512 Changes in Dielectric Constant produced by Strain. 



and Wien enable us to determine the values of o\ and § 2 » or 

 at least to find the limiting values between which they must 

 lie. 



A silvered glass tube, of internal radius 5 cm., wall- 

 thickness 0*1 cm., and length 55 cm., was twisted through 

 5°*1 before it broke. The capacity remained constant. 



Identifying this kind of glass with Wiillner and Wien's 

 " N.Th.R." we have, from Table I., in the paper referred to : 



K = 8'60, 

 a = 0'2, 

 S*-o-(8i + S«) = -0-94. 



8C 



Since the smallest measurable value of ^- was 10~ 3 (3) 

 gives as the limits between which o\ — $ 2 lies 



c=8i— 8»= ±1*06, 

 and we get 



c. 



8i. 



8 2 . 



8i + 8 2 . 



+ 1-06 



-0-16 



-1-22 



-1-38 



-1-06 



-2-96 



-1-90 



-4-86j 







-1-57 



-1-57 



-3-14 



Similar results were obtained with a Jena glass tube of 

 length 61 cm., internal radius 1*75 cm., and wall-thickness 

 0*1 cm. This tube was twisted through 4° before it broke. 

 Identifying this with Wiillner and Wien's No. 10, we have 



K=6-61, 



8 2 -o-(8i + 8 8 )=-2-54, 

 and we find 





C — 01 — 2 



= ±0-62; 





and we can write: 









c. 



oY 



oV 



81 + 82 



+ 0-62 



-3-4 



-4-0 



-7-4 



-0-62 



-5-0 



-4-4 



-9'4 







-4-2 



-4-2 



-8-4 



We can now compare the elongation to be expected on 

 the erroneous theory of eleetrostriction, i. e. 



2 K 

 8ttE' 

 with the elongation given by equation (1). Their ratio is 



e (8i+8 8 )q- 



e'~ K ' 



'-©' 



