48 Mr. C. V. Burton on 



Professor Lodge considers that the E.M.F. between chemi- 

 cally active substances is due to a tendency to chemical action. 

 But mere tendency to chemical action at a finite junction 

 could not be the source of an indefinite amount of energy, 

 and could not, therefore, account for the phenomena in the 

 case discussed above. There must be actual chemical action. 

 It appears in fact (excluding all idea of a Peltier E.M.F.) 

 that when two chemically active conductors are brought into 

 contact, electricity in general crosses the junction, and 

 establishes a difference of potential between them, and the 

 amount of chemical action which takes place is precisely the 

 equivalent of the electricity which crosses the junction determined 

 in accordance with Faraday's Electrolytic Law. 



Let K = number of absolute units of energy evolved when 

 one gram of (say) the substance A enters into the kind 

 of combination which takes place in the case considered. 



Let k = number of grams of A which enters into combina- 

 tion per unit flow of electricity. 



Then for every unit of electricity which crosses the junc- 

 tion, E units of chemical energy are absorbed ; .\ E/K grams 

 of A are dissolved. 



.-. E/K=& or E = kK. 



That is, the E.M.F. is equal to the energy of combination of one 

 electrochemical equivalent. 



Of course this is only the case where all the energy of the 

 chemical action is converted into electrical energy ; as in the 

 case of pure zinc dipped in dilute sulphuric acid. From this 

 it follows that in the case of the typical cell, for instance, 

 where pure zinc and pure copper are immersed in dilute 

 sulphuric acid, the principal E.M.F is at the contact of the 

 zinc and acid, an opposing E.M.F. being set up at the con- 

 tact of acid and copper. 



If we construct a cell consisting of zinc and carbon dipping 

 in dilute H 2 S0 4 , there will be no power of chemical action at 

 the liquid-carbon junction, so that the only E.M.F. there is 

 a Peltier E.M.F. Neglecting Peltier E.M.F.'s altogether, 

 the E.M.F. of such a cell would be equal to that of the zinc- 

 liquid junction. 



The apparent contact E.M.F.'s of metals, as measured in 

 air, must be due chiefly to the air-metal contacts, since a metal- 

 metal contact can only be the seat of a Peltier E.M.F. It 

 also appears that if the apparent E.M.F. between two 

 metals were measured inductively in some chemically inactive 

 gas (or liquid), the result would be the sum of three Peltier 

 E.M.F.'s, and would probably be small. There even seems 

 to be some reason for supposing it to be zero. 



