Contact Theory of the Galvanic Cell. 



407 



which shall yet exhibit difference of potential and continuous 

 current; for if this is possible, it must follow that chemical 

 affinity alone is capable of creating electromotive force as well as 

 of maintaining a current, and that, in an ordinary cell-series, 

 some part at least of the electromotive force is due to this cause, 

 whilst the remainder is the result of the metallic contacts that 

 may exist. Or (ii) if we can establish directly that the two 

 plates in one cell are not at the same potential, as stated by 

 more than one authority. 



With regard to the first point, it will be remembered that au 

 old experiment of Faraday's proved that a current can be main- 

 tained and decomposition effected by a single cell where there is 

 no dissimilar contact. It is not easy to see how this experiment 

 can be explained by any form of contact theory ; indeed it ap- 

 pears unanswerable. But in order to leave no point unsettled 

 by experiment, it seemed desirable to try and arrange a series of 

 cells in which all dissimilar contact was absent, so that the dif- 

 ference of potential due to chemical action might be separated 

 from that due to the contacts and rendered visible by the elec- 

 troscope. 



It is obvious that we can make no attempt to do this unless 

 we can in some way or other obtain a battery with terminals of 

 the same metals ; for otherwise the very junctions with the electro- 

 scope introduce what we want to eliminate, viz. dissimilar metallic 

 contact. But the following is a method by which this can be 

 accomplished. If plates of lead and copper be placed in nitric 

 acid the lead is positive to the copper, since it is most acted 

 upon ; but if lead and copper be placed in solutions of alkaline 

 persulphides, then the copper is most readily acted upon aud is 

 positive to the lead ; that is, the positions are reversed. 



Now, if we place in a cell A dilute nitric acid and a copper 



Fig. 1. 



.A. IB 



and a lead plate, we cannot join up another cell of the same sort 



