J g- 1^5 



Electric Strength of Solid Dielectrics. 



125 



Consider, first, a dielectric free from absorption. The 

 charge q on unit area of a slab of thickness t placed in a 

 field vjt, where v is the voltage between opposite faces, is 



k v 

 -. — .-, k beino- the dielectric constant. The work w done in 



4-7T t ° 



establishing the polarization is 



J 



q dv = 



«7T t 



and 



k_ v 2 



c57r' W 



(1) 



Malcles has shown that pure paraffin has no residual 

 effects*, and it may thus be regarded as a typical perfect 

 dielectric. It is found below, from the measurement of 

 "Weickertj that in the case of paraffin the relation between 

 thickness and breakdown voltage can be very accurately 

 represented by t = Br 2 . 



Table I.— Paraffin Wax. 13 = 1-32 . 10" 10 . 





Bi 



■eakdown Voltage. 



v*/t. 











Thickness. 



Observed. 



Calcukited. 



Difference 









275 . 103 



per cent. 





O'l cm. 



27 . 10 3 



+ 17 



■730. 10" 



•2 



30. 



38-8. 



-Oo 



•761. 



•4 



56. 



55*0. 



-1-8 



•785 



•6 



68. 



67-5 . 



-0-75 



•771 . 



•8 



78. 



777. 



-0-4 



■752. 



10 



87. 



87-0. 



-00 





1-2 



95. 



95-4. 



+0-5 



■752 . 



1-4 



102. 



1030 . 



+ 1-0 



•745. 



Mean 



•759 . 



It follows that k/Sirw is constant, so that the icork done in 

 breakdown is the same for any thickness of dielectric. 



2. Inorganic Insulators having residual effects. 



All other solid insulators but paraffin, whether crystalline 

 or not, show residual charge. Even paraffin, unless very 



* L. Malcles, Ann. Chim. Phys. xvi. pp. 153-236 (1907). 

 t Weicker, Electrotechniache Zeitschrift, 1903. p. 800. 



