43 MEASUREMENT OF ELECTROMOTIVE FORCES. 



with the equation of condition 



v.-v.-.v,. 



The method requires that the plates are exactly at the same 

 distance in the three experiments, and that the electrometer is 

 graduated. By means of the stops a and b this first condition is 

 satisfied. 



This method has been employed in analogous conditions by 

 different physicists. Pellat* greatly improved it by connecting the 

 two plates by a conductor which contains, instead of a fixed electro- 

 motive force, a variable one, obtained by means of a contact moving 

 along a wire traversed by a permanent current. The contact is regu- 

 lated so that the charge of the two plates is zero. The electromotive 

 force of contact is then equal and opposite in sign to that which 

 the wire comprises. A single experiment is sufficient, and both the 



Fig. 210. 



distance of the plates and the graduation of the electrometer need 

 not be regarded. 



1025. We may mention, further, an ingenious arrangement 

 used by Professors Ayrton and Perry, f although it is more complex 

 and less certain. 



Four plates, A, A', B, B' (Fig. 210), form two systems of con- 

 densers. The lower plates, A and A', are made of the two metals 

 of which we desire to know the electromotive force of contact. 

 The upper plates B and B', both of brass, are insulated, and com- 

 municate respectively with the quadrants of an electrometer, the 

 needle of which is kept at a very high potential. 



* PELLAT. Ann. de Phys. et Chimie [5], Vol. xxiv., p. 5. 1881. Journal 

 de Physique, Vol. IX., p. 145. 1880. 



t AYRTON and PERRY. Transactions of the Royal Society ', i88o t p. 15. 



