(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 thus 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+. I have followed the reverse order and deduced the mechanical action from the laws of induction. <s>(18)<\s> I have then <s>explain<\s> 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 the same field with a magnet, I show the distribution of its equipotential magnetic surfaces, cutting the lines of force at right angles. In order to bring these results within the power of symbolical calculation, I then express them in the form of the General Equations of the Electromagnetic Field. These equations express (A) the relation between electric displacement, true conduction and the total current, compounded of both. (B) The relation between the lines of magnetic force and the inductive coefficients of a circuit, as already deduced from the laws of induction. (C) The relation between the strength of a current and its magnetic effects, according to the electromagnetic system of measurement. *Conservation of Force. Physical Society of Berlin, 1847 & Taylors Scientific Memoirs 1853 p 114 +Trans British Association 1848 Phil Mag. Dec 1851
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Manuscript details
 Author
 James Clerk Maxwell
 Reference
 PT/72/7
 Series
 PT
 Date
 1864
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Cite as
J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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