PYROELECTRICITY AND PIEZOELECTRICITY 155 



Let E be the capacity of the electrometer and tinfoil, though 

 the latter is probably negligible. Let q be the charge due to F. 

 This charges capacity C + E to D volts, where D is the E.M.F. of 

 the DanielFs cell, or to D/300 E.S. units. 



Then 



q =(C+E)D/300. 



Now C was removed and the force was adjusted to F', again 

 giving zero reading. Let q be the charge. 



Then 

 and 



2'=ED/300 

 q-q = CD/300 



which gives us q q in terms of known quantities. 



But q is proportional to F. Put, then, q q = A(F F') and 

 we find 



\ - 



F - F' 



which determines A. 



The Curies found that a load of I kgm. on a certain tourmaline 



FIG. 103. 



gave </, charging a capacity of 14*2 cm. to 1*12 volts, the E.M.F. 

 of the Daniel^ cell. 



- X 1-12 



so that 1 dyne would give a charge 5'41 X 10 ~ 8 . On a quart/ 

 plate cut perpendicular to one of the three axes which are perpen- 

 dicular to the axis of the prism a load of 1 kgm. gave 



q = -062. 



Then 1 dyne would give a charge 6'32 X 10 8 . 



The authors pointed out that the capacities of small condensers 

 might be compared by piezoelectricity. For suppose that, as 

 above, we have found with a condenser capacity C x 



g, - <( = X(l'\ - F) = C^D/300. 



