156 



STATIC ELECTRICITY 



Now substitute a second condenser capacity C 2 . If the first 

 charge is now q 2 and the force is F 2 , the second will still be q with 

 force F', and we have 



" 9 = X (F 2 - F') = C 2 1)/300 



and 



The piezoelectric electrometer. The Curies were led 

 from this to the invention of a very convenient instrument for 

 producing definite small quantities of electricity. 



Let Fig. 104 represent a rectangular block of quart/ with the 

 optic axis the axis of the prism parallel to the edge AC. Let 

 one of the pyro- and piezoelectric axes be parallel to the edge 

 BF. They found that load F on the face ACDE gave charge on 

 the face DEFG 



cLF 



Liectnc. 



FlG. 104. 



where L is the length AB, e is the thickness AD. and v has the 



value already found, via. 6'32 X 10 8 

 E.S. unit per dyne. 



In the inurnment n (jiiart/ plate is 

 cut with length L several centum -I n . 

 with breadth AC two or three centi- 

 metres, and with thickness c as small 

 as is consistent with strength suflic u nt 

 to carry loads of the order of 1 kgm. 

 This plate is fixed in brn-s pieces at 

 the top and bottom and hung vertically 

 from a support above, with a scale pan 

 below, as shown diagrammatic-ally in 

 Fig. 105, to carry a load. 



Tinfoil tt is pasted on to the front 



and back of the plate, and wires pass from the tinfoils to the >\>tem 

 to be charged. The charge is proportional to the total load and 

 if the load be increased the additional charge is proportional to 

 the additional load. The instrument may be graduated by the use 

 of an electrometer and condenser in the manner already explained. 

 Voigt's theory connecting pyroelectricity and 

 piezoelectricity.* The Curies found that pressure produces 

 electrification at the ends of an axis the same in sign as that 

 produced by cooling, and tension the same in sign as heating. 

 This suggests that the change in dimensions of the crystal may 

 alone be concerned in the two classes of phenomena, and Voigt 

 has investigated the question. The following is a mere indication 

 of the method. 



* Abhand. Gesell. d. Wins. Gottingen, 36 (1890). 



