l8o ELECTROMETRY. 



potentials that of the ring, for instance having a fixed value Vj, 

 the plate is successively raised to the potentials V and V, and its 

 distance is increased until the attraction on the plate is the same 

 in both cases. If e and e' are the corresponding distances, we 

 shall have 



a 



8^F 



and therefore 



In this way we measure, not the difference of potentials between 

 two opposite faces, but that of the two potentials to which the 

 plate A has been successively raised. It is to be observed that 

 with this method the electrical densities on the two opposite sur- 

 faces, which are necessarily of opposite signs, have always the 

 same absolute value. 



)( 808. In the apparatus of Sir W. Thomson the plate is sup- 

 ported by a flexible system consisting of three springs placed 

 symmetrically, and shaped like carriage springs. These springs 

 are contained in the box D, and are themselves supported by an 

 insulating piece, which may be raised or lowered by means of a 

 micrometric screw C. 



For each observation the plate must be brought back to a fixed 

 position, as exactly as possible in the centre of the ring. For this 

 purpose the piece attached to the plate and the springs is provided 

 with a cross wire stretched horizontally. A lens / gives a real image 

 of. this wire, which is formed between the two very fine points V, 

 which are so regulated that they include the image of the wire when 

 the plate is in the plane of the ring. The points and the image are 

 observed with the lens /'. The position of the plate is thus fixed 

 without error of parallax. 



In order to adjust the instrument, the plate is brought back 

 to the mark by means of the micrometer screw. It is then charged 

 with marked weights, distributed symmetrically (o'6 grains in 

 Sir W. Thomson's apparatus), and the screw is turned until the 

 plate is again brought to its mark. These weights obviously re- 

 present the electrical attraction which would keep the plate in 



