FUNDAMENTAL FORMULATIONS OF ELECTRODYNAMICS. 
2II 
expression for the rate of change of the magnetic force vector at any point of the 
held. The first of these has the usual form 
c = 
dt 
+ P>'m + Ci, 
wherein is the density of the true conduction current, is the velocity of the material 
medium at the typical field point, and 
D = —E + P 
Att 
is the total dielectric displacement of Maxwell’s theory which consists in part of the 
dielectric polarisation P and in part of an sethereal constituent proportional to the 
electric force. 
The time rate of change of the composite vector D requires careful specification ; it 
consists in the main of the terms 
Att dt dt ’ 
but when the dielectric media are in motion there is in addition a term arising on 
account of the convection of the polarisation. This term has been shown* to be 
equal to 
Curl [Pv], 
SO that 
d^ 
dt 
1 (iE dP p 1 rp -] 
The equation expressing the rate of change of the magnetic 
dt 
^ —Att^—Att Curl [Ipm]. 
dt dt 
This latter equation must be specially emphasised as it has apparently never yet 
been explicitly introduced in the theory, although it is necessary to secure greater 
consistency in the dynamical theory. 
of the magnetic polarisation at a fixed point in the field only when the magnetic 
media as a whole are at rest. When these media are in motion there will be a 
contribution to this rate due to convection just as in the electric case, and the 
argument for its exact form may be developed on the same lines. The vectors B and 
H are, so to speak, attached to the aether, just as were the vectors D and E,t whilst 
* Cf. my ‘ Theory of Electricity,’ pp. 365-367, or Larmor, ‘ .^ther and Matter,’ Chap. IV. 
t The vector H being the.composite vector of the magnetic theory is analogous to the vector D of the 
electric theory; the {ethereal vector B is analogous to the aethereal vector E. This is the reverse of the 
usual convention, but see below § 10. 
