Contact Theory of the Galvanic Cell. 409 



circuit for 2\ hours its electromotive force had fallen off 50 per 

 cent. ; it then required about 8 cells to bring the needle to zero. 

 This gives for the electromotive force of two cells at first about 

 *5 of a volt; or the whole sixty cells are equal nearly to 15 Da- 

 rnell's cells. It readily effects the decomposition of many elec- 

 trolytes, and exhibits therefore every property of an ordinary cell- 

 series. Above all, it will be noticed that since there is a regular 

 rise in potential in passing from cell to cell, and as all parts of 

 each plate must be at the same potential, that rise can only take 

 place at the surfaces where the active metals are in contact with 

 the electrolyte (that is, at the seat of the chemical action), and that 

 therefore two metals in one electrolyte cannot be at exactly the 

 same potential. But I find that more direct evidence still of this 

 fact is to be found in an experiment of Faraday's, which seems 

 to have escaped the notice of the contact theorists. 



In his 'Experimental Researches' he gives the following fact. 

 "I took a voltaic apparatus, consisting of a single pair of large 

 plates, namely a cylinder of amalgamated zinc and a double cy- 

 linder of copper. These were put into a jar containing dilute 

 sulphuric acid, and could at pleasure be placed in metallic com- 

 munication by a copper wire connecting the two plates. Being 

 thus arranged, there was no chemical action whilst the plates 

 were not connected; on making the contact a spark was ob- 

 tained. In this case it is evident that the first spark must have 

 occurred before metallic contact was made, for it passed through 

 an interval of air ; and also that it must have tended to pass 

 before the electrolytic action began, for the latter could not take 

 place until the current passed, and the current could not pass 

 before the spark appeared." " Hence," he says, " I think there 

 is sufficient proof that the zinc and water were in a state of 

 powerful tension previous to the actual contact"*. It is difficult 

 to reconcile this with the experiment of the half disks and drop 

 of water made by Sir W. Thomson. But, at any rate, a consi- 

 deration of the whole of the facts would seem to point out that 

 the only safe conclusion is, that in any series of cells of any sort 

 the electromotive force is a complex effect, being due to the alge- 

 braical sum of all the differences of potential due to dissimilar 

 contacts plus the algebraical sum of the differences of potential 

 due to the chemical affinities of the metals and electrolytes minus 

 any opposing force due to polarization &c. ; and that so far 

 from being the exclusive cause, the contacts can only be said 

 strictly to have a share in producing the difference of potentials 

 between the extremities of a battery f. And, lastly, we may with 



* Experimental Researches in Electricity, Series viii. ^[ 956. 

 t Amounting in a Daniell's cell perhaps to 60 or 70 per cent, of the 

 whole electromotive force. 



