FLUXES OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
453 
P N = (E.DN - NU) + (H.BN -NT),.(136) 
divided into electric and magnetic portions. This is with restriction to symmetrical 
fx aad c, and with persistence of their forms as a particle moves, but is otherwise 
unrestricted. 
Neither stress is of the symmetrical or irrotational type in case of eolotropy, and 
there appears to be no getting an irrotational stress save by arbitrary assumptions 
which destroy the validity of the stress as a correct deduction from the electro¬ 
magnetic equations. But, in case of isotropy, with consequent directional identity 
of E and D, and of H and B, we see, by taking N in turns parallel to, or 
perpendicular to E in the electric case, and to H in the magnetic case, that the 
electric stress consists of a tension U parallel to E combined with an equal lateral 
pressure, whilst the magnetic stress consists of a tension T parallel to H combined 
with an equal lateral pressure. There are, in fact. Maxwell’s stresses in an isotropic 
medium homogeneous as regards p, and c. The difference from Maxwell arises when 
fx and c are variable (including abrupt changes from one value to another of g and c), 
and when there is intrinsic magnetisation, Maxwell’s stresses and forces being then 
different. 
The stress on the plane whose normal is YEH, is 
E.DVEH + H.BYEH - (U + T) YEH 
V 0 EH 
E.HYDE + H.EVHB - (U + T) YEH 
Y 0 EH 
(137) 
reducing simply to a pressure (U + T) in lines parallel to YEH in case of isotropy. 
§ 20 . To find the force F, we have 
FN = div d x = div (D.EN - NU + B.HN - NT) 
= EN./> + DV.EN - fE.NV.D - ^D.NV.E + &c. 
= EN.p + D (V.EN - NV.E) + f (D.NV.E - E.NV.D) + &c. 
= N [E/j + Y curl E.D - VIT C + Ac.],.(138) 
where the unwritten terms are the similar magnetic terms. This being the N 
component of F, the force itself is given by (122), as is necessary. 
It is Y curl h 0 . B that expresses the translational force on i ntrinsically magnetised 
matter, and this harmonises with the fact that the flux B due to any impressed 
force h 0 depends solely upon curl h 0 . 
Also, it is — VT^ that explains the forcive on elastically magnetised matter, e.g., 
Faraday’s motion of matter to or away from the places of greatest intensity of the 
field, independent of its direction. 
