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.)
Electro-magnet, Iron Clad.
A magnet whose coil and core are encased in a iron jacket, generally
connected to one end of the core. This gives at one end two poles, one
tubular, the other solid, and concentric with each other. It is
sometimes called a tubular magnet.
Electro-magnet, One Coil.
An electro-magnet excited by one coil. In some dynamos the field magnets
are of this construction, a single coil, situated about midway between
the poles, producing the excitation.
Electro-magnetic Leakage.
The leakage of lines of force in an electro-magnet; the same as magnetic
leakage. (See Magnetic Leakage.)
Electro-magnetic Lines of Force.
The lines of force produced in an electro-magnetic field. They are
identical with Magnetic Lines of Force, q. v. (See also Field of
Force-Line of Force.)
Electro-magnetic Stress.
The stress in an electro-magnetic field of force, showing itself in the
polarization of light passing through a transparent medium in such a
field. (See Magnetic Rotary Polarization.)
Electro-magnetic Theory of Light.
This theory is due to J. Clark Maxwell, and the recent Hertz experiments
have gone far to prove it. It holds that the phenomena of light are due
to ether waves, identical in general factors with those produced by
electro-magnetic induction of alternating currents acting on the ether.
In a non-conductor any disturbance sets an ether wave in motion owing to
its restitutive force; electricity does not travel through such a
medium, but can create ether waves in it. Therefore a non-conductor of
electricity is permeable to waves of ether or should transmit light, or
should be transparent. A conductor on the other hand transmits
electrical disturbances because it has no restitutive force and cannot
support an ether wave. Hence a conductor should not transmit light, or
should be opaque. With few exceptions dielectrics or non-conductors are
transparent, and conductors are opaque.