178 ELECTROM ETR Y. 



^805. Allowance should in strictness be made for the rods 

 which support the balls. In the case in which the charges are 

 measured, the needle and the support of the ball are insulating 

 substances. Electricity can scarcely be prevented from gradually 

 escaping along these surfaces, which introduces sources of error 

 difficult to estimate. When the balance is used to measure 

 potentials, the wire and the needle themselves act as conductors, 

 as well as the rod which supports the fixed ball. Their influence 

 can only be neglected when the distances are great and the 

 diameter of the conducting parts is very small, so that their 

 capacity is very small. 



/ 806. THOMSON'S ABSOLUTE ELECTROMETER. This instrument 

 weighs the attraction which is exerted between a plane and a plate 

 parallel to it when they are raised to different potentials (81). 



The force P, measured in absolute units, which balances elec- 

 trical attraction, is given by the formula 



in which a represents the surface of the plate, e the distance of 

 the two planes, V l and V 2 their respective potentials. This ex- 

 pression only applies, however, to the case in which the surface 

 of the plate is covered with a layer of uniform density, and is 

 therefore in the same condition as if it formed part of an infinite 

 plane. 



We have seen how Sir W. Thomson realises these conditions 

 by the use of a guard-plate. The movable plate is surrounded 

 by a ring B (Fig. 150) in the same plane and in connection with 

 it, the interval 0^04 cm. to 0*06 cm. being just sufficient to allow 

 freedom of motion without the risk of touching. The system is 

 completed by a closed box D in connection with the guard-ring. 

 The object of this box is to prevent electricity from being produced 

 on the top of the movable plate, and to keep it from any electrical 

 action from adjacent bodies. The distribution is sensibly uniform 

 on the lower surface of the plate. The density is however a little 

 greater at the edges, and a factor of correction must be introduce^ 

 in the calculation of the surface a. It is clear that the corrected 

 surface must be greater than the real surface and smaller than 

 the opening of the guard-ring. We shall have then a closer value 



