<s>Electrical Quantity (e) Let e represent the quantity of free electricity in unit of volume (either positive or negative) then the equation of continuity is e + df/dx + dg/dy + dh/dz = 0 (5) <\s> Electromotive force (P Q R) (56) Let P, Q, R represent the components of the Electromotive Force at any point. Then P represents the difference of potential per unit of length in a conductor placed in the direction of <u>x<\u> at the given point. We may suppose an indefinitely short wire placed parallel to x at the given point and touched, during the action of the force P by two small conductors which are then insulated and removed from the influence of the electromotive force. The value of P might then be ascertained by measuring the charges of the conductors. Thus if l be the length of the wire the difference of potential of its ends will be Pl and if C be the capacity of each of the small conductors the charge on each will be [half]CPl. Since the capacities of moderately large conductors, measured on the electromagnetic system are exceedingly small, ordinary electromotive forces arising from electromagnetic actions could hardly be measured in this way. In practice such measurements are always made with long conductors forming closed or nearly closed circuits. Electromagnetic Momentum (F G H) (57) Let F G H represent the components of Electromagnetic Momentum at any point of the field, due to any system of magnets or currents. Then F is the total impulse of the electromotive force that would be generated by the removal of these magnets or currents from the field, that is if P be the electromotive force at any
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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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