ELECTROMOTIVE FORCE OF CONTACT OF TWO LIQUIDS. 433 



The vapour from the liquid, and which condenses on the upper 

 plate, introduces a source of error which it is difficult to get rid of, 

 and which cannot be allowed for. y *f 



1027. Two LIQUIDS. The method of Ayrton and Perry may 

 be applied to the case of two liquids. The two lower plates A and A' 

 are replaced by the surfaces of the two liquids, which are connected 

 by a siphon. In order to allow the vessels to be reversed, while only 

 leaving a very small distance between the surfaces of the liquid and 

 the upper plates, these are suspended by a frame in the shape of a 

 parallelogram, so that they can be raised parallel to themselves. This 

 complication adds to the difficulty of bringing the surfaces to exactly 

 the same distance. Condensation of vapour on the upper plates is 

 also to be feared. 



MM. Bichat and Blondlot* used a method which is not open to 

 these objections. 



Let L and L' be the two liquids to be compared. The two vessels 

 X and Y, both containing the liquid L, communicate respectively by 

 a platinum wire with the quadrants of an electrometer. The liquid 

 L', contained in the vessel Z, communicates with the vessel X by a 

 siphon closed by a porous diaphragm, and filled with liquid L. The 

 liquids of the two vessels Y and Z being brought to the same 

 potential by a special device, the difference of potentials between 

 the vessels X and Y is equal to the electromotive force of contact 

 L|L'. The device in question consists in allowing the liquid L' to 

 flow out in a fine stream, and divide into droplets in the centre of a 

 large tube, on the inside of which the liquid L flows continuously. 

 The two vessels from which the liquids flow, communicate respectively 

 with the vessels Y and Z, by means of siphons filled with the same 

 liquid, and which therefore introduce no difference of potential. 



1028. Maxwell was the first to draw attention to the nature of 

 the phenomenon, which is measured in all methods in which bodies 

 are separated by a dielectric layer. In the experiment on the electro- 

 motive force of contact of two metals M and M', for instance, there 

 are in reality three contacts to be considered that of the two metals 

 with each other, and that of each of the metals with the medium A 

 in which it is immersed. We really measure then the sum 



BICHAT and BLONDLOT. Comptes rendus, Vol. xc., pp. 1202 and 1293. 

 . Journal de Physiqtie\2\ Vol. II., p. 533. 1883. 

 t MAXWELL. Electrician. April 26th, 1879. 

 VOL. II. F F 



