ON ELECTRICAL THEORIES 143 
observed by Kerr, and also by the fracture of the dielectric when the 
field is made sufficiently intense. So that whenever an electromotive force 
acts on a dielectric it produces a change in its structure which we shall 
always speak of as polarisation. This, strictly speaking, has only been 
directly proved for electromotive forces produced by charges of statical 
electricity ; but, unless we are prepared to say that the electromotive 
force due to statical electricity is in some way different from that due to 
a changing current, we must admit that when an electromotive force of 
the latter kind acts on a dielectric it polarises it. And we are not with- 
out experimental evidence that the electromotive force due to variations 
in the vector potential does produce some of the effects of the electromo- 
tive force due to a charge of statical electricity. Rowland’s experi- 
ments have shown that a moving electrified body will set a magnet 
placed near to it in motion. It follows from this, by dynamical prin- 
ciples, that if we have the charged body initially at rest and move the 
magnet it will, if no other forces act upon it, be set in motion; so that 
in this case there is an electromotive force due to the motion of the 
magnet, t.e., the variation in the vector potential produces the same 
effect on the electrified body as the electromotive force due to a charge of 
statical electricity. For this reason we shall suppose that the electro- 
motive force due to the variation in the vector potential always produces 
effects on a dielectric on which it acts of the same type as those which 
have been observed to arise from the action of an electromotive force due 
to a charge of statical electricity. 
Let us now consider a magnet surrounded by a dielectric. If we set 
the magnet in motion, we produce an electromotive force which polarises 
the dielectric. Let us, to fix our ideas, consider an element of the dielec- 
tric and the magnet. When the magnet moves it polarises the dielectric ; 
it follows from dynamical principles (an extension of the principle of 
action and reaction),! that if the polarisation of the dielectric be 
altered, the magnet will move, so that a change in the polarisation of a 
dielectric produces a magnetic force. 
Again, let us instead of the magnet consider a coil of wire conveying 
a current. A change in the rate of flow of the current produces a 
change in the polarisation of the dielectric; it follows that a change in 
the rate of change of the polarisation of the dielectric will produce a 
change in the current, i.e., will produce an electromotive force. 
It follows too, from dynamical principles, that as the change in the 
polarisation of an element of the dielectric due to the change in the 
current depends on the distance of the element from the current, there 
must be a force between the current and the element when the polari- 
sation of the latter is changing. Thus we see that a change in the 
polarisation of the dielectric must produce all the effects of an ordinary 
conduction current, so that it is only absolutely necessary to consider 
how the experimental evidence affects those theories which take the 
action of the dielectric into account. As, however, the experiments 
which have been made are few in number, and are all concerned with 
interesting points, we shall consider them in their relation to all the 
theories, and not only to those which take the dielectric into account. 
_* See a paper by the author of this report ‘On some Applications of Dynamical 
Principles to Physical Phenomena,’ Phil. Trans., 1885. 
