154 REPORT—1885. 
Appenpix II. 
ON THE STRESS IN THE DIELECTRIC. 
In the preceding Report we have had so frequently to refer to the 
action of the dielectric that it may be convenient to give a very brief ac- 
count of the work which has been done on the stresses which are supposed 
to exist in it. We shall confine ourselves to the work which has been 
done on the stresses in the electrostatic field; those existing in the electro- 
magnetic field are of a similar nature, so that any remark applying to 
one will also apply to the other. The idea of explaining the forces 
in the electrostatic field by means of stresses in the dielectric seems to be 
due to Faraday, who describes ! the stress in the medium by saying that 
the lines of force tend to contract and also to repel one another. The 
magnitude and distribution of this stress was investigated by Maxwell; ? 
he found that in a medium whose specific inductive capacity was K, and 
at a point where the electromotive force is R, a tension equal to KR?/8 
per unit area along the lines of force combined with a pressure of the 
same amount at right angles to these, would produce the effects observed 
in the electrostatic field, that is, at a pointin a dielectric, the resultant of 
these stresses would be a force whose components, parallel to the axes of — 
w, y, 2, are eX, eY, eZ respectively, e being the charge of electricity at the 
point, and X, Y, Z the components of the electromotive force. It may be 
observed that this system of stress could not be produced by the strain 
in an elastic solid at rest: this points to the kinetic origin of electrostatic 
phenomena. 
These stresses are in equilibrium at a point in a dielectric where there 
is no free electricity. At the junction of two media, whose specific inductive 
capacities are K, and K,, and in which the electromotive forces are 
R, and R,, and whose interface is perpendicular to the lines of forces, 
the stresses are not in equilibrium, but there is an unbalanced stress 
(K, R,? — K, R,”) /87 which will tend to’ make the boundary move 
towards the medium whose specific inductive capacity is K, ; if these 
dielectries are liquids, their interface may become curved so that the forces 
due to surface tension balance this stress. 
Quincke,® who has experimentally investigated the effects of electrifi- 
cation on various dielectrics, such, for example, as the effects on the glass 
of a Leyden jar, has found that the effects on different bodies are very 
different ; he finds, for example, that though the effect of the electrification 
on the dielectric of the Leyden jar is generally to produce an expansion, 
yet in some substances, such as the fatty oils, contraction takes place.‘ 
This diversity in the effects of electrification on different dielectrics shows 
that the distribution of stress cannot be so simple as was supposed by 
Maxwell. It also shows that there must be forces in the electric field 
which are not recognised either by Maxwell’s theory or the theory of 
action at a distance. More general theories have been given in order to 
meet this difficulty. 
1 Experimental Researches, § 1297. 
* Hlectricity and Magnetism, 2nd edition, p. 149. 
3 Wied. Ann., x. pp. 161, 374, 513; Jbdid., ix. p. 105; Phil. Mag., vol. x. p. 30° 
(1880). 
* The fatty oils are also an exception to the rule that the index of refraction 
equals the square root of the specific inductive capacity. . a 
