160 STATIC ELECTRICITY 



dm 



- 



The Curies found that for tourmaline (p. 155) 



a = 5-41 X ID- 8 . 



Now let us suppose that F is kept constant and that the potential 

 is altered. Then -p = + fl, and for tourmaline this should be 



(!,(' 



equal to 5*41 X lO' 8 . 



Similarly for quartz ^- should be equal to 6*32 X 10 8 , 



If the quartz plate is cut and loaded as in the piezoelectric 

 electrometer, it is evident that a similar investigation would 

 give 



dm dl 



Verification by the Curies. This result was verified at 

 once by J. and P. Curie.* It will be sufficient to describe one 

 experiment. 



A thin quartz plate cut perpendicular to an electric axis, like 

 that in the piezoelectric electrometer, was covered with tinfoil on 

 its two faces and fixed so that any variation in its length could be 

 measured by means of a magnifying lever. When a traction of 

 258 grm. or 258 X 981 dynes was put on, the charge produced \\.i^ 

 found to be 0'18b', so that one dyne would give 7*39 X 10' 7 . 



A difference of potential estimated at 65*2 E.S. units was then 

 put on between the tinfoil plates. This should give 



dl = 7-39 x 65-2 x 10' 7 = 480 x 10 7 cm. 

 Direct measurement gave 



dl= 500 X 10 7 . 



Dielectric subjected to uniform pressure. If a dielect He 

 is subjected to uniform pressure p instead of to end thrust, a 

 precisely similar investigation to that above gives 



dm = cdx adp 

 dv = adx bdp 



where dv is the volume change. The diagram of Fig. 106 may be 

 taken to represent the relation between p and v for values of m and x. 

 Quincke (ante, p. 142) found that the total change of volume of a 

 jar for a rise of potential a: was nearly proportional to .r 2 // 2 , where 

 t was the thickness of the dielectric. We may therefore put 



* (Euvres de Pierre Carle, p. 26 and p. 30, where another form of experiment is 

 described as well as that in the text. 



