Changes in Dielect?ic Constant produced by Strain. 511 



Capacity, C = 1005 e. s. units. 



Length of armatures 40 cm. 



Internal radius a 1*78 cm. 



Wall thickness d O'l cm. 



K=3'24 



Stretching weight = 33'6 kg. 

 81/1 = 1-9 10" 3 , 

 8C/C = 1-4 10- 3 , 

 E = l-7 10 10 . 



Poisson's ratio was determined by measuring the angle of 

 twist for a known twisting moment M : 



2M 



The results were : 



^ = 7*05 10 9 , 



a=0-2. 

 Using (2) we find 



&-^(8i + &)=-0'77. 



When the condenser was twisted no change in its capacity 

 could be detected up to a whole angle of twist of 10° in a 

 length of 44*3 cm. The smallest value of SC/C that could 

 be detected was 10 " 3 . By (3) we find that the absolute 

 value of Si~ & is l ess than 0'09, or we can write 



c=S 1 -&=±0-09, 



as the limiting values of the difference between § x and § a . 

 We then find the limiting values of & and & separately by 

 combining this result with the value of & —cr (8i + &) obtained 

 from the stretching experiments : — 



c. &. &,. Si+&. 



+ 0-09 -1*25 -1-16 -2-41 



-0-09 -1-32 -1-41 -2-73 



-1*28 -1-28 -2-56 



Experiments with Glass Condensers. 



The only experiments with glass condensers were made 

 with the object of determining a change in capacity when 

 twisted. As no change was found the experiments of Wiillner 



