A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 533 



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 con- 

 nexions, 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 connexion with the magnetization of the field. 

 The first result of this connexion that I 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 conductors carrying currents. The phenomenon of the induction of 

 currents has been deduced from their mechanical action by Helmholtz * and 

 Thomson t. 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 magnetic force which indicate its magnetic properties. By exploring 



* "Conservation of Force," Physical Society of Berlin, 1847; and Taylor's Scientific Memoirs, 1853, 

 p. 114. 



t Reports of the British Association, 1848; Philosophical Magazine, Dec. 1851. 



