136 On the Electric Strength of Solid Dielectrics. 



According to this a very small electrical displacement 

 relatively to the diameter of a molecule is sufficient to 

 strain the atomic structure to breaking point. 



If, as in porcelain, a/v is large, =17*2, -^ = 0'016. In the 



case of paraffin, N . ¥e ,x J2 = w, where w is the work of 

 breakdown per unit volume and N the number of molecules 

 in unit volume. N is nearly 2/d 3 , so that 



x wd 2 • 



d = ¥e~~ 

 The breakdown gradient is 9000 E.S.U. and i6- = 8500 ergs. 



Therefore with rf = 5.10- 8 , ^ =-002. 



d 



8. Total work of Electric Breakdown. 



The total work done upon a material up to the point of 

 rupture is w = k/$7rB. It therefore depends upon the di- 

 electric constant k. The energy of the first polarization at 

 any voltage is kv 2 /Sirt, of the second kav/4:7rt, in each case 

 divided by 9.10 4 to convert to ergs, when v is in volts and t 

 in centimetres. The ratio of these energies is given in 

 Table IX. and the total work in Table X. 



Table X.— 



Energy absorbed before failure. 



Materia!. 



k. 



Total energy 

 per sq. cm. 



Energy on one 

 molecular 







m ergs. 



area. 



Mica 



6-0 



265 .10 3 



-10 

 5*3 .10 erg. 



Porcelain 



5-3 



68 .„ 



1-36. 1<T 10 



V.I. Rubber 



' 3-0 



34 .„ 



•68. 10- 10 



Glass 



6-0 



11-6 



2-3 .MT 11 



Paraffin 



2-3 



8*5 .„ 



17 .lO"" 



Presspahn 



4-9 



1-58 . „ 



3-1 .10 



Oiled Canvas 



2-7 

 4-3 



1-25 . ,; 



10 .„ 



2-5 .10 _1 



— 12 



2-0 . 10 



Impregnated Jute 



Glass fails under mechanical tension at about 5.10 8 ergs 

 per cubic centimetre, so that it is 50,000 times stronger 

 mechanically than electrically. The energy to break down 

 a single chain of molecules of sectional area 2.10 -15 sq. cm. 

 in the above typical insulators is of the mean order of 10 ~ u erg. 

 This may be compared with the energy required to change 

 the charge in an atom by an amount e. That required to 

 ionize a hydrogen atom is 1*7. 10 -11 erg. 



