instant during the removal of the system F = [integral]Pdt Hence the part of the electromotive force which depends on the motion of magnets or currents in the field or their alteration of intensity is P =  dF/dt Q =  dG/dt R =  dH/dt (29) Electromagnetic Momentum of a Circuit (58) Let s be the length of the circuit, then if we integrate [integral](F dx/ds + G dy/ds + H dz/ds) ds (30) round the circuit we shall get the total electromagnetic momentum of the circuit, or the number of lines of magnetic force which pass through it the variations of which measure the total electromotive force in the circuit. This Electromagnetic momentum is the same thing to which Prof Faraday has applied the name of the Electronic State If the circuit be the boundary of the elementary area dy dz then its electromagnetic momentum is (dH/dy  dG/dz)dy dz and this is the number of lines of magnetic force which pass through the area dy dz. Magnetic Force ([alpha] [beta] [gamma]) (59) Let [alpha], [beta], [gamma] represent the force acting on a unit magnetic pole placed at the given point resolved in the direction of x, y and z. Coefficient of Magnetic Induction ([mu]) (60) Let [mu] be the ratio of the magnetic induction in a given medium So that in air under an equal magnetizing force, then the number of lines of force in unit of area perpendicular to x will be [mu][alpha] ([mu] is a quantity depending on the nature of the medium, its temperature the amount of magnetization already produced and in crystalline bodies varying with the direction)
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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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