Coefficients of Induction for Two Circuits 17. (26) In the electromagnetic field the values of L M N depend on the distribution of the magnetic effects due to the two circuits and this distribution depends only on the form and relative position of the circuits. Hence L M N are quantities depending on the form and relative position of the circuits and are subject to variation <s>when<\s> with the motion of the conductors It will be presently seen that L, M, N are geometrical quantities of the nature of lines, that is, of one dimension in space L depends on the form of the first conductor which we shall call A N on that of the second, which we shall call B, and M on the relative position of A & B (27) Let [xi] be the electromotive force acting on A <s>and<\s> x the strength of the current and R the resistance, then Rx will be the resisting force. In steady currents the electromotive force just balances the resisting force, but in variable currents the resultant force [xi] - Rx is expended in increasing the “electromagnetic momentum” using the word momentum merely to express that which is generated by a force acting during a certain time, that is, a velocity existing in a body. In the case of electric currents, the force in action is not ordinary mechanical force, at least we are not as yet able to measure it as common force, but we call it electromotive force and the body moved is not merely the electricity in the conductor but something outside the conductor capable of being affected by other conductors in the neighbourhood carrying currents. In this it resembles rather the reduced momentum of a driving point of a machine as influenced by its mechanical connexions, than that of a simple moving body like a cannon ball or water in a tube.
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J. C. Maxwell’s, ‘Dynamical theory of the electromagnetic field’, 1864. From The Royal Society, PT/72/7
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