# Page:A Dynamical Theory of the Electromagnetic Field.pdf/26

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PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD.

will be increased by the following increments,

$a\left({\frac {dF}{dx}}{\frac {dx}{dt}}+{\frac {dF}{dy}}{\frac {dy}{dt}}+{\frac {dF}{dz}}{\frac {dz}{dt}}\right)$ , due to motion of conductor

$-a{\frac {ds}{dt}}\left({\frac {dF}{dx}}{\frac {dx}{ds}}+{\frac {dG}{dx}}{\frac {dy}{ds}}+{\frac {dH}{dx}}{\frac {dz}{ds}}\right)$ , due to lenghtening of circuit.

The total increment will therefore be

$a\left({\frac {dF}{dy}}-{\frac {dG}{dx}}\right){\frac {dy}{dt}}-a\left({\frac {dH}{dx}}-{\frac {dF}{dz}}\right){\frac {dz}{dt}};$ or, by the equations of Magnetic Force (B),

$-a\left(\mu \gamma {\frac {dy}{dt}}-\mu \beta {\frac {dz}{dt}}\right)$ If P is the electromotive force in the moving conductor parallel to $x$ referred to unit of length, then the actual electromotive force is P$a$ ; and since this is measured by the decrement of the electromagnetic momentum of the circuit, the electromotive force due to motion will be

 $P=\mu \gamma {\frac {dy}{dt}}-\mu \beta {\frac {dz}{dt}}$ (36)

(65) The complete equations of electromotive force on a moving conductor may now be written as follows:—

Equations of Electromotive Force.

 $\left.{\begin{array}{l}P=\mu \left(\gamma {\frac {dy}{dt}}-\beta {\frac {dz}{dt}}\right)-{\frac {dF}{dt}}-{\frac {d\Psi }{dx}},\\\\Q=\mu \left(\alpha {\frac {dz}{dt}}-\gamma {\frac {dx}{dt}}\right)-{\frac {dG}{dt}}-{\frac {d\Psi }{dy}},\\\\R=\mu \left(\beta {\frac {dx}{dt}}-\alpha {\frac {dy}{dt}}\right)-{\frac {dH}{dt}}-{\frac {d\Psi }{dz}}.\end{array}}\right\}$ (D)

The first term on the right-hand side of each equation represents the electromotive force arising from the motion of the conductor itself. This electromotive force is perpendicular to the direction of motion and to the lines of magnetic force; and if a parallelogram be drawn whose sides represent in direction and magnitude the velocity of the conductor and the magnetic induction at that point of the field, then the area of the parallelogram will represent the electromotive force due to the motion of the conductor, and the direction of the force is perpendicular to the plane of the parallelogram.

The second term in each equation indicates the effect of changes in the position or strength of magnets or currents in the field.

The third term shows the effect of the electric potential It has no effect in causing a circulating current in a closed circuit. It indicates the existence of a force urging the electricity to or from certain definite points in the field. 