1877.] 



Contact of different Substances. 



309 



the cyanide to 5 parts by weight of distilled water), exhibits at its ter- 

 minals a difference of potential equal to 1*102 D, or 1*189 volt. 



In order to test the constancy of this last-mentioned cell, it was con- 

 structed in the form usually adopted for Grove's battery : the inner 

 porous vessel contained 60 cub. centims. of the potassium-cyanide solu- 

 tion, and in it was inserted a plate of copper ; the outer vessel contained 

 a solution of copper sulphate maintained in a state of saturation by the 

 presence of crystals of the salt, and the copper plate in this vessel was 

 bent round the flat porous vessel so as nearly to touch it on both sides. 

 The terminals were now connected by short wires with a tangent galva- 

 nometer, and the current allowed to pass uninterruptedly, with the fol- 

 lowing results : — 



Deflection of the 

 needle. 



o 



At the time the circuit was closed 34 



After 6 minutes 58 



„ 15 „ 73 



„ 22 „ 75 



„ 30 „ 75 



„ 45 „ 68*5 



„ 62 „ 46 



„ 75 „ 28 



„ 90 „ . -. 25 



,,105 „ 23*5 



,,120 „ 21*5 



It appears, therefore, that though, for a voltaic element containing 

 only one metal, this cell has a considerable electromotive force, it is far 

 from constant in its action. 



In endeavouring to compare the differences of potential arising in pairs 

 of metals in contact, I have employed the method of Kohlrausch (Wiede- 

 mann's ' G-alvanismus,' 1861, vol. i. p. 28), in which, by the aid of a con- 

 denser having its plates of two metals, the difference of potential of these 

 metals, due to their contact, is compared with the difference of potential 

 exhibited by the terminals of a constant voltaic element. 



For the purpose of comparison I used a Clark's standard cell (Phil. 

 Trans, vol. clxiv. p. 1), relying upon the reported constancy of the differ- 

 ence of potential of its terminals ; and although I find that this difference 

 is very far from constant, yet the rate of change is slow, and for experi- 

 ments performed at intervals of only a few days it may be considered 

 constant within the limits of the experimental errors to which my present 

 apparatus seems liable. 



I will denote by C the difference of potential of the terminals of this 

 cell at the time the following measures were made : — 



