'358 Dr. P. Sacerdote on tlie supposed Elongation of 



2. Discussion of the Second Experiment. — In this experiment 

 ■we have to do with a condenser of which the dielectric is the 

 glass and the armatures are adherent to the dielectric and 

 follow its deformations ; these conditions are quite analogous 

 to those in the experiments of Righi, Quincke, and Cantone*. 

 On the ha sis of the figures obtained by this last author, we 

 can calculate a priori the elongation which the tube should 

 undergo in the experiments of Prof. More. Let 



I be the length of the condenser, 



AZ be its elongation when charged to the potential V, 

 e be the distance between the armatures, which is here 

 equal to the thickness of the dielectric. 



Cantone's experiments give for the expression I —=- x ^ 2 Jf 



values i varying between 4*6 and 7*1 x 10 -13 : let us take the 

 mean, 6 x 10~ 13 . Denoting by sc the elongation to be pre- 

 dicted for the tube in Prof. More's experiment (for a spark- 

 length of 12 mm., V=110c.G.s.), we have 



45 



(0-25) ! 

 (110) 2 



=■ 6xl0- 18 , 



* In a long criticism of the experimental work on this question 

 (Sacerdote, " Pecherches theoriques sur les deformations electriques des 

 . d ielectriques solides isotropes," Annates de phys, et chimie, ser. 7, t. xx. 

 pp. 289-877 : abstract, Journ. de Phys. ser. 3, t. viii. Sept.-Oct. 1899) 

 I have already shown (pp. 344-369) that the experiments of Duter, 

 Righi, and Quincke should only be considered from the qualitative point of 

 view ; while those of Prof. Cantone (Rendiconti delta R. Ace. dei Lincei, 

 ser. 4, t. iv. pp. 344-353, 471-477, 1888), in which the elongation is 

 measured directly by the displacement of interference-fringes, present 

 far greater guarantees of accuracy, and that these are the only ones the 

 numerical results of which can be depended on. Prof. More does not 

 appear to have been acquainted either with my paper or with the 

 experiments of Prof. Cantone. 



T The formula for the electric deformation of a thin cylindrical con- 

 denser with adherent armatures is 



in which a, k lt K are coefficients depending solely on the nature of the 

 dielectric (for the proof of this formula and the meaning of the co- 

 efficients see Sacerdote, loc. cit. p. 307) ; the quantity — Xy- 2 should thus 



i v 



be sensibly the same in the experiments of Prof. More and in those of 

 Prof. Cantone. 



X See Sacerdote, loc. cit. table on p. 368; these numbers have just been 

 confirmed by recent researches (Cantone & Sozzani, "Nuovo Ricerche 

 intorno alia Deformazione dei Condensatori," Rendiconti della It. Istituto 

 Lombardo, ser. 2, t. xxxiii. 1900). 



