Fig. 149. ELECTRO-MAGNET, HINGED
Electro-magnetic Attraction and Repulsion.
The attraction and repulsion due to electromagnetic lines of force,
which lines always tend to take as short a course as possible and also
seek the medium of the highest permeance. This causes them to
concentrate in iron and steel or other paramagnetic substance and to
draw them towards a magnet by shortening the lines of force connecting
the two. It is exactly the same attraction as that of the permanent
magnet for its armature, Ampére's theory bringing the latter under the
same title. In the case of two magnets like poles repel and unlike
attract. In the case of simple currents, those in the same direction
attract and those in opposite directions repel each other. This refers
to constant current reactions. Thus the attraction of unlike poles of
two magnets is, by the Ampérian theory, the attraction of two sets of
currents of similar direction, as is evident from the diagram. The
repulsion of like poles is the repulsion of unlike currents and the same
applies to solenoids, q. v. (See Magnetism and do. Ampére's Theory
of--Induction, Electro-dynamic--Electro-magnetic Induction.)
218 STANDARD ELECTRICAL DICTIONARY.
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 When withdrawn, in compliance with the natural tendency of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.
219 STANDARD ELECTRICAL DICTIONARY.
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.
220 STANDARD ELECTRICAL DICTIONARY.
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.
221 STANDARD ELECTRICAL DICTIONARY.
Fig. 150 HUGHES' POLARIZED ELECTRO-MAGNET
Electro-magnets, Interlocking.
Electro-magnets so arranged that their armatures interlock. Thus two
magnets, A A and B B, may be placed with their armatures, M and N, at
right angles and both normally pulled away from the poles. When the
armature M is attracted a catch on its end is retained by a hole in the
end of the other armature N, and when the latter armature N is attracted
by its magnet the armature M is released. In the mechanism shown in the
cut the movements of the wheel R are controlled. Normally it is held
motionless by the catch upon the bottom of the armature M, coming
against the tooth projecting from its periphery. A momentary current
through the coils of the magnet A A releases it, by attracting M, which
is caught and retained by N, and leaves it free to rotate. A momentary
current through the coils of the magnet B B again releases M, which
drops down and engages the tooth upon R and arrests its motion.
Fig. 151. INTERLOCKING ELECTRO-MAGNETS.
222 STANDARD ELECTRICAL DICTIONARY.
Electro-magnet, Stopped Coil.
An electro-magnet consisting of a tubular coil, in which a short fixed
core is contained, stopping up the aperture to a certain distance, while
the armature is a plunger entering the aperture. This gives a longer
range of action than usual.
Electro-magnet, Surgical.
An electro-magnet, generally of straight or bar form, fitted with
different shaped pole pieces, used for the extraction of fragments of
iron or steel from the eyes. Some very curious cases of successful
operations on the eyes of workmen, into whose eyes fragments of steel or
iron had penetrated, are on record.
Electro-medical Baths.
A bath for the person provided with connections and electrodes for
causing a current of electricity of any desired type to pass through the
body of the bather. Like all electro-therapeutical treatment, it should
be administered under the direction of a physician only.
Electro-metallurgy.
(a) In the reduction of ores the electric current has been proposed but
never extensively used, except in the reduction of aluminum and its
alloys. (See Reduction of Ores, Electric.)
(b) Electro-plating and deposition of metal from solutions is another
branch. (See Electroplating and Electrotyping.)
(c) The concentration of iron ores by magnetic attraction may come under
this head. (See Magnetic Concentration of Ores.)
Electrometer.
An instrument for use in the measurement of potential difference, by the
attraction or repulsion of statically charged bodies. They are
distinguished from galvanometers as the latter are really current
measurers, even if wound for use as voltmeters, depending for their
action upon the action of the current circulating in their coils.
Electrometer, Absolute.
An electrometer designed to give directly the value of a charge in
absolute units. In one form a plate, a b, of conducting surface is
supported or poised horizontally below a second larger plate C, also of
conducting surface. The poised plate is surrounded by a detached guard
ring--an annular or perforated plate, r g r' g'--exactly level and even
with it as regards the upper surface. The inner plate is carried by a
delicate balance. In use it is connected to one of the conductors and
the lower plate to earth or to the other. The attraction between them is
determined by weighing. By calculation the results can be made absolute,
as they depend on actual size of the plates and their distance, outside
of the potential difference of which of course nothing can be said. If S
is the area of the disc, d the distance of the plates, V-V1 the
difference of their potential, which is to be measured, and F the force
required to balance their attraction, we have:
F = ( ( V - V1 )^2 * S ) / ( 8 * PI * d^2 )
223 STANDARD ELECTRICAL DICTIONARY.
If V = 0 this reduces to
F = ( V^2 * S ) / ( 8 * PI * d^2 ) (2)
or
V = d * SquareRoot( (8 * PI * F ) / S ) (3)
As F is expressed as a weight, and S and a as measures of area and
length, this gives a means of directly obtaining potential values in
absolute measure. (See Idiostatic Method--Heterostatic Method.)
Synonyms--Attracted Disc Electrometer--Weight Electrometer.
Fig. 152. SECTION OF BASE OF PORTABLE ELECTROMETER.
In some forms the movable disc is above the other, and supported at the
end of a balance beam. In others a spring support, arranged so as to
enable the attraction to be determined in weight units, is adopted. The
cuts, Figs. 152 and 154, show one of the latter type, the portable
electrometer. The disc portion is contained within a cylindrical vessel.
Fig. 153. DIAGRAM ILLUSTRATING
THEORY OF ABSOLUTE ELECTROMETER.
Referring to Fig. 152 g is the stationary disc, charged through the
wire connection r; f is the movable disc, carried by a balance beam
poised at i on a horizontal and transverse stretched platinum wire,
acting as a torsional spring. The position of the end k of the balance
beam shows when the disc f is in the plane of the guard ring h h. The
end k is forked horizontally and a horizontal sighting wire or hair is
fastened across the opening of the fork. When the hair is midway between
two dots on a vertical scale the lever is in the sighted position, as it
is called, and the disc is in the plane of the guard ring.
224 STANDARD ELECTRICAL DICTIONARY.