Strain alone/ the Lines of Force. 683 



matter. My claim is. not that Faraday stresses may not 

 exist in the aether, but that these stresses are not transferred 

 in any appreciable degree to matter as unbalanced mechanical 

 forces. The fact is, a dynamic system in a hypothetical 

 medium may be either affirmed or denied, but it cannot be 

 proved as the question is a metaphysical one. The absence 

 of the electrostrictive effect does not preclude the existence 

 of aetherial stresses, but it does prevent it from serving as a 

 fundamental experimental verification of the stress-problem. 



The recent discussion of Maxwell's stress theory by 

 Bjerknes* is opportune to this subject, and his conclusion 

 should be quoted : — " In the meanwhile it will therefore be 

 safest to consider the Maxwell stresses as only fictitious 

 stresses that might have produced the required forces, and 

 not as the real stresses which do produce tbem. Other 

 authors have also termed them fictitious stresses, especially 

 Lorentz, who also considers the stress formulae only as 

 useful analytical formulae, but not as representing any 

 physical reality. The reason, however, why he has come to 

 this opinion is quite different from the reason brought forward 

 here. His view is that the stress-problem in itself has no 

 physical meaning. For, according to the doctrine of the 

 immobility of the aether, it is an absurdity to speak of forces 

 acting upon or stresses existing in the aether." 



Dielectric Strain along the Lines of Force. 



Maxwell's theory, which supposes an expansion perpendi- 

 cularly to the lines of force, also demands an equal contraction 

 in their direction,. A test of this latter effect has never, so 

 far as I know, been attempted : it is obviously a much more 

 difficult piece of work, but it also should prove to be of greater 

 importance. In the former method, the effect sought and 

 the extraneous effects are all in the same direction and are 

 difficult to separate. But the effect along the lines of force 

 should be a contraction, and so opposite in effect to the heating 

 and other sources of error. Furthermore, it does not 

 introduce a correction for the little-known value for Poisson's 

 ratio. It is besides possible to reduce the magnitude of the 

 disturbances themselves. 



It is evident that a sheet of dielectric placed between two 

 metal electrodes will be compressed by the attraction of these 

 electrodes when they are charged. We must neutralize this 

 attractive force by making a system of balanced electric 

 forces. This can be best done by building up a pile of alter- 

 nate metallic and dielectric plates, each held horizontal and 

 * V. Bjerknes, Phil. Mag. vol. ix. p. 491 (1905). 

 3 A2 



