FUNDAMENTAL FORMULATIONS OF ELECTRODYNAMICS. 
and it has been shown on a previous occasion* that the only form of specification of 
the energy distribution which is consistent with our usual ideas on these matters is 
that which makes the density of the magnetic energy equal to BVStt, and as the 
present disciission sliows that our only hope of discrimination lies in that direction, we 
may assume that the evidence in favour of this special form is conclusive, at least bn- 
the present ; it is besides the only form in which the most general case is completely 
representative of the distribution in any volume of the field without requiring the 
introduction of boundary terms involving surface distributions. 
11. We next turn to a consideration of the expression for the forcive of electro¬ 
magnetic origin acting on the polarised media in the field, e have seen that the 
mechanical forcive on the dielectrically polarised media is such that its ./’-component 
per unit volume at any place is of the form 
(pv)E.+i([paff)4 
dF p 
or as it first appears in the analysis 
a.r 
[P CurlF4-f-t 
0E 1 d 
b' 
_dt 
1 /ro 1 
c \ ox 
ox cat c 
0a;/ 
This result is in complete agreement with that derived by L armor in the electron 
theory,! l)ut the present derivation indicates clearly the origin of the difterent terms 
in it. The expression 
is that corresponding to the expression derived in the statical theory from energy 
considerations and corresponds to Maxwell’s magnetic expression ; the second term, 
viz., 
-[P Curl EJ, 
is one of the terms arising as a result of the convection of the media, and this is the 
term which is effective in reducing the electric part of the forcive to the form 
(PV) E, 
which is the result derived in the elementary theory by regarding the forcive as the 
resultant of the forces on the elementary bi-poles. 
* ‘ Phil. Mag.,’ vol. U (1917), p. 885. Cf. also ‘Phil. Mag.,’ vol. 32 (1916), p. 162. 
t ‘ ^ther and Matter,’ p. 104. 
2 K 2 
