Electric Strength of Solid Dielectrics. 135 



Table IX. — Values of - and - volts per centimetre 

 z z 



for the smallest thicknesses. 



1 



Material. Internal field. 

 aft. 



Applied field. Ratioa/v> 



Paraffin 



Glass 0-17. 10 5 



Porcelain 11*2. ,, 



Mica 165"0. „ 



2-7 . 10 5 

 0-32.,, 

 1-3. „ 

 34-0. „ 

 10. „ 

 0-23 . ,, 







054 



8-6 



4-85 



0-65 



2-4 



Vulcanized Rubber 065 . ,, 



Jute 0'55. „ 



Presspahn 3 - 2. ,, 



Oiled Canvas 2'85. ,, 



096 . „ 3-4 

 15. „ 









fails. Rupture is chiefly an internal effect, an electric ex- 

 plosion or recombination of atomic charges of such violence 

 that the molecular structure is permanently damaged along 

 the line of action. That electric breakdown is of the nature 

 of an explosion along the line of failure is supported by the 

 appearance of the point of passage of the spark, which has a 

 burr at both ends, and by the fact that in many cases, unless 

 the testing transformer or generator is large, a spark 

 ' through ' insulation is not followed by permanent break- 

 down of insulation resistance. This, on the contrary, may 

 actually be improved by the passage of a spark intended to 

 break down an incipient fault*. 



7. Amplitude of Electrical Displacement. 

 The area OAC, fig. 3, is jF*a? , that of CDE is 



e 2 x z 



19^(7.^-57^, 



the second term beino- the area CD', fig. 3. The ratio 

 the two areas is v/a, so that 



76 -^.r 3 / JF«# =a/fl. 



From Table IX. the mean value of a/i\ omitting paraffin 

 and porcelain as exceptional, is 2'2. So that with 

 e=4-7.10- 10 , F = 300E.S.U., d=5.1Q~\ 



feV , 25. 10? 3 00. 2*2 



\a) ~ 152. 4-7. 10 16 



or 



X 4 =0-005. 



See Beaver, I. E. E. " Cables," 1914, discussion by Bridge. 



