m1] METHODS OF MEASUREMENT 85 
Since the electrometer can readily measure a current corre- 
sponding to a movement of half a scale division per second, 
it is easily seen that an electrometer can measure a current of 
3 x 10-“ amperes, which is considerably below the range of the 
most sensitive galvanometer. 
The capacity of the electrometer itself must not be considered 
as only that of the pair of quadrants and the needle when in a 
position of rest. The actual capacity is very much larger than this, 
on account of the motion of the charged needle. Suppose, for 
example, the needle is charged to a high negative potential, and 
kept at the zero position by an external constraint. If a quantity Q 
of positive electricity is given to the electrometer and its connections, 
the whole system is raised to a potential V, such that Q=CV, 
where C is the capacity of the system. When however the néedle 
is allowed to move, it is attracted into the charged pair of quad- 
rants. This corresponds to the introduction of a negatively charged 
body between the quadrants, and in consequence the potential of 
the system is lowered to V’. The actual capacity C’ of the system 
when the needle moves is thus greater than C, and is given by 
CiVe— OVE 
The capacity of the electrometer is thus not a constant, but 
depends on the potential of the needle, z.e. on the sensibility of the 
electrometer. 
An interesting result of practical importance follows from the 
variation of the capacity of the electrometer with the potential of 
the needle. If the external capacity attached to the electrometer 
is small compared with that of the electrometer itself, the rate of 
movement of the needle for a constant current is, In some cases, 
independent of the sensibility. An electrometer may be used for 
several days or even weeks to give nearly equal deflections for 
a constant current, without recharging the needle, although its 
potential has been steadily falling during the interval. In such 
a case the decrease in sensibility is nearly proportional to the 
decrease in capacity of the electrometer, so that the deflection for 
a given current is not much altered. The theory of this action has 
been given by J. J. Thomson. 
1 Phil. Mag. 46, p. 537, 1898. 
