Again, the relation between the electrostatic and electro-magnet units
of quantity is expressed by 1 : 30,000,000,000; the latter figure in
centimeters gives approximately the velocity of light. The
electro-magnetic unit depending on electricity in motion should have
this precise relation if an electro-magnetic disturbance was propagated
with the velocity of light. If an electrically charged body were whirled
around a magnetic needle with the velocity of light, it should act in
the same way as a current circulating around it. This effect to some
extent has been shown experimentally by Rowland.
A consequence of these conclusions is (Maxwell) that the specific
inductive capacity of a non-conductor or dielectric should be equal to
the square of its index of refraction for waves of infinite length. This
is true for some substances--sulphur, turpentine, petroleum and benzole.
In others the specific inductive capacity is too high, e. g., vegetable
and animal oils, glass, Iceland spar, fluor spar, and quartz.
Electro-magnetic Unit of Energy.
A rate of transference of energy equal to ten meg-ergs per second.
Electro-magnetism.
The branch of electrical science treating of the magnetic relations of a
field of force produced by a current, of the reactions of
electro-magnetic lines of force, of the electromagnetic field of force,
of the susceptibility, permeability, and reluctance of diamagnetic and
paramagnetic substances, and of electro-magnets in general.
Electro-magnet, Long Range.
An electro-magnet so constructed with extended pole pieces or otherwise,
as to attract its armature with reasonably constant force over a
considerable distance. The coil and plunger, q. v., mechanisms
illustrate one method of getting an extended range of action. When a
true electro-magnet is used, one with an iron core, only a very limited
range is attainable at the best. (See Electro-magnet, Stopped Coil--do.
Plunger.)
Electro-magnet, Plunger.
An electro-magnet with hollow coils, into which the armature enters as a
plunger. To make it a true electro-magnet it must have either a yoke,
incomplete core, or some polarized mass of iron.
Electro-magnet, Polarized.
An electro-magnet consisting of a polarized or permanently magnetized
core wound with magnetizing coils, or with such coils on soft iron cores
mounted on its ends. The coils may be wound and connected so as to
cooperate with or work against the permanent magnet on which it is
mounted. In Hughes' magnet shown in the cut it is mounted in opposition,
so that an exceedingly feeble current will act to displace the armature,
a, which is pulled away from the magnet by a spring, s.