278 
PROFESSOR J. H. POYNTING- ON ELECTRIC CURRENT AND THE 
where the electrical level surfaces cut the wire perpendicularly to the axis, it appears 
that the energy dissipated in the wire as heat comes in from the surrounding medium, 
entering perpendicularly to the surface. 
In that paper I made no assumption as to the transfer of the electric and magnetic 
inductions—the electric and magnetic conditions—through the medium, merely con¬ 
sidering the movement of energy. I now propose to develop a hypothesis as to the 
transfer of the inductive condition in the medium, and its movement inwards upon 
current-bearing wires. 
The value of the electric induction at any point in an isotropic medium is equal to 
K X RT./47T, and the direction of the induction coincides with that of the intensity. 
Maxwell terms this electric induction “displacement,” but I think that “induction” 
is preferable, as it implies no hypothesis beyond that of some alteration in the medium, 
which can be described by a vector. The value of the magnetic induction is equal to 
p, X M.I., and its direction coincides with that of the magnetic intensity. 
If we symbolise the electric and magnetic conditions of the field by induction tubes 
running in the directions of the intensities, the tubes being supposed drawn in each 
case so that the total induction over a cross ; section is unity, then we have reason to 
suppose that the electric tubes are continuous except where there are electric charges, 
while the magnetic tubes are probably in all cases continuous and re-entrant. 
In the neighbourhood of a wire containing a current, the electric tubes may in 
general be taken as parallel to the wire while the magnetic tubes encircle it. The 
hypothesis I propose is that the tubes move in upon the wire, their places being 
supplied by fresh tubes sent out from the seat of the so-called electromotive force. 
The change in the point of view involved in this hypothesis consists chiefly in this, 
that induction is regarded as being propagated sideways rather than along the tubes 
or lines of induction. This seems natural if we are correct in supposing that the 
energy is so propagated, and if we therefore cease to look upon current as merely 
something travelling along the conductor carrying it, and in its passage affecting the 
surrounding medium. As we have no means of examining the medium, to observe 
what goes on there, but have to be content with studying what takes place in 
conductors bounded by the medium, the hypothesis is at present incapable of 
verification. Its use, then, can only be justified if it accounts for known facts better 
than any other hypothesis. 
The basis of Maxwell’s Electromagnetic Theory. 
Maxwell’s Electromagnetic Theory rests on three general principles. 
I. The first principle consists in the assumption that energy has position, i.e., that 
it occupies space. The electric and magnetic energies of an electromagnetic system 
reside therefore somewhere in the field. It is an inevitable conclusion that they are 
present wherever the electric and magnetic intensities can be shown to exist. For 
