1878.] 



Contact Theory of Voltaic Action. 



197 



treatises on electricity is felt to be incomplete, even by the writers of 

 snch treatises, and the present investigation has been entered npon in 

 conseqnence. 



Sir W. Thomson's result, and our own experiments lead ns to 

 imagine that when zinc and copper are immersed in water there are three 

 successive states to be noticed : — At the instant of immersion the zinc 

 and copper may be reduced to the same potential, so that the electro- 

 motive force of the voltaic cell E is equal to the difference of potential 

 ZC between zinc and copper in contact ; the zinc now becomes nega- 

 tive to the copper, so that E reaches a limit which is greater than ZC ; 

 lastly, if a current passes, polarization occurs and the zinc becomes 

 gradually less negative to the copper, E diminishing, therefore, from 

 its maximum value. But when a saturated solution of zinc sulphate 

 is employed instead of water, the first state, if it exists at all, exists 

 for so short a time that practically, zinc and copper in zinc sulphate 

 are never at the same potential. Thus (see Table X) when care is 

 taken to keep the zinc and copper in a water cell well insulated from 

 one another, E is found to increase from a value very little greater 

 than ZC, the electromotive force of contact of zinc and copper, to a 

 limit, but in a zinc sulphate cell no such great increase is observed. 



In our present experiments we are not concerned with instantaneous 

 electromotive forces of contact, a short time elapsing in every case 

 between the contact and the determination of its electromotive force ; 

 and with polarization in the ordinary sense we have equally no con- 

 cern. In fact, our experiments relate to the maximum electromotive 

 forces of simple and compound cells. 



We find that zinc and copper connected by the electrolyte zinc 

 sulphate, dilute sulphuric acid, or water are not at the same potential, 

 the zinc being negative to the copper. In the case of dilute sulphuric- 

 acid, where polarization cannot be prevented, the negative charge of 

 the zinc rapidly diminishes so that there is an instant at which the 

 metals are reduced to the same potential ; from Sir TV. Thomson's 

 result we may suppose that there are two such instants. 



We did not measure the potentials of the surfaces which we mention 

 but really of air in the neighbourhood of these surfaces, but the dis- 

 crepancies of our results are sufficiently small for us to neglect what- 

 ever difference of potential may exist between a metallic or liquid 

 surface, and the air in contact with it. In fact we have good reason 

 to believe that there is no great difference of potential between a 

 metallic or liquid surface, and the air in contact with it. 



Our experiments show, but not with as much accuracy as might be 

 desired, that the electromotive force of contact of two metals or two 

 electrolytes, or of a metal and an electrolyte is in each case a constant ; 

 that is to say, if AB means the electromotive force of contact of the 



