THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
751 
in fact as mechariical energy of tlie charged conductors, which determines the 
inechanica] forcive between them. It is desirable to attempt a closer examination of 
the nature of the action by which this transfer of energy takes })lace Ijetween 
the tether and the material of the conductors, and by which the similar transfer takes 
place at a transition between one dielectric substance and another. 
In the disjilacement of a conductor through an excited dielectric there is thus an 
overflow of electromotive energy, and in the absence of viscous agencies and radiation 
it simply displays itself in ordinary mechanical forces acting on the surfrce of the 
conductor. The magnitude of these forces has been examined experimentally in 
different media, and has been found to correspond precisely with this account of 
then? origin ; good reason can be assigned to show that their intensity changes from 
point to point of the surface according to a law^ (KF^/Stt, where F is electric force) 
which suggests that the energy is absorbed by the conductor at its surface. In a 
similar way, when a dielectric body is moved through the electric field the trans¬ 
formation of energy takes place at the interface between the two dielectrics. 
The statical distribution of electromotive stress in the excited mtliereal medium 
is definite and has just been determined : it involves on each element of interface 
in the dielectric eether a purely tangential traction at right angles to the tan¬ 
gential component of the electric force and equal to it. This is the denomination 
of stress that corresponds to the displacement (^, rj, Q, just as an ordinary force 
corresponds to a translation of matter or a couple to a rotation. If we have no 
direct knowledge of the sethereal displacement (^, rj, Q w'e cannot actually recognize 
this stress ; but when (£, rj, Q is taken as here to be a linear displacement, this 
electromotive stress must be a mechanical stress in the sether such as does work in 
making a linear displacement. 
45. The mechanical traction along the normal, which is distributed over the 
surfaces of two conductors separated by an excited dielectric, as for example tlie 
coatings of a charged Leyden jar, may be balanced by supports applied to the 
conductors; or if there is a dielectric body between them, it may be mechanically 
balanced by a stress in the material of this dielectric. This is the only kind of 
mechanical stress in a dielectric of which we have direct cognizance : its amount has 
been calculated by KiECHHOFFt and others for some cases, and compared with 
experimental measures of change of volume of dielectrics under electrification. The 
stress in the aether itself has been here deduced by a wholly different path. 
It will possibly be a true illustration of what occurs to imagine each element 
* Cf. “ On the theory of Electrodynamics, as affected by the nature of the mechanical stresses in 
excited dielectrics,” ‘ Roy. Soc. Proc.,’ 1892. 
t G. Kirchhopf, “ Ueber die Formanderung, die ein fester elastischer Kdrjmr erfiihrt, wcnu er magne- 
tisch odor dielectrisch polarisirt wird,” ‘ Wied. Ann.,’ 24, 1885, p. 52 ; 25, 1885, p. GOl. Such a stress, 
involving the square of the electric intensity instead of its first poAver, must of necessity be of secondary 
character, and cannot take direct part in wave-iAi'ojAagation in the electx’ic medium. 
