476 



Mr. A. P. Ghattock on an 



the lines of force enter and leave the substance being, as 

 before, normal to the lines). Hence, by combining these 



equations, 



9 = 



dN (K-l)d 2 M 



ds 



ttT 



dV 



-j- being here the maximum slope of potential which the 



substance will stand dielectrically. M is unknown ; but, 

 judging by what is known of the behaviour of solids under 

 stress, it is probably considerably less than the value of M 

 measured within the limits of elasticity. 



To test this formula I made a few rough experiments on 

 the dielectric strength of glass, in the form of the thin plates 

 used for the covers of microscope-slides. The plates, which 

 were 1 J inch diam., were held at their centres between platinum 

 knobs of 2^ mm. diam. Parallel with these knobs was a 

 micrometer spark-gauge, the length of which was gradually 

 increased until the glass was pierced. The following table 

 gives the results obtained : — 



Thickness (s) of 

 glass in cm. 



Max. value of V 

 in E.S. units. 



ds. 



0023 

 0023 

 0-025 



70 



77 

 78 



3000 

 3300 

 3100 







3130 



Taking the value of K for glass as 7, T=6*5xl0 8 , and 

 M = 5*5 x 10", we have 



? < 1-2 x HP 10 . 



Now, in addition to the fact that M is probably less than M, 

 T is also likely to be too small on account of flaws in the 

 material. These would not affect the dielectric strength nearly 

 so much as they affect the tenacity, because the stress in the 

 latter case has to be transmitted from point to point in the 

 material, and may therefore have to be shared by more or 

 fewer molecules according as the material is truly homo- 

 geneous or not, whereas in the dielectric the stress is applied 



