436 MEASUREMENT OF ELECTROMOTIVE FORCES. 



we shall have, in like manner, the value of Hg | L' by the electro- 

 motive force E', which produces the maximum tension. 



Let us now put the two liquids L and L' in two vessels, above a 

 layer of mercury, and let them be connected by a siphon filled with 

 either of the two liquids, and provided with a diaphragm. By 

 measuring the electromotive force E x of the couple thus formed, and 

 of which the two layers of mercury are the electrodes, we have 



from which is deduced the value of L|L' as a function of the three 

 electromotive forces determined directly. 



This method gives results which differ completely, not only in 

 magnitude, but also in sign, from those which the ordinary methods 

 furnish. Such a divergence can only be explained by the fact that 

 there is an electric difference between a liquid and air, as had been 

 pointed out by Maxwell. This point of view is confirmed by results 

 deduced from the consideration of the Peltier effects. 



1031. The explanation of thermoelectric currents on the principle 

 of Volta alone (295) leads to this conclusion that all couples should 

 have a uniform course ; and in this case (286) the electromotive 

 force of contact of the two metals should be proportional to the 

 absolute, temperature. M. Potier* arrives directly at this result. 



Suppose two plates of zinc and copper, for instance, forming a 

 condenser, are joined by a conducting wire, and kept at a constant 

 temperature. The capacity of the system being C, and the difference 

 of potential of contact H, the plates are brought nearer each other 

 by an indefinitely small quantity, and the capacity then increases by 

 dC ; if the value of H is constant, a quantity of electricity passes 

 from one to the other 



and the thermal energy absorbed at the junction is 



On the other hand, the electrical work of the system, which is at 

 constant potential (97), is equal to the change of energy that is 

 to say, 



POTIER. Journal de Physique [2], Vol. iv., p. 220. 1885. 



