Strain along the Lines of Force. 681 



with mechanical pressure. In the first place, no such varia- 

 tion has been found experimentally, as the results so far are 

 discordant and subject to serious criticism. The question 

 still remains, whether it' it were found that K changes when 

 a body is distorted mechanically, electrostriction necessarily 

 exists. "We know already that as a general rule this constant 

 is least for gases, greater for liquids and greatest for solids, 

 or it varies approximately as the density of the matter. We 

 may then readily imagine that an increase in density caused 

 by pressure will change the dielectric constant slightly 

 because of a variation in the elasticity ; but the theorists main- 

 tain that electrostriction is due to a direct action of electricity 

 and not to a variation of the coefficient of elasticity, as shown 

 by my former quotation from Lippmann. Wiillner and Wien, 

 who alone regard the effect as being due to such a variation 

 in elasticity, have certainly mistaken the nature of the problem. 

 Their results,, as stated before, show large variations, equal at 

 times to the value of the whole coefficient of elasticity, in the 

 elasticity of electrified and unelectrified glass. And though 

 I believe that this difference must have been exaggerated by 

 extraneous effects, still such a variation may exist in a less 

 degree, but it has no bearing on the subject of electrostriction. 

 The question may be put in this form. Does a mechanical 

 pressure (a stretching force which produces a decrease in 

 density should theoretically increase the dielectric power) 

 cause a variation in the mutual relations of the aether and 

 matter ? This is the same question discussed with respect to 

 the polarization theory. 



There seems to be a considerable degree of confusion pre- 

 valent in regard to the significance of electrostriction. It 

 does not involve the mechanical pressure produced by the 

 attraction of two charged bodies, but is supposed to be a 

 property of the electric flux in a non-conductor. For 

 example, two charged bodies attract each other, and if they 

 touch a dielectric, glass for example, placed between them, the 

 glass must be deformed mechanically whatever theory is 

 adopted. But those who believe in electrostriction maintain 

 that if the charged plates be separated from the glass, and if 

 the intervening spaces be filled with a fluid dielectric of the 

 same dielectric constant as the glass and filled in such a 

 manner that the mechanical force of attraction of the electrified 

 armatures cannot be communicated by the fluid to the glass, 

 still the latter will contract in the direction of the lines of 

 force and expand perpendicularly to them. This mechanical 

 deformation, called electrostriction, is due then entirely to 

 the electrical stresses in an aether and communicated bv it to 



Phil. Mag. S. 6. Vol. 10. No. 60. Dec. 1905. 3 A 



