ELECTROMOTIVE FORCE OF CONTACT OF METAL AND LIQUID. 431 



A and A' being in permanent connection, B and B' are con- 

 nected. The needle of the electrometer comes to zero. The 

 connection between B and B' being broken, the system of the 

 two plates A and A' is turned through 180, so that the plate A' 

 comes under the plate B and A under B'. The deflection of the 

 needle is proportional to the difference of potential between A 

 and A'. 



For let c and c f be the capacities of the plates B and B', in- 

 cluding the corresponding quadrants, q and q' the quantities of 

 electricity taken by each of them in the first experiment, V the 

 potential of A, V + SV that of A', and V the common potential 

 of B and of B'; we have 



q = c(V' - V), q' = c'(V - V - 8V). 



After being turned through 180, the four plates A, A', B, and B' 

 have potentials represented respectively by U, U + 5V, U', and U", 

 which, since the charges and capacities of the two systems B and B' 

 have not altered, gives 



From this follows 



V'-V V'-V-SV 8V" 



U' - U - 8V U" - U U' - U" - 8V ' 

 or 



8V = -(U'-tT); 



2 



the difference U' - U" is given by the electrometer. 



This method presupposes that the capacity of the electrometer 

 is independent of the deflection (813). Moreover, the necessity 

 of bringing the plates exactly to the same distance requires a per- 

 fection in the mechanism which is realised with difficulty. 



1026. METAL AND LIQUID. In order to obtain the electro- 

 motive force of contact between a solid and a liquid, Hankel* 



* HANKEL. Abhhandlung. der Konig. Sachs. Gesell.; Math.-Phys. Klasse. 

 1861 and 1865. Pogg. Ann., Vol. cxv., p. 57, 1862; Vol. cxxvi., pp. 286, 440, 

 1865; Vol. cxxxi., p. 607, 1867. 



