THOMSON'S ABSOLUTE ELECTROMETER. 181 



the plane of the ring for the same position of the screw. A scale 

 on the side indicates the number of turns, and a divided head 

 fractions of turns. We may thus ascertain if the displacement due 

 to a given weight varies with the temperature, or even with the 

 time. 



X 809. In the absolute electrometer the system of the plate of the 

 ring and of the box is kept at constant potential. It is permanently 

 connected with the inner coating of a Leyden jar, the dielectric 

 of which is the glass shade of the apparatus, on the two faces 

 of which sheets of tinfoil are fixed. Pieces of pumice, soaked 

 with strong sulphuric acid, are placed in lead dishes, and keep 

 the interior of the bell dry. Two accessory apparatus, the re- 

 plenisher R and the gauge J, enable us to bring the potential 

 each time a constant value. 



Fig- IS'- 



The replenisher (Fig. 151) is one of the induction apparatus, the 

 general theory of which we have already discussed (195). The 

 transmitter T, when turned in one direction, increases the charge 

 of the jar ; and diminishes it when turned in the opposite direction. 



The gauge enables us to tell when the potential has acquired 

 a constant value. This apparatus, which depends on the same 

 principle as the electrometer itself, consists of a movable plate p 

 (Fig. 152), with its guard-ring G and a plate which attracts 

 it, the whole contained in a box J (Fig. 150). The plate and the 

 ring are connected with the external armature of the electrometer. 

 The plate is at a fixed distance from the disc, and is in connection 

 with the internal armature. The potential acquires the same value 

 whenever the plate is brought into the plane of the ring. The 



