PYROELECTRICITY AND PIEZOELECTRICITY 161 



whence 



He found that for a particular case the total volume change/ dv 

 was equal to about 10 ~ 5 , the original volume, when the potential 

 was such as to spark across 2 mm., say, 20 E.S. units, and the 

 thickness was *014 cm. ; whence, if we deal with unit volume of 

 dielectric, K is of the order 10 ~ u . 



If we keep x constant, 



dm = adp, 



which shows that there will be a change in m depending on the 

 pressure but, for any moderate values of x and dp, exceedingly 

 minute. 



Change of temperature of a pyroelectric crystal on 

 changing the potential. Lippmann, in the paper already 

 cited, pointed out that there should be a change of temperature in 

 a pyroelectric crystal when the electric strain in it is changed, and 

 the investigation applies to any condenser. 



We assume that the pressure and volume remain constant, so 

 that the only interchanges of energy are between the electrical and 

 the heat forms. We further assume reversibility. 



It is convenient to employ the entropy temperature diagram. 

 Let us draw on it equal charge lines, AD at m-dm and BC at wi, and 



3D 



FIG. 107. 



equipotential lines AB at x and DC at x-dx. Let us take a 

 dielectric with charges on its two faces through the cycle ABCD. 

 The net electrical energy given out in the cycle is dmdx. 



The heat put in is the area ABCD = a/3CD = (31)^' dxty, and 

 equating the two quantities, 



<dn? 



,r 



