FUNDAMENTAL FORMULATIONS OF ELECTRODYNAMICS. 
223 
This result is consistent with that generally obtained in these theories. The last 
term arising on account of the current due to the convection of the polarisations is 
however probably of kinetic origin. 
Of course, in the general case, all the potential energy put into the field cannot l)e 
got out of it again in the form of useful mechanical work, or,, in other words, it is not 
all available. The effectively available energy in the present case consists In the main 
of the part 
For the magnetic polarisations the results are somewhat difterent. Tn this case the 
kinetic energy of the field is assumed to be 
to which we must add the intrinsic kinetic energy involved in the induced magnetic 
polarisations to obtain the total energy in the field ; reckoned as potential energy the 
intrinsic energy is 
fdv[(BV^I), 
and therefore as kinetic energy it is 
- j(B'rfl), 
giving a total for the field equal to 
= ijrfi-j(BrfH) + iJd»|(E + E„ [<il, rj), 
a result which is again practically equivalent to that usually given in this theor 5 \ 
If we treat the convection of the dielectric polarisation as effectively equivalent to 
a magnetic polarisation of intensity 
r = i [PnJ, 
and the convection of the magnetic polarisation as effectively equivalent to a dielectric 
polarisation of intensity 
p' = - ^ 
2 I 
YOL. CCXX.-A. 
