*) 
J PRESIDENT’S ADDRESS—-SECTION A. 
induction of currents, and is really an extension of Faraday’s 
law. It is that the line integral of the electric intensity round 
any closed curve is equal to the rate of decrease of the total 
magnetic induction through the curve. The line integral of 
electric intensity taken round a conducting circuit is what we 
are generally accustomed to call the picermmmnnive force acting in 
the circuit. The above principle has been established in many 
ways, in so far as it refers to conductors—but Maxwell supposes 
that it is generally true, whether there be conductors in the 
field or not. This amounts to saying that change of magnetic 
induction can produce electric polarisation without the presence 
of charged bodies at all, and moreover, states the amount of 
pol: RIT which will be produced in any case. As a matter 
of fact, the principle just enunciated is in a sense the converse of 
the pr inciple of the magnetic action of polarisation currents, and 
may be deduced from that principle by the method of Lagrange, 
and so is not really an independent principle at all. Before we 
go any further it will perhaps be as well to give some idea of 
Maxwell’s views as to magnetism ; this is a subject which as far 
as I know has not been much treated by reviewers. The first 
fact which Maxwell always seems to have had before him—at all 
events from the time he considered he discovered that the energy 
of a magnetic field is kinetic—is that the energy in a magnetic 
tield is due to a rotational motion of some kind around the lines 
of magnetic force. This idea he obtained from a consideration of 
the action of magnetic forces on a beam of polarised light. The 
next point was to explain the action between magnets, and this 
was accomplished by imagining a stress in the medium analogous 
to the electric stress in a medium of unit specific inductive 
capacity. In the case of a homogeneous isotropic solid or of 
liquid which is non-magnetisable in the ordinary sense, the stress 
1 
is to amount to a hydrostatic pressure of H? across the 
5 
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lines of force combined with a longitudinal tension of the same 
amount along these lines, H being. the magnetic force. If the 
substance Peach is permeated by the medium is magnetic, a 
distinction arises between the magnetic force in the medium and 
the force in the substance, and we have to take the magnetic 
induction instead of the magnetic force. The expressions, too, 
are complicated, but exact for any medium, magnetisable or not. 
In some speculations as to the cause of the energy and stress in 
the medium, Maxwell considers that the rotatory motion referred 
to is due to the action of ether vortices and the stresses to their 
centrifugal action. Electric currents produce magnetic action as 
well as magnets, consequently we must imagine that current 
action is probably the expression of ether Somte motion, if it be 
admitted that magnetic action is so. Though it would be 
improper to ignore the action of conductors to the extent that I 
