Prof. E. P. Adams on Electrostriction. 895 



outside. The elongation is given by* 



We thus have the important result that the elongation of 

 a charged cylindrical condenser is due wholly to the changes 

 in the dielectric constant resulting from the strain in the 

 dielectric. 



We have no means at present of determining the amount 

 of elongation, expressed by (12), to be expected. The com- 

 bination 6 2 —cr(8 1 +S 2 ) maybe determined from the experi- 

 ments of Wiillner and Wien t on the change in capacity of 

 cylindrical glass condensers when stretched. The capacity 

 is given by 



IK 



U = 



2 log b/a' 



where I is the length of the coated part. Let T be the 

 tension applied. The ratio bja is unaltered by a simple 

 longitudinal traction, so that the whole change in the capacity 



is given bv 



Hence we get 



SC= Tr ^ rr (K8Z+Z8K) 

 2 log bja v 



/T 

 $K=^(8 2 -a8 i -o$ 2 ). 



^ > = 1 ^(K + S 2 -aS 1 - ff 8 2 ). • • • (17) 



The following table gives the average values of 

 S 2 — cr(8 1 + B 2 ) calculated from the results of their experi- 

 ments. The first column gives the designation of the glass, 

 the second Young's modulus determined by stretching, and 

 the third the dielectric constant. 



* Love, I c. p. 141. 



t Annalen tier Phjsik, xi. p. 619 (1003). 



