112 Relation of Volta Electromotive Force to Pressure Sfc. 



Effect of Pressure upon Voltaic Couples. 



In all the foregoing experiments the investigation was 

 limited to the influence of pressure upon the electromotive 

 force generated bj a single kind of metal and a single kind of 

 liquid, and did not include its influence upon that of ordinary 

 voltaic couples composed of pairs of metals or pairs of liquids ; 

 it is evident, however, that the latter are only compound cases 

 of the former. Gibault has already experimentally examined 

 the effect of a pressure of 100 atmospheres on several kinds of 

 voltaic cells, and obtained the following amounts of electro- 

 motive force in volts : — Daniell's cell (20 per cent. ZnS0 4 ) 

 + •0005 ; (27-56 per cent. ZnSO 4 ) + '0002 ; Warren De la 

 Rue cells (l'O per cent. ZnCl 2 ) + *0007 ; (40'0 per cent. 

 ZnCl 2 )-*0005 ; Volta's cells -'06; Bunsen's -*04 ; Gas 

 battery + 0'8 (Comptes Rendu s, 1891, vol. cxiii. p. 465 ; The 

 Electrician, 1891, vol. xxvii. p. 711). Owing to the greater 

 complexity of the conditions in these experiments, the pro- 

 portion of cases giving reverse effects was 30 per cent., or six 

 times larger than in those with single metals in the present 

 research. 



In all such experiments with voltaic cells, we have to con- 

 sider not only the effect of pressure at the positive metal, but 

 also that at the negative one. The results obtained in the 

 present research show that the direction of the current which 

 occurs with zinc and other positive metals is the same as that 

 with gold and platinum ; so that the effect of pressure upon 

 the negative metal of a voltaic couple would probably be in 

 nearly all cases to produce a greater or less amount of counter 

 electromotive force, which would either diminish or reverse 

 the effect due to the positive one. The amount or balance of 

 effect, therefore, obtained with a voltaic couple would usually 

 be very much less than that with a single kind of metal ; this 

 conclusion is confirmed by the results of Gibault's experiments, 

 in which the amounts of electromotive force obtained by a 

 pressure of 100 atmospheres were very much less than those 

 usually obtained in my experiments by a difference of pressure 

 of only about 2 or 3 atmospheres. As the pressure alone 

 attending the height of the liquid of a voltaic cell affects the 

 electromotive force, it necessarily follows that the energy of 

 such a cell is affected by gravity and varies with the altitude 

 and geographical position of the cell. 



