88. METHODS OF MEASUREMENT [CH. 
the crystal is horizontal and at right angles to the plane of the 
paper. 
The two faces A and B are normal to one of the binary axes 
(or electrical axes) of the crystal. The tension must be applied m 
a direction normal to the optic and electric axes. The two faces 
A and Bare silvered, but the main portion of the plate is electrically 
To Support 
Earth 

To Weight 
Fig. 19. 
insulated by removing a narrow strip of the silvering near the upper 
and lower ends of the plate. One side of the plate is connected to 
the electrometer and to the conductor, the rate of leak of which is 
to be measured. The quantity of electricity set free on one face of 
the plate is accurately given by 
Q = 0:063 : vi 
where L is the length of the insulated portion of the plate, b the 
thickness AB, and F the weight attached in kilogrammes. Q is 
then given in electrostatic units. 
Suppose, for example, that it is required to measure the current 
between the plates CD (Fig. 19), due to some radio-active material 
on the plate C, for a given difference of potential between C and D. 
