FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
445 
- div VEH, 
making VEH the flux of energy.* 
All attempts to construct an elastic solid analogy with a distortional stress fail to 
give satisfactory results, because the energy is wrongly localised, and the flux of 
energy incorrect. Bearing this in mind, the above analogy is at first sight very 
enticing. But when we come to remember that the d/dt in (98) and (99) should be 
0/3 t, and find extraordinary difficulty in extending the analogy to include the conduc¬ 
tion current, and also remember that the electromagnetic stress has to be accounted 
for (in other words, the known mechanical forces), the perfection of the analogy, as 
far as it goes, becomes disheartening. It would further seem, from the explicit 
assumption that q = 0 in obtaining W above, that no analogy of this kind can be 
sufficiently comprehensive to form the basis of a physical theory. We must go 
altogether beyond the elastic solid with the additional property of rotational elasticity. 
I should mention, to avoid misconception, that Sir W. Thomson does not push the 
analogy even so far as is done above, or give to p and c the same interpretation. The 
particular meaning here given to p is that assumed by Professor Lodge in his “ Modern 
Views of Electricity,” on the ordinary elastic solid theory, however. I have found it 
very convenient from its making the curl of the electric force be a Newtonian force 
(per unit volume). When impressed electric force e 0 produces disturbances, their real 
source is, as I have shown, not the seat of e 0 , but of curl e 0 . So we may with facility 
translate problems in electromagnetic waves into elastic solid problems by taking the 
electromagnetic source to represent the mechanical source of motion, impressed New¬ 
tonian force. 
Examination of the Flux of Energy in a moving Medium, and Establishment of the 
Measure of “ True ” Current. 
§ 17 . Now pass to the more general case of a moving medium with the equations 
curl - curl (H-h 0 -h) = J = C + b + q y9 , .(101) 
— curl Ej = — curl (E — e () — e) = G = K + B + qc,.(192) 
where E : is, for brevity, what the force E of the flux becomes after deducting the 
intrinsic and motional forces ; and similarly for H 1 . 
From these, in the same way as before, we deduce 
(e 0 + e) J + (h 0 + h) G — EJ + HG + div VE^;.(103) 
and it would seem at first sight to be the same case again, but with impressed forces 
* This form of application of the rotating ether I gave in ‘ The Electrician,’ January 23, 1891, p. 360. 
