462 PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD. 
field. When a body is moved across the lines of magnetic force it experiences what is 
called an electromotive force ; the two extremities of the body tend to become oppo- 
sitely electrified, and an electric current tends to flow through the body. When the 
electromotive force is sufficiently powerful, and is made to act on certain compound 
bodies, it decomposes them, and causes one of their components to pass towards one 
extremity of the body, and the other in the opposite direction. 
Here we have evidence of a force causing an electric current in spite of resist- 
ance; electrifying the extremities of a body in opposite ways, a condition which is 
sustained only by the action of the electromotive force, and which, as soon as that force 
is removed, tends, with an equal and opposite force, to produce a counter current through 
the body and to restore the original electrical state of the body ; and finally, if strong 
enough, tearing to pieces chemical compounds and carrying their components in oppo- 
site directions, while their natural tendency is to combine, and to combine with a force 
which can generate an electromotive force in the reverse direction. 
This, then, is a force acting on a body caused by its motion through the electro- 
magnetic field, or by changes occurring in that field itself ; and the effect of the force is 
either to produce a current and heat the body, or to decompose the body, or, when it 
can do neither, to put the body in a state of electric polarization, — a state of constraint 
in which opposite extremities are oppositely electrified, and from which the body tends 
to relieve itself as soon as the disturbing force is removed. 
(10) According to the theory which. I propose to explain, this “electromotive force” 
is the force called into play during the communication of motion from one part of the 
medium to another, and it is by means of this force that the motion of one part causes 
motion in another part. When electromotive force acts on a conducting circuit, it pro- 
duces a current, which, as it meets with resistance, occasions a continual transformation 
of electrical energy into heat, which is incapable of being restored again to the form of 
electrical energy by any reversal of the process. 
(11) But when electromotive force acts on a dielectric it produces a state of polari- 
zation of its parts similar in distribution to the polarity of the parts of a mass of iron 
under the influence of a magnet, and like the magnetic polarization, capable of being 
described as a state in which every particle has its opposite poles in opposite con- 
ditions *. 
In a dielectric under the action of electromotive force, we may conceive that the 
electricity in each molecule is so displaced that one side is rendered positively and the 
other negatively electrical, but that the electricity remains entirely connected with the 
molecule, and does not pass from one molecule to another. The effect of this action on 
the whole dielectric mass is to produce a general displacement of electricity in a cer- 
tain direction. This displacement does not amount to a current, because when it has 
attained to a certain value it remains constant, but it is the commencement of a current, 
and its variations constitute currents in the positive or the negative direction according 
* Faeaday, Exp. Res. Series XI. ; Mossotti, Mem. della Soc. Italiana (Modena), vol. xxiv. part 2. p. 49. 
