THOMSON'S ABSOLUTE ELECTROMETER. 



179 



by taking the mean 



of the two surfaces. As they are very 



little different from each other, this correction may be considered 

 sufficient. 



The plate and the guard-ring in connection with each other are 

 only at the same potential provided the metals of which they are 

 made are the same (9). In most instruments the plate is of 



Fig. 150. 



aluminum and the ring of brass ; hence there is a difference of 

 potential between them. This gives rise to an action which would 

 be appreciable in very delicate instruments. 



j[ 807. The exact measurement of the distance e of the movable 

 plate from the plate A would present great difficulties. Sir W. 

 Thomson avoids them by modifying the method so as only to 

 have to measure the difference of the two distances. One of the 



N 2 



