OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
351 
for the line integral of the magnetic intensity 27 tv a. round the wire is 47 t X current 
through it, and P l=Y. 
But by Ohm’s law Y=tR and iY = rR, or the heat developed according to Joule’s 
law. 
It seems then that none of the energy of a current travels along the wire, but that 
it comes in from the nonconducting medium surrounding the wire, that as soon as it 
enters it begins to be transformed into heat, the amount crossing successive layers of 
the wire decreasing till by the time the centre is reached, where there is no magnetic 
force, and therefore no energy passing, it has all been transformed into heat. A con¬ 
duction current then may be said to consist of this inward How of energy with its 
accompanying magnetic and electromotive forces, and the transformation of the energy 
into heat within the conductor. 
We have now to inquire how the energy travels through the medium on its way to 
the wire. 
(2.) Discharge of a condenser through a wire. 
We shall first consider the case of the slow discharge of a simple condenser consist¬ 
ing of two charged parallel plates when connected by a wire of very great resistance, 
as in this case we can form an approximate idea of the actual path of the energy. 
Fig. 2. 
Let A and B, fig. 2, be the two plates of the condenser, A being positively and B 
negatively electrified. Then before discharge the sections of the equipotential surfaces 
will be somewhat as sketched. The chief part of the energy resides in the part of the 
dielectric between the two plates, but there will be some energy wherever there is 
electromotive intensity. Between A and B the E.M.I. will be from A to B, and every- 
