150 rEPoRT—1885. 
be an experimental investigation of the question of the continuity of these 
dielectric currents; we have experimental proof that they exist, but we | 
do not know whether Maxwell’s assumption that they always form closed 
circuits with the other currents is true or not. If Maxwell’s assumption 
should turn out to be true, we should have a complete theory of electrical 
action; if, on the other hand, it should turn out to be wrong, then we 
should have to go on to determine the quantity /. This quantity is diffi- 
cult to determine, as its influence on all closed circuits disappears. It 
influences, as v. Helmholtz has shown, the rate of propagation of the 
electric potential along conducting wires, and I think we can see that it 
would influence the time of oscillation of an irregular distribution of elec- 
tricity over a conducting shell. The easiest way, however, of determin- 
ing this quantity would seem to be the straightforward one of measuring 
electrostatically the value of the electromotive force due to a variation 
in the charge of a condenser; the expression for the vector potential, as 
we saw on p. 140, involves /, so that if we measure the electromotive 
force, which is equal to the rate of variation of the vector potential, we 
shall determine the value of the vector potential, and consequently of k. 
AppEeNnpDIx I. 
Since the Report was written I have had through the kindness of the 
author an opportunity of seeing the advance proofs of a paper by Pro- 
fessor J. H. Poynting, of Mason’s College, Birmingham, ‘ On the Connexion 
between Electric Current and the Electric and Magnetic Induction in 
the Surrounding Medium,’ which is about to appear in the ‘ Philosophical — 
Transactions.’ 
The views expressed in this paper are rather a new way of looking at 
Faraday and Maxwell’s theory than a new theory of electrodynamic 
action, as however it brings the action of the dielectric into great 
prominence it is instructive to consider it. 
The paper is largely based on a previous one by the same author on 
the ‘ Transference of Energy in the Electromagnetic Field,’! it is therefore 
necessary to give a brief account of this paper. 
In it the author shows that the rate of increase of the energy inside 
any closed surface equals 
EL [|e — Ox) + mOP’— aR) +m (a! — BPD} AS, 
where dS is an element of surface, /, m,n the direction cosine of the 
normal te dS, a, 2, y the components of magnetic induction, and 
P’, Q’, R’ given by the following equations :— 
dk db 
em 
fsa ge Oe 
a dt dy’ 
dH dy) 
ey erncab os! 
1 Phil. Trans., 1884, part ii 
