ELECTROMETERS. 163 



53. A small aluminium plate P is fitted to a square 

 hole in the metal plate 6r, like a trap-door, without 

 touching the edges. To one side of P is attached an 

 arm A, of the same material, enlarged at its junction 

 with P, and bent, so that when the surfaces of P and 

 Gr are in the same plane, the arm is elevated a little 

 above 6r, and is parallel with it. 



A platinum wire / stretched between two supports, 

 attached to (7, passes through the enlarged part of the 

 arm ^, over a slight projection ; supporting P, and, by 

 its torsion, regulating its movements. At the outer 

 end of the arm is a fork 

 F ; and between its 

 prongs is a little white 

 enameled standard, at- 

 tached to G- ; having, 

 on its outer face, two 

 black dots, close to- 



gether, and in the same Fig 53 _ The Idiostatic ^T 

 vertical line. A black 

 hair, stretched across the fork, and viewed through the 

 lens Z, moves up and down in front of the dots ; and 

 conies exactly between them, when the surfaces of P 

 and G- are in the same plane. This is called the sighted 

 position. 



Under the plate G- is seen, in Fig. 52, a circular 

 metal plate F, supported on a metal rod, attached to 

 the metal plate A, which is in contact with the inner 

 coating of the Ley den jar; so that A and Pare always 

 at the same potential V, as this coating. The distance 

 between F and 6r is so regulated, that when the poten- 

 tial of F is V, its attraction for the plate P overcomes 

 the torsion of the platinum wire, and keeps P in the 



