11] METHODS OF MEASUREMENT 89 
At a given instant the connection of the quadrants of the electro- 
meter with the earth is broken. The weight is attached to the 
quartz plate, and is held in the hand so as to gradually apply the 
tension. This causes a release of electricity opposite in sign to 
that given to the plate D. The electrometer needle is kept at the 
position of rest as nearly as possible by adjusting the tension by 
hand. The tension being fully applied, the moment the needle 
commences to move steadily from zero is noted. The current 
between the plates CD is then given by 2 where ¢ is the time of 
the observation. The value of Q is known from the weight attached. 
In this method the electrometer is only used as a detector to 
show that the system is kept at zero potential. No knowledge of 
the capacity of the insulated system is required. With practice, 
measurements of the current can be made in this way with rapidity 
and certainty. 
