678 Prof. L. T. More on Dielectric 



the value -j . ^- 2 = 6'5 X 10 -13 ; when the best value for 



Young's modulus, fi, and the dielectric constant for the glass 

 are substituted in the right-hand member, there results a value 

 of the same order and about one-half as large. Quincke's 

 average results are in somewhat closer agreement. This 

 discrepancy they assumed to be due to the inadequacy of 

 Maxwell's formula, which should contain another term, 

 signifying the relation between mechanical and electrical 

 pressures. 



Passing over the cruder work of early writers, the best 

 direct results were obtained by Quincke * and Cantone f. In 

 previous papers J I attempted to show, by their own results, 

 that both neglected to consider extraneous effects which were 

 of the same magnitude as their recorded values and which, if 

 introduced, would have accounted for the entire deformations 

 observed. These errors are due chiefly to heating, lateral 

 displacements, distortions caused by using thin tubes more or 

 less irregular in diameter and thickness, or by the lack of 

 sphericity in any blown bulb of glass. Quite recently 

 Wiillner and Wien § attacked the problem in an indirect 

 manner, attempting to show that the elasticity of glass is 

 greater when obtained from electrical stress than when found 

 in the usual manner by mechanical or acoustical methods. It 

 is interesting to note the contradictions of these different 

 observers. Wiillner and Wien confirm Quincke's conclusions, 

 though not his values, and condemn Cantone's and mine ; 

 while Sacerdote and Cantone consider Quincke's to have 

 little value. In the first place, the measurement of the rise or 

 fall of an electrified capillary water column, such as Wiillner 

 and Wien use, is in my opinion a faulty method, and their 

 agreement with Quincke, who often employed the same method, 

 is natural. I may add that this method is condemned by 

 other investigators. Wiillner and Wien themselves have to 

 introduce elaborate corrections to obviate the irregularities in 

 the motion of the water column. Secondly, their results show 

 great variations in the value of the elasticity, measured 

 mechanically and electrically, for different kinds of glass. 

 For some glass the latter is not more than one-half of one per 

 cent., but for others it is as much as one hundred per cent. 



* Quincke, Wied. Ann. Bd. x. pp. 161, 374, 513 ; Bd. xix.pp. 545, 705 ; 

 Bd. xxviii. p. 529 ; Bd. xxxii. p. 503. 



t Cantone, Rend. d. R. Accad. Line. t. iv. pp. 341, 471 ; Rend. d. R. 

 1st. Lomb. [2] t. xxxiii. ; Nuovo Cim. [4 n i t. xii. p. 150. 



\ More, Phil. Mag. [5] vol. i. p. 198, [6] vol. ii. p. 527, [6] vol. vi. p. 1 ; 

 Elect. World and Engineer, vol. xliii. p. 127. 



§ Wiillner and Wien, Drude's Ann. [4] Bd. ix. p. 123 7. 



