Equations of a Moving Material Medium, fyc. 371 



aberration worked out in 14 16, pp. 225 229, will be found to 

 fit into this scheme. I take the opportunity of correcting an erratum 

 in p. 226, lines 16, 17, which should read 



with of course different values of the constants. 



2. In a material dielectric the bodily mechanical forcive is derived 

 from a potential (K l)F 2 /87r, and there is also a normal inward 

 traction KP 2 /87r where it abuts on conductors. For the thin dielec- 

 tric shell of a condenser this forcive could be balanced by a hydro- 

 static pressure (K l)F 2 /8:r together with a Maxwell stress 

 consisting of a pressure F 2 /8;r along the lines of force and an equal 

 tension at right angles to them : in fact this reacting system gives 

 the correct traction over the faces of the sheet and the correct 

 forcive throughout its substance. If the sheet has an open edge the 

 tractions on that edge are however not here attended to ; when the 

 sheet is thin these are of small amount, and their effect is usually 

 local, as otherwise the nature of the edge would be an important 

 element. Moreover, in the most important applications of the 

 formula the edge is of small extent, so that they form a local 

 statically balanced system. The stress above specified will thus 

 represent the material elastic reaction, provided the strains in the 

 different elements of volume, which correspond to it, can fit together 

 without breach of continuity of the solid material. This condition 

 will be secured if the shell is of uniform thickness so that F is constant 

 all over it : in that case, therefore, the elastic reaction in the material 

 will make up a pressure KF 2 /8w along the lines of force and a 

 pressure (K 2)F 2 /87r in all directions at right angles to them, which 

 is the result obtained for solids in 76. 



If, however, the coatings of the condenser are not supported by the 

 dielectric shell, the elastic reaction in the shell will be simply a 

 pressure (K l)F 2 /87r uniform in all directions. This is what 

 actually occurs in the case of a fluid dielectric, where such support 

 is not mechanically possible. 



It appeared from 79 that in glass there is actually an increase of 

 volume under electric excitation, while the mechanical forces would 

 produce a diminution : and the same is true for most dielectric 

 liquids, the fatty oils being exceptions,* though by a confusion 

 between action and reaction the result was there stated as the 

 opposite. It thus appears that in general an intrinsic expansion, in 

 addition to the effects of the mechanical force, accompanies electric 



* In the cognate case of magnetisation of ferrous sulphate solution, Hurmuzescu 

 iiiuls a contraction of volume. 



VOL. LX1I1. 2 E 



