OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
361 
any point in the direction of x is —v 3 £ where £ is the component of the true displace¬ 
ment (Phil, Mag., June, 1881). It seems to me then that our use of the term is 
somewhat unfortunate, as suggesting to our minds so much that is unverified or false, 
while it is so difficult to bear in mind how little it really means. 
I have therefore given several cases in considerable detail of the application of the 
mode of transfer of energy in current-bearing circuits according to the law given 
above, as I think it is necessary that we should realise thoroughly that if we accept 
Maxwell’s theory of energy residing in the medium, we must no longer consider 
a current as something conveying energy along the conductor. A current in a 
conductor is rather to be regarded as consisting essentially of a convergence of 
electric and magnetic energy from the medium upon the conductor and its transforma¬ 
tion there into other forms. The current through a seat of so-called electromotive 
force consists essentially of a divergence of energy from the conductor into the 
medium. The magnetic lines of force are related to the circuit in the same way 
throughout, while the lines of electric force are in opposite directions in the two parts 
of the circuit,—with the so-called current in the conductor, against it in the seat of 
electromotive force. It follows that the total E.M.I. round the circuit with a steady 
current is zero, or the work done in carrying a unit of positive electricity round the 
circuit with the current is zero. For work is required to move it against the E.M.I. 
in the seat of energy, this work sending energy out into the medium, while an equal 
amount of energy comes in in the rest of the circuit where it is moving with the E.M.I. 
This mode of regarding the relations of the various parts of the circuit is, I am aware, 
very different from that usually given, but it seems to me to give us a better account 
of the known facts. 
It may seem at first sight that we ought to have new experimental indications of 
this sort of movement of energy, if it really takes place. We should look for proofs at 
points where the energy is transformed into other modifications, that is, in conductors. 
Now in a conductor, when the field is in a steady state, there is no electromotive 
intensity, and therefore no motion and no transformation of energy. The energy 
merely streams round the outside of the conductor, if in motion at all in its neighbour¬ 
hood. If the field is changing energy can pass into the conductor, as there may be 
temporary E.M.I. set up within it, and there will be transformation. But we already 
know the nature of this transformation, for it constitutes the induced current. Indeed, 
the fundamental equation describing the motion of energy is only a deduction from 
Maxwell’s equations, which are formed so as to express the experimental facts as far 
as yet known. Among these are the law^s of induction in secondary circuits, and they 
must therefore agree with the law of transfer. We can hardly hope, then, for any 
further proof of the law beyond its agreement with the experiments already known 
until some method is discovered of testing what goes on in the dielectric independently 
of the secondary circuit. 
3 A 
MDCCCLXXX1V. 
