ELECTROMETERS. 243 



The disc c, when " in position," is placed so that its lower 

 face is as nearly as possible in the same plane with the lower 

 surface of the "guard-ring" B. The interstice between B 

 and c is extremely narrow in comparison with the diameter 

 of c and with the distance between the plates. This is 

 the electrical part of the absolute electrometer. 



The lever L is pivotted on a torsion wire, which is stretched 

 between two insulated metallic pillars PP. Q is a counter- 

 poise. At the end of the lever L there is an index hair ; 

 and a lens I is placed so as to view the position of the hair 

 relatively to two black dots on an index plate. When the 

 hair is between the two black dots the plate c is in position, 



with its lower surface in the plane with the lower surface 

 of the guard-ring B. The guard-ring B is kept metallically 

 connected with the metal pillars PP, and thus with the lever L 

 and with the plate c, which, however, is not in direct contact 

 with B, but is perfectly free to move. The lever L is hung 

 so that when A and c are at the same potential the counter- 

 poise preponderates, and the hair at the extremity of L 

 rises above the sighted position ; and before each series of 

 experiments with this electrometer the force required to 

 bring the hair to the sighted position is determined by 

 placing a small weight on c and a rider on the arm L. 

 When this has been done the weights are removed. But 

 when A and c are at different potentials the attraction 



B 2 



