464 PROFESSOR CLERK MAXWELL OX TJIE ELECTROMAGNETIC FIELD. 
same substances, if dielectrics, may possess the analogous electrical property, and if 
magnetic, may have corresponding properties relating to the acquisition, retention, and 
loss of magnetic polarity. 
(15) It appears therefore that certain phenomena in electricity and magnetism lead 
to the same conclusion as those of optics, namely, that there is an sethereal medium 
pervading all bodies, and modified only in degree by their presence ; that the parts of 
this medium are capable of being set in motion by electric currents and magnets ; that 
this motion is communicated from one part of the medium to another by forces arising 
from the connexions of those parts ; that under the action of these forces there is a 
certain yielding depending on the elasticity of these connexions ; and that therefore 
energy in two different forms may exist in the medium, the one form being the actual 
energy of motion of its parts, and the other being the potential energy stored up in the 
connexions, in virtue of their elasticity. 
(16) Thus, then, we are led to the conception of a complicated mechanism capable 
of a vast variety of motion, but at the same time so connected that the motion of one 
part depends, according to definite relations, on the motion of other parts, these motions 
being communicated by forces arising from the relative displacement of the connected 
parts, in virtue of their elasticity. Such a mechanism must be subject to the general 
laws of Dynamics, and we ought to be able to work out all the consequences of its 
motion, provided we know the form of the relation between the motions of the parts. 
(17) We know that when an electric current is established in a conducting circuit, 
the neighbouring part of the field is characterized by certain magnetic properties, and 
that if two circuits are in the field, the magnetic properties of the field due to the two 
currents are combined. Thus each part of the field is in connexion with both currents, 
and the two currents are put in connexion with each other in virtue of their con- 
nexion with the magnetization of the field. The first result of this connexion that 1 
propose to examine, is the induction of one current by another, and by the motion of 
conductors in the field. 
The second result, which is deduced from this, is the mechanical action between con- 
ductors carrying currents. The phenomenon of the induction of currents has been 
deduced from their mechanical action by Helmholtz* and Thomson f. I have followed 
the reverse order, and deduced the mechanical action from the laws of induction. I 
have then described experimental methods of determining the quantities L, M, N, on 
which these phenomena depend. 
(18) I then apply the phenomena of induction and attraction of currents to the 
exploration of the electromagnetic field, and the laying down systems of lines of mag- 
netic force which indicate its magnetic properties. By exploring the same field with a 
magnet, I show the distribution of its equipotential magnetic surfaces, cutting the lines 
of force at right angles. 
* “ Conservation of Force,” Physical Society of Berlin, 1847 ; and Taylor’s Scientific Memoirs, 1853, 
p. 114. 
f Reports of the British Association, 1848; Philosophical Magazine, Dec. 1851. 
