MEASUREMENT OF THE ELECTROMOTIVE FORCE OF CONTACT. 429 



mining the electromotive force of contact of two metals consists 

 in using them as armatures of a condenser which is joined by a 

 wire, and the charge of which is measured after they have been 

 insulated from each other and put at a great distance apart (187). 

 The charge is proportional to the product of the capacity by the 

 difference of potential, or merely by the latter if the capacity is 

 constant. 



Kohlrausch* measures the charge by means of a Dellmann's 

 electrometer ; and, in order to reduce the measurements to a given 

 standard, he makes three experiments by joining the two plates 

 A and B (Fig. 209) ; first by a wire only ; then by a wire in which 

 is interposed a DanielPs element; and, thirdly, by the same wire 



Fig. 209. 



and the same element, but turned in opposite directions. He thus 

 obtains three deflections corresponding to the differences of potential 

 V,, V 2 , and V 3 . If SV is the electromotive force of contact of the 

 two metals, D that of the DanielPs element, we have 



from which is deduced 



SV= 2 D 



* R. KOHLRAUSCH. Pogg. Ann.,Vo\. Lxxxn.,pp. 1,40, 1851 ; Vol. LXXXVIII., 

 p. 465. 1853. 



