of combination is at the surface of the 

 zinc, while at the surface of the copper there must be a smaller 

 opposition force corresponding to the tearing of the S0 4 away 

 from the Cu there. 



Now this concentration in one compartment and weakening 

 in the other compartment corresponds much more nearly than 

 the first hypothesis does with what is observed, the truth being 

 of course something intermediate between the two extremes. 

 Anions and cations both travel, and so the total E.M.F. is 

 shared in certain proportions between all the junctions : 



