1. electro-magnetic induction
When two currents of unlike direction are brought towards each other, against their natural repulsive tendency work is done, and the consequent energy takes the form of a temporary increase in both currents. When withdrawn, in compliance with the natural tendency of repulsion, the currents are diminished in intensity, because energy is not expended on the withdrawal, but the withdrawal is at the expense of the energy of the system. The variations thus temporarily produced in the currents are examples of electro-magnetic induction. The currents have only the duration in each case of the motion of the circuits. One circuit is considered as carrying the inducer current and is termed the primary circuit and its current the primary current, the others are termed the secondary circuit and current respectively. We may assume a secondary circuit in which there is no current. It is probable that there is always an infinitely small current at least, in every closed circuit. Then an approach of the circuits will induce in the secondary an instantaneous current in the reverse direction. On separating the two circuits a temporary current in the same direction is produced in the secondary.
A current is surrounded by lines of force. The approach of two circuits, one active, involves a change in the lines of force about the secondary circuit. Lines of force and current are so intimately connected that a change in one compels a change in the other. Therefore the induced current in the secondary may be attributed to the change in the field of force in which it lies, a field maintained by the primary circuit and current. Any change in a field of force induces a current or change of current in any closed circuit in such field, lasting as long as the change is taking place. The new current will be of such direction as to oppose the change. (See Lenz's Law.)
The action as referred to lines of force may be figured as the cutting of such lines by the secondary circuit, and such cutting may be brought about by moving the secondary in the field. (See Lines of Force--Field of Force.) The cutting of 1E8 lines of force per second by a closed circuit induces an electro-motive force of one volt. (See Induction, Mutual, Coefficient of.)
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Definitions of electro-magnetic induction
