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XY. On the Transfer of Energy in the Electromagnetic Field. 
By J. H. Poynting, M.A., late Fellow of Trinity College, Cambridge, Professor of 
Physics, Mason College, Birmingham. 
Communicated by Lord Rayleigh, M.A., D.C.L., F.R.S. 
Received December 17, 1883,—Read January 10, 1884. 
A space containing electric currents may be regarded as a field where energy is trans¬ 
formed at certain points into the electric and magnetic kinds by means of batteries, 
dynamos, thermoelectric actions, and so on, while in other parts of the field this energy 
is again transformed into heat, work done by electromagnetic forces, or any form of 
energy yielded by currents. Formerly a current was regarded as something travelling 
along a conductor, attention being chiefly directed to the conductor, and the energy 
which appeared at any part of the circuit, if considered at all, was supposed to be con¬ 
veyed thither through the conductor by the current. But the existence of induced 
currents and of electromagnetic actions at a distance from a primary circuit from 
which they draw their energy, has led us, under the guidance of Faraday and 
Maxwell, to look upon the medium surrounding the conductor as playing a very 
important part in the development of the phenomena. If we believe in the continuity 
of the motion of energy, that is, if we believe that when it disappears at one point 
and reappears at another it must have passed through the intervening space, we are 
forced to conclude that the surrounding medium contains at least a part of the energy, 
and that it is capable of transferring it from point to point. 
Upon this basis Maxwell has investigated what energy is contained in the 
medium, and he has given expressions which assign to each part of the field a quantity 
of energy depending on the electromotive and magnetic intensities and on the 
nature of the matter at that part in regard to its specific inductive capacity and 
magnetic permeability. These expressions account, as far as we know, for the whole 
energy. According to Maxwell’s theory, currents consist essentially in a certain 
distribution of energy in and around a conductor, accompanied by transformation and 
consequent movement of energy through the field. 
Starting with Maxwell’s theory, we are naturally led to consider the problem, 
How does the energy about an electric current pass from point to point—that is, by 
what paths and according to what law does it travel from the part of the circuit where 
