a 
* 
- 
ON ELECTRICAL THEORIES. 155 
y. Helmholtz ' has supposed that a change in the density of a dielec- 
tric might alter its specific inductive capacity, and he has investigated 
the consequences of this supposition. Korteweg and Lorberg ? have 
‘inyestigated the more general case, when the specific inductive capacity 
of a strained dielectric is supposed to be a function of the strains. 
Korteweg supposes that if the body suffers dilatation e along the lines of 
force, and dilatations fand g at right angles to them, then the specific 
inductive capacity = K—ae—/ (f+g). Helmholtz assumed that 
a =f. The presence of strain in a dielectric must influence the specific 
indifctive capacity, for Quincke has shown that the various coefficients of 
_ elasticity are altered under the influence of electricity. Lorberg, lL.c., has 
_ found the distribution of stress in the medium when the specific inductive 
capacity alters in this way. He finds that there is a tension along the 
line of force equal to 
K tL 6 
dese 18 et RS 
(= sf 3) 
and a pressure at right angles to them equal to 
dy 
= Sy A 
p dz ot 
where 
R? dk @ aise d 4, dp? d do do 
a — 2S (GENRE pe (a —— ye 9 pea 
An cz Ef daz (/ D sah dx (aot) da a dy G58) da dy 
d- ay dd do 
Seer ee dhe 
Where ¢ is the potential, and p the volume density of the free electricity. 
The part A of this force exists even when there is no free electricity at 
_ the place under consideration ; if the dielectric were a fluid, these terms 
would indicate forces tending to move the fluid when placed in a variable 
electric field; this motion, however, seems not to have been observed. 
The supposition made by Korteweg and Lorberg is not the most general 
one that could be made; we might assume that the specific inductive 
capacity of the strained body became different in different directions, so 
that the body would behave like a crystal. Dr. Kerr’s experiments on 
the double refraction in liquids placed between the poles of a powerful 
electrical machine seem to point to this conclusion. 
Kirchhoff? has made similar assumptions to those of Korteweg and 
Lorberg on the effect of strain on the specific inductive capacity, and has 
arrived at similar equations ; in the second paper he applies these equations 
to some cases which Quincke investigated experimentally. 
} vy. Helmholtz, Wied. Ann., xiii. p. 385; Wissenschaftl. Abh. vol. i. p. 298. 
* Korteweg, Wied. Ann., ix. p. 48; Lorberg, Wied. Awn., xxi. p. 300. 
8 Wied. Ann., xxiv. p. 52, 1885; Tbid., xxv. p. 601, 1885. 
