Changes in Dielectric Constant produced by Strain. 509 



under a state of simple longitudinal tension and also under 

 a state of simple torsion make it possible to determine Si 

 and § 2 separately. 



It is also possible to determine ^ and £ 2 for parallel plate 

 condensers. The capacity is : C = KA/4tt^, so that 



SC _ SA gK $d 

 C ~ A + K ~~d' 



We shall assume that the armatures are adherent, rectangular 

 in shape, of dimensions a and b. 



First, let the condenser be stretched by a tension T applied 

 to the side b, at right angles to the lines of force. We find : 



£A 

 A : 



T 



«■), 







d '' 



o-T 



-~ E 









K 



T f 

 ~EK ( 



i- 



<»1 



+ *)}. 



SO 



c = 



4P* 



^ 



$2 — 



■<8i + $ 



E being Young's modulus. 



Next, let the condenser be compressed by a pressure P, 

 along the lines of force, applied to its whole area, the edges 

 being free from traction. 



SA 

 A ' 



=2|., 







$d 

 d : 



P 



~"~E' 







K : 



P 

 ~ EK 



(&- 



•2<r&), 



c = 



■ft* 



■+1- 



-i» 



2<r& 



»}■ 



As §! and S 2 enter into these two expressions for &C/C in 

 different combinations, they may each be determined. 



Experiments with Hard Rubber. 



The first experiments were made with hard rubber in the 

 thought that as larger strains could be produced the effects 

 would be larger than with glass. A hard rubber tube, of 

 internal radius 1*78 cm. and wall-thickness O'l cm., was 



