Electro-magnetic Control.
Control of a magnet, iron armature, or magnetic needle in a
galvanometer, ammeter, voltmeter or similar instrument by an
electro-magnetic field, the restitutive force being derived from an
electro-magnet. The restitutive force is the force tending to bring the
index to zero.
Electro-magnetic Field of Force.
A field of electro-magnetic lines of force, q. v., established through
the agency of an electric current. A wire carrying a current is
surrounded by circular concentric lines of force which have the axis of
the wire as the locus of their centres. Electro-magnets produce lines of
force identical with those produced by permanent magnets. (See Field of
Force--Magnetic Field of Force--Controlling Field--Deflecting Field.)
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 When withdrawn, in compliance with the natural tendency of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.