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XVII
FUNDAMENTAL EQUATIONS OF ELECTROMAGNETICS
f(cos
moments of the magnets per unit length measured in electro-
magnetic units, and of the integral (cos e/r)dede', where de,
de' denote elements of the axes of the magnets, e the inclina-
tion of these elements to one another.
The potential thus determined is of the same form as the
mutual potential of electric currents, and therefore represents
the same actions. Two ring-magnets, which are placed close
together and side by side, will attract each other at the moment
when they both lose their magnetism, if they are magnetised
in the same direction; they will repel each other if oppositely
magnetised. In the usual¹ electromagnetics this action is
missing. To describe it more simply we shall introduce a
new name. We call the change of magnetic polarisation a
magnetic current, and take as unit that magnetic current in-
tensity which corresponds to unit change per unit time of the
polarisation per unit voluine measured in absolute magnetic
units. So far as we can conclude from the phenomena of
unipolar induction as yet known, magnetic poles distributed
continuously along a closed curve and moving along it exert
the same electromagnetic action at outside points as a ring-
magnet coinciding with that curve and of suitably changing
moment. If this relation can be looked upon as true in
general, the name "magnetic current" includes all the different
cases of magnetism in motion; and we may speak of constant
magnetic currents just as we speak of constant electric cur-
rents. But here that name is only to be regarded as a simple
contraction for "changing polarisation." Our result may now
be stated in this form :-Magnetic currents act on each other
according to the same laws as electric currents; in absolute
magnitude the action between magnetic currents of S magnetic
units is equal to that between electric currents of S electrical
units.
This theorem may not be capable of experimental
verification. It may be possible to show that electrically
charged bodies are set in motion by a ring-magnet whose
moment is diminishing; perhaps even that the magnet itself
is turned by electrostatic force so that its plane sets itself
1 By usual I mean, here and in what follows, that electromagnetics which
regards the forces deduced from Neumann's laws of the potential as exactly
applicable, even when we consider the attraction of variable currents. Every
such system of electromagnetics is necessarily opposed to Maxwell's.