106 STATIC ELECTRICITY 



dO j %7T(T Z "K j 



d Q*-TF m *'TP"3S M 



and ^Q : 



JV IV U< I/ 



dK 

 or the heat energy added per unit volume is + ^ -yg of the 



energy added by the work done in charging the system. 



In some experiments by Cassie* the following values of -^ -^ 



and ^ were obtained at about 30 C., say 300 A 

 A a(7 



!_ j/K dl\ 



K ,/tf K do 

 Glass <HMW <H> 



Mica ()()()() I- \<> 



Ebonite 00007 Ml 



We see that the heat supplied to keep the temperature constant 

 when glass of the kind used by Cassie is electrically strained is 0*6 

 of the energy supplied by the work done. 



If this heat is not supplied and the charging takes place under 

 adiabatic conditions the temperature falls. K is dec T MM <1. and the 

 medium becomes as it were electrically stronger, and \\e ha\e the 

 analogue to the increased elasticity of solids under adiabatic 1 

 strain. But it can be shown that the adiabatic capacity of a 

 condenser bears to its isothermal capacity a ratio which differs from 

 unity by a quantity quite negligible in practical meaMiivnu-nK 

 even though the excess over unity is proportional, as investigatlOD 

 shows, to the square of the potential diffi rence.f 



Conditions to be satisfied where tubes of strain pass 

 from one dielectric to another. Law of refraction. When 

 tubes of strain pass from one medium to another with different 

 dielectric constant, no charge being on the surface, they change 

 their direction unless they are normal to the separating surface, 

 and they are said to be refracted. 



There are two conditions to be satisfied. We may describe the 

 first as (1) continuity of potential on passing through the surface. 

 This continuity of potential implies that the potential is the same 

 at two points indefinitely near to each other, one on each side of the 

 separating surface ; and this implies that the intensity in any 

 direction parallel to the surface, and close to it, is the same in each 



* Thomson's Applications of Dynamics to Physics and Chemistry, p. 102. 

 | If C , and C0 are the two capacities, 



^0 = ! V_ 2 _l_rfK_*_ 

 <J0 4ir K d8 Jp<r 



where V is the potential difference, /> is the density, and <r the specific heat of the 

 medium. 



