Fig. 221. ONE-HALF PORTION OF A DOUBLE MAGNETIC CIRCUIT.
Magnetic Concentration of Ores.
The concentration of ores or the freeing them from their gangue by
magnetic attraction. It is only applicable to those cases in which
either the ore itself or the gangue is attracted by the magnet. Its
principal application is to the concentration of magnetic iron sands.
(See Magnetic Concentration.)
Magnetic Concentrator.
An apparatus similar to a magnetic separator, q. v., but used to
concentrate magnetic iron sands. By the action of electro-magnets the
magnetic iron sand (magnetite) is separated from the sand with which it
is mixed.
Magnetic Conductivity and Conductance.
The first notion of permeance and of the magnetic circuit included the
idea of magnetic conductivity, which conducted lines of force urged by
magneto-motive force through a magnetic circuit. The terms are displaced
by permeability and permeance.
341 STANDARD ELECTRICAL DICTIONARY.
Magnetic Continuity.
The completeness of a magnetic circuit, as when the armature of a
horseshoe magnet is in contact with both poles. It is an attribute of a
paramagnetic substance only and is identical for permanent magnets or
for electro-magnets. An air space intervening between armature and
magnet poles, or a space filled with any diamagnetic substance prevents
continuity, although the lines of force to some extent still find their
way around. The leakage is increased by discontinuity.
Magnetic Control.
Control of a magnetic needle, magnet, iron index or armature, in a
galvanometer, ammeter or voltmeter by a magnetic field; the restitutive
force being derived from a permanent magnet.
Magnetic Couple.
The couple of magnetic force which tends to bring the magnetic needle
into the plane of the magnetic meridian. One force is represented by the
imaginary pull upon the north pole, and the other by the opposite pull
upon the south pole of the needle. The moment of the couple varies from
a maximum when the needle is at right angles to the plane of the
magnetic meridian to zero when it is in such plane.
Magnetic Creeping.
Viscous hysteresis; the slow increase of magnetism in a paramagnetic
body when exposed to induction.
Fig. 222. MAGNETIC CURVES OR FIGURES.
Magnetic Curves.
The pictorial representation of magnetic lines of force. It is generally
produced by scattering filings on a sheet of paper or pane of glass held
over a magnet. The filings arrange themselves in characteristic curves.
Tapping the paper or pane of glass facilitates the arrangement, or
jarring the filings off a smaller magnet, so that they fall polarized
upon the paper, is thought by some to improve the effect. The group of
curves forms what are termed magnetic figures, q. v.
342 STANDARD ELECTRICAL DICTIONARY.
Magnetic Declination.
The angular deviation of the magnetic needle, causing it to rest at an
angle with the true meridian; the variation of the compass. (See
Magnetic Elements.)
Magnetic Density.
The intensity of magnetization expressed in lines of force per stated
area of cross-section in a plane at right angles to the lines of force.
Magnetic Dip.
The inclination from the horizontal assumed by a magnetic needle free to
move in the vertical plane. (See Magnetic Elements.) The angle of dip or
inclination is entirely a function of the earth, not of the needle.
Magnetic Discontinuity.
A break or gap in a magnetic circuit. To make a complete circuit the
iron or other core must be continuous. If the armature of a horseshoe
magnet is in contact with both poles the continuity is complete. If the
armature is not in contact magnetic continuity gives place to
discontinuity. It is an attribute of a paramagnetic substance only, and
is identical for permanent magnets, or for electro-magnets.
Magnetic Elements.
The qualities of the terrestrial magnetism at any place as expressed in
its action upon the magnetic needle. Three data are involved.
I. The Declination or Variation.
II. The Inclination or Dip.
III. The Force or Intensity.
I. The Declination is the variation expressed in angular degrees of the
magnetic needle from the true north and south, or is the angle which the
plane of the magnetic meridian makes with that of the geographical
meridian. It is expressed as east or west variation according to the
position of the north pole; east when the north pole of the needle is to
the east of the true meridian, and vice versa. Declination is different
for different places; it is at present west in Europe and Africa, and
east in Asia and the greater part of North and South America. The
declination is subject to (a) secular, (b) annual and (c) diurnal
variations. These are classed as regular; others due to magnetic storms
are transitory and are classed as irregular, (a) Secular variations. The
following table shows the secular variations during some three hundred
years at Paris. These changes are termed secular, because they require
centuries for their completion.
343
STANDARD ELECTRICAL DICTIONARY.
Table of Declination or Variation at Paris.
Year. Declination.
1580 11º 30' E.
1663 0°
1700 8° 10' W.
1780 19º 55' W.
1785 22º 00' W.
1805 22º 5' W.
1814 22º 34' W.
1825 22° 22' W.
1830 22º 12' W.
1835 22º 4' W.
1850 20º 30' W.
1855 19º 57' W.
1860 19º 32' W.
1865 18º 44' W.
1875 17º 21' W.
1878 17º 00' W.
[Transcriber's note The value for 2008 is about 0° 48' W, changing by
0° 7' E/year.]
On scrutinizing these figures it will be seen that there is part of a
cycle represented and that the declination is slowly returning to the
zero point after having reached its maximum western variation in 1814.
Upwards of 300 years would be required for its completion on the basis
of what is known. In other places, notably the coast of Newfoundland,
the Gulf of the St. Lawrence and the rest of the North American seaboard
and in the British Channel, the secular variations are much more rapid
in progress. (b) Annual variations--These were first discovered in 1780
by Cassini. They represent a cycle of annual change of small extent,
from 15' to 18' only. In Paris and London the annual variation is
greatest about the vernal equinox, or March 21st, and diminishes for the
next three months, and slowly increases again during the nine following
months. It varies during different epochs. (c) Diurnal variations were
discovered in 1722 by Graham. A long needle has to be employed, or the
reflection of a ray of light, as in the reflecting galvanometer, has to
be used to observe them. In England the north pole of the magnetic
needle moves every day from east to west from sunrise until 1 or 2 P.
M.; it then tends towards the east and recovers its original position by
10 P. M. During the night the needle is almost stationary. As regards
range the mean amplitude of diurnal variations at Paris is from April to
September 13' to 15'; for the other months from 8' to 10'. On some days
it amounts to 25' and sometimes is no more than 5'. The amplitude of
diurnal variations decreases from the poles to the equator. Irregular
variations accompany earthquakes, the aurora borealis and volcanic
eruptions. In Polar regions the auroral variations may be very great;
even at 40° latitude they may be 1° or 2°. Simultaneous irregularities
sometimes extend over large areas. Such are attributed to magnetic
storms. II. The Inclination is the angle which the magnetic needle makes
with the horizon, when the vertical plane in which the needle is assumed
to be free to move coincides with the magnetic meridian. It is sometimes
called the dip of the needle. It varies as does the declination, as
shown in the following table of inclinations of London.
344 STANDARD ELECTRICAL DICTIONARY.
Table of Inclination or Dip at London
Year. Inclination.
1576 71° 50'
1600 72°
1676 73° 30'
1723 74° 42'
1773 72° 19'
1780 72° 8'
1790 71° 33'
1800 70° 35'
1821 70° 31'
1828 69° 47'
1838 69° 17'
1854 68° 31'
1859 68° 21'
1874 67° 43'
1876 67° 39'
1878 67° 36'
1880 67° 35'
1881 67° 35'
III. Force or Intensity is the directive force of the earth. It varies
with the squares of the number of oscillations the magnetic needle will
make if caused to oscillate from a determined initial range. The
intensity is supposed to be subject to secular change. According to
Gauss the total magnetic intensity of the earth is equal to that which
would be exerted if in each cubic yard there were eight bar magnets,
each weighing one pound. This is, of course, a rough way of expressing
the degree of intensity. Intensity is least near the magnetic equator
and greatest near the magnetic poles; the places of maximum intensity
are termed the magnetic foci. It varies with the time of day and
possibly with changes in altitude.
Magnetic Elongation.
The elongation a bar of iron or steel undergoes when magnetized. By
magnetization it becomes a little longer and thinner, there being no
perceptible change in volume. The change is accompanied by a slight
sound--the magnetic tick. An exceedingly delicate adjustment of
apparatus is required for its observation.
Magnetic Equator.
A locus of the earth's surface where the magnet has no tendency to dip.
It is, approximately speaking, a line equally distant from the magnetic
poles, and is called also the aclinic line. It is not a great circle of
the earth.
345 STANDARD ELECTRICAL DICTIONARY.
Magnetic Field of Force.
The field of force established by a magnet pole. The attractions and
repulsions exercised by such a field follow the course of the electro-
magnetic lines of force. (See also Field of Force.) Thus the tendency of
a polarized needle attracted or repelled is to follow, always keeping
tangential to curved lines, the direction of the lines of force, however
sweeping they may be. The direction of magnetic lines of force is
assumed to be the direction in which a positive pole is repelled or a
negative one attracted; in other words, from the north pole of a magnet
to its south pole in the outer circuit. The direction of lines of force
at any point, and the intensity or strength of the field at that point,
express the conditions there. The intensity may bc expressed in terms of
that which a unit pole at unit distance would produce. This intensity as
unitary it has been proposed to term a Gauss. (See Weber.)
The direction of the lines of force in a magnetic field are shown by the
time-honored experiment of sprinkling filings of iron upon a sheet of
paper held over a magnet pole or poles. They arrange themselves, if the
paper is tapped, in more or less curved lines tending to reach from one
pole of the magnet to the other. Many figures may be produced by
different conditions. Two near poles of like name produce lines of force
which repel each other. (See Magnetic Curves.)
A magnetic and an electro-magnetic field are identical in all essential
respects; the magnetic field may be regarded as a special form of the
electro-magnetic field, but only special as regards its production and
its defined north and south polar regions.
Synonyms--Magnetic Spin (not much used).
Magnetic Field, Uniform.
A field of identical strength in all parts, such as the earth's magnetic
field. If artificially produced, which can only be approximately done,
it implies large cross-section of magnet pole in proportion to the
length of the magnetic needle affected by it, which is used in
determining its uniformity.
Magnetic Figures.
The figures produced by iron filings upon paper or glass held near
magnetic poles. By these figures the direction of lines of force is
approximately given, and a species of map of the field is shown. (See
Magnetic Field of Force--Magnetic Curves.)
Magnetic Filament.
The successive rows of polarized molecules assumed to exist in
magnetized iron. Each molecule represents an infinitely small magnet,
and its north pole points to the south pole of the next molecule. Such a
string or row is a theoretical conception based on the idea that the
molecules in a magnet are all swung in to parallelism in the magnetizing
process. A magnetic filament may be termed the longitudinal element of a
magnet. (See Magnetism, Hughes' Theory of.)
[Transcriber's note: This description parallels the modern
notion of electron spin as the basis of magnetism in materials.]
Magnetic Fluids.
A two-fluid theory of magnetism has been evolved, analogous to the
two-fluid theory of electricity. It assumes north fluid or "red
magnetism" and a south fluid or "blue magnetism." Each magnetism is
supposed to predominate at its own pole and to attract its opposite.
Before magnetization the fluids are supposed to neutralize each other
about each molecule; magnetization is assumed to separate them,
accumulating quantities of them at the poles.
Magnetic Flux.
Magnetic induction; the number of lines of force that pass through a
magnetic circuit.
Synonym--Magnetic Flow.
346 STANDARD ELECTRICAL DICTIONARY.
Magnetic Force.
The forces of attraction and repulsion exercised by a magnet. By
Ampere's theory it is identical with the forces of attraction and
repulsion of electric currents.
Magnetic Friction.
The damping effect produced on the movements of a mass of metal by
proximity to a magnet; the phenomenon illustrated in Arago's wheel, q.
v. When a mass of metal moves in the vicinity of a magnet it cuts the
lines of force emanating from its poles, thereby producing currents in
its mass; as the production of these currents absorbs energy a damping
effect is produced upon the movements of the mass.
Magnetic Gear.
Friction gear in which electro-magnetic adherence is employed to draw
the wheels together. (See Adherence, Electro-magnetic--Electro-magnetic
Friction Gear.)
Magnetic Inclination.
The inclination from the horizontal of a magnetic needle placed in the
magnetic meridian. (See Magnetic Element--Inclination Map.)
Synonym--Magnetic Dip.
Magnetic Induction.
The force of magnetization within an induced magnet. It is in part due
to the action of the surrounding particles of polarized material; in
part to the magnetic field. (See Magnetic Induction, Coefficient of.)
In a more general way it is the action of a magnet upon bodies in its
field of force. In some cases the magnetism induced causes the north
pole of the induced magnet to place itself as far as possible from the
north pole of the inducing magnet and the same for the south poles. Such
substances are called paramagnetic or ferromagnetic. They lie parallel
or tangential to the lines of force. In other cases the bodies lie at
right angles or normal to the lines of force. Such bodies are called
diamagnetic.
Some bodies are crystalline or not homogeneous in structure, and in them
the lines of magnetic induction may take irregular or eccentric paths.
(See AEolotropic.)
Synonym--Magnetic Influence.
Magnetic Induction, Apparent Coefficient of.
The apparent permeability of a paramagnetic body as affected by the
presence of Foucault currents in the material itself. These currents act
exactly as do the currents in the coils surrounding the cores of
electro-magnets. They produce lines of force which may exhaust the
permeability of the iron, or may, if in an opposite direction, add to
its apparent permeability.
Magnetic Induction, Coefficient of.
The number, obtained by dividing the magnetization of a body, expressed
in lines of force produced in it, by the magnetizing force which has
produced such magnetization, expressed in lines of force producible by
the force in question in air. It always exceeds unity for iron, nickel
and cobalt. It is also obtained by multiplying the coefficient of
induced magnetization by 4 PI (4 * 3.14159) and adding 1. (See Magnetic
Susceptibility--Magnetization, Coefficient of Induced.)
347 STANDARD ELECTRICAL DICTIONARY.
The coefficient of magnetic induction varies with the material of the
induced mass, and varies with the intensity of the magnetizing force.
This variation is due to the fact that as the induced magnetism in a
body increases, the magnetizing force required to maintain such
induction, increases in a more rapid ratio. The coefficient of magnetic
induction is the same as magnetic permeability, and in a certain sense
is the analogue of conductivity. It is also termed the multiplying power
of the body or core magnetized. It is the coefficient of induced
magnetization (see Magnetization, Coefficient of Induced) referred to a
mass of matter. For diamagnetic bodies the coefficient has a negative
sign; for paramagnetic bodies it has a positive sign.
Synonyms--Permeability--Multiplying Power--Magnetic Inductive Capacity.
Magnetic Induction, Dynamic.
The induction produced by a magnetic field which moves with respect to a
body, or where the body if moving moves at a different rate, or where
the body moves and the field is stationary. In the case where both move,
part of the induction may be dynamic and part static. (See Magnetic
Induction, Static.)
Magnetic Induction, Static.
Magnetic induction produced by a stationary field acting upon a
stationary body.
Magnetic Induction, Tube of.
An approximate cylinder or frustrum of a cone whose sides are formed of
lines of magnetic induction. (See Magnetic Induction, Lines of.) The
term tube is very curiously applied in this case, because the element or
portion of a magnetic field thus designated is in no sense hollow or
tubular.
Magnetic Inertia.
A sensible time is required to magnetize iron, or for it to part with
its magnetism, however soft it may be. This is due to its magnetic
inertia and is termed the lag. Permanent or residual magnetism is a
phase of it. It is analogous to self-induction of an electric circuit,
or to the residual capacity of a dielectric.
Magnetic Insulation.
Only approximate insulation of magnetism is possible. There is no
perfect insulator. The best ones are only 10,000 times less permeable
than iron. Hence lines of force find their way through air and all other
substance, being simply crowded together more in paths of iron or other
paramagnetic substance.
348 STANDARD ELECTRICAL DICTIONARY.
Magnetic Intensity.
The intensity of the magnetization of a body. It is measured by the
magnetic lines of force passing through a unit area of the body, such
area being at right angles to the direction of the lines of force.
Magnetic Lag.
In magnetism the tendency of hard iron or steel especially to take up
magnetism slowly, and to part with it slowly. (See Magnetic Inertia.)
The lag affects the action of a dynamo, and is a minor cause of those
necessitating the lead of the brushes.
Synonym--Magnetic Retardation.
Magnetic Latitude.
Latitude referred to the magnetic equator and isoclinic lines.
Magnetic Leakage.
The lines of force in a field magnet which pass through the air and not
through the armature are useless and represent a waste of field. Such
lines constitute magnetic leakage.
Magnetic Limit.
The temperature beyond which a paramagnetic metal cannot be magnetized.
The magnetic limit of iron is from a red to a white heat; of cobalt, far
beyond a white heat; of chromium, below a red heat; of nickel at about
350° C. (662°F.) of manganese, from 15° C. to 20° C. (59° to 68° F.)
Magnetic Lines of Force.
Lines of force indicating the distribution of magnetic force, which is
due presumably to whirls of the ether. A wire or conductor through which
a current is passing is surrounded by an electro-magnetic field of
force, q. v., whose lines of force form circles surrounding the
conductor in question. A magnet marks the existence of a similar
electro-magnetic field of force whose lines form circuits comprising
part of and in some places all of the body of the magnet, and which are
completed through the air or any surrounding paramagnetic or diamagnetic
body. They may be thought of as formed by the Ampérian sheet of current,
and analogous to those just mentioned as surrounding a conductor.
Fig. 223. MAGNETIC LINES OF FORCE, DIRECTION OF.
A magnetic line of force may be thought of as a set of vortices or
whirls, parallel to each other, and strung along the line of force which
is the locus of their centres.
If as many lines are drawn per square centimeter as there are dynes (per
unit pole) of force at the point in question, each such line will be a
unitary c. g. s. line of force.
349 STANDARD ELECTRICAL DICTIONARY.
Magnetic Mass.
A term for a quantity of magnetism. Unit mass is the quantity which at
unit distance exercises unit force.
Magnetic Matter.
Imaginary matter assumed as a cause of magnetism. Two kinds, one
positive and one negative, may be assumed as in the two fluid theory of
electricity, or only one kind, as in the single fluid theory of
electricity. Various theories of magnetic matter have been presented
whose value is only in their convenience.
[Transcriber's note: See "magnet" and Edward Purcell's explanation of
magnetism using general relativity.]
Magnetic Memory.
The property of retaining magnetism; coercive force; magnetic inertia;
residual magnetism.
[Transcriber's note: Small ferrite magnetic donuts were used as computer
main memory from 1950 to 1970.]
Magnetic Meridian.
A line formed on the earth's surface by the intersection therewith of a
plane passing through the magnetic axis. It is a line determined by the
direction of the compass needle. The meridians constantly change in
direction and correspond in a general way to the geographical meridians.
Magnetic Moment.
The statical couple with which a magnet would be acted on by a uniform
magnetic field of unit intensity if placed with its magnetic axis at
right angles to the lines of force of the field. (Emtage.) A uniformly
and longitudinally magnetized bar has a magnetic moment equal to the
product of its length by the strength of its positive pole.
Magnetic Needle.
A magnet with a cup or small depression at its centre and poised upon a
sharp pin so as to be free to rotate or oscillate in a horizontal plane.
The cup is often made of agate. Left free to take any position, it
places its magnetic axis in the magnetic meridian.
Magnetic Parallels.
Lines roughly parallel to the magnetic equator on all parts of each of
which the dip of the magnetic needle is the same; also called Isoclinic
Lines. These lines mark the places of the intersection of equipotential
surfaces with the earth's surface. They are not true circles, and near
the poles are irregular ellipses; the magnet there points toward their
centres of curvature. They correspond in a general way with the
Geographical Parallels of Latitude.
Magnetic Permeability.
The specific susceptibility of any substance, existing in a mass, for
magnetic induction. (See Magnetic Induction, Coefficient of, synonym for
Magnetic Permeability and Magnetization, Coefficient of Induced.)
Synonyms--Magnetic Inductive Capacity--Multiplying Power--Coefficient of
Magnetic Induction.
350 STANDARD ELECTRICAL DICTIONARY.
Magnetic Perturbations.
Irregular disturbances of the terrestrial magnetism, as by the aurora
and in electric storms.
Magnetic Poles.
The points where the equipotential surfaces of the terrestrial field of
force graze the earth's surface; the points toward which the north or
south poles of the magnetic needle is attracted. Over a magnetic pole
the magnetic needle tends to stand in a vertical position. There are two
poles, Arctic or negative, and Antarctic or positive. Magnetic needles
surrounding them do not necessarily point toward them, as they point to
the centres of curvature of their respective magnetic parallels. The
poles constantly change in position. The line joining them does not
coincide with anything which may be termed the magnetic axis of the
earth.
Magnetic Poles, False.
Poles on the earth's surface other than the two regular magnetic poles.
There seem by observation to be several such poles, while analogy would
limit true magnetic poles to two in number.
Magnetic Potential.
The potential at any point of a magnetic field is the work which would
be done by the magnetic forces of the field upon a positive unit of
magnetism as it moves from that point to an infinite distance. (Emtage.)
Magnetic Proof Piece.
A piece of iron used for testing magnets and the distribution of
magnetism in bars, by suspending or supporting above or near the magnet,
by detaching after adherence, and in other ways.
Magnetic Proof Plane.
An exploring coil used for testing the distribution of magnetism. It is
connected in circuit with a galvanometer, and exposed to alternation of
current, or to other disturbing action produced by the magnet or field
under examination. This affects the galvanometer, and from its movements
the current produced in the coil, and thence the magnetic induction to
which it was exposed, are calculated.
Synonym--Exploring Coil.
Magnetic Quantity.
The magnetism possessed by a body; it is proportional to the action of
similar poles upon each other, or to the field produced by the pole in
question. It is also called the strength of a pole.
The force exercised by two similar poles upon each other varies with
their product and inversely with the square of the distance separating
them; or it may be expressed thus (m * m) / (L^2). This is a force, and
the dimensions of a force are ML/(T^2). Therefore, (m^2)/(L^2) =
ML/(T^2) or m = (M^.5)*(L^1.5)/T.
351 STANDARD ELECTRICAL DICTIONARY.
Magnetic Reluctance.
The reciprocal of permeance; magnetic resistance; the relative
resistance to the passage of lines of force offered by different
substances. The idea is derived from treating the magnetic circuit like
an electric one, and basing its action on magneto-motive force acting
through a circuit possessing magnetic reluctance.
Magnetic Reluctivity.
The reciprocal of magnetic permeability, q. v.
Synonym--Magnetic Resistance.
Magnetic Retentivity.
The property of steel or hard iron by which it slowly takes up and
slowly parts with a magnetic condition--traditionally (Daniell) called
coercitive force.
Magnetic Rotary Polarization.
If a plane polarized beam of light is sent through a transparent medium
in a magnetic field its plane of polarization is rotated, and this
phenomenon is denoted as above. (Compare Refraction, Electric, and see
Electro-magnetic Stress.) This has been made the basis of a method for
measuring current. A field of force varies with the current; the
polarization produced by such field is therefore proportional to the
current. (Becquerel & Rayleigh.)
A plane polarized beam of light passing through the transparent medium
in the magnetic field by the retardation or acceleration of one of its
circular components has its plane of polarization rotated as described.
The direction of the lines of force and the nature of the medium
determine the sense of the rotation; the amount depends upon the
intensity of the field resolved in the direction of the ray, and on the
thickness and nature of the medium.
Magnetic Saturation.
The maximum magnetic force which can be permanently imparted to a steel
bar. A bar may be magnetized beyond this point, but soon sinks to it.
The magnetism produced in a bar is prevented from depolarization by the
retentivity or coercive force of the bar. The higher the degree of
magnetization the greater the tendency to depolarization.
It is also defined as the maximum intensity of magnetism produced in a
paramagnetic substance by a magnetic field as far as affected by the
permeability of the substance in question. The more lines of force
passed through such a substance the lower is its residual permeability.
It is assumed that this becomes zero after a certain point, and then the
point of saturation is reached. After this point is reached the addition
of any lines of force is referred entirely to the field and not at all
to the permeability of the substance. But such a zero is only definable
approximately.
Magnetic Screen.
A box or case of soft iron, as thick as practicable, for protecting
bodies within it from the action of a magnetic field. The lines of force
to a great extent keep within the metal of the box on account of its
permeability, and but a comparatively few of them cross the space within
it.
Such screens are used to prevent watches from being magnetized, and are
a part of Sir William Thomson's Marine galvanometer.
A magnetic screen may be a sphere, an infinite or very large plane, or
of the shape of any equipotential surface.
Synonym--Magnetic Shield.
352 STANDARD ELECTRICAL DICTIONARY.
Magnetic Self-induction.
The cause of a magnet weakening is on account of this quality, which is
due to the direction of the lines of force within a magnet from the
positive towards the negative pole. "A magnet thus tends to repel its
own magnetism and to weaken itself by self-induction." (Daniell.)
Magnetic Separator.
An apparatus for separating magnetic substances from mixtures. Such
separators depend on the action of electro-magnets. In one form the
material falls upon an iron drum, magnetized by coils. Any magnetic
substance adheres to the drum and is thereby separated. They are used by
porcelain makers for withdrawing iron particles from clay, by machinists
to separate iron filings and chips from brass, and for similar purposes.
Fig. 224. MAGNETIC SEPARATOR.
Magnetic Shell.
A theoretical conception of a cause of a magnetic field or of a
distribution of magnetism. If we imagine a quantity of very short
magnets arranged in contact with their like poles all pointing in the
same direction so as to make a metal sheet, we have a magnetic shell.
Its magnetic moment is equal to the sum of the magnetic moment of all
its parts. If the shell is of uniform strength the magnetic moment of a
unit area gives the strength of the shell; it is equal to the magnetic
quantity per unit of area, multiplied by the thickness of the shell.
If its strength is uniform throughout a magnetic shell is called simple;
if its strength varies it is termed complex.
Emtage thus defines it: A magnetic shell is an indefinitely thin sheet
magnetized everywhere in the direction normal to itself.
Magnetic Shell, Strength of.
The magnetic quantity per unit of area of the shell multiplied by the
thickness of the shell.
353 STANDARD ELECTRICAL DICTIONARY.
Magnetic Shield.
In general a magnetic screen, q. v. Sometimes a strong local field is
made to act as a shield, by its predominance overcoming any local or
terrestrial field to which the needle to be protected may be exposed.
Magnetic Shunt.
The conception of a magnetic circuit being formed, the shunt is a
corollary of the theory. It is any piece of iron which connects points
of a magnet differing in polarity, so as to divert part of the lines of
force from the armature or yoke. The shunt is especially applicable in
the case of horseshoe magnets. Thus a bar of iron placed across from
limb to limb a short distance back from the poles would act as a shunt
to the armature and would divert to itself part of the lines of force
which would otherwise go through the armature and would weaken the
attraction of the magnet for the latter. In dynamos a bar of iron used
as a magnetic shunt has been used to diminish the lines of force going
through the armature and hence to weaken the field and diminish the
electro-motive force. By moving the shunt nearer or further from the
poles the dynamo is regulated.
In the cut the projections between the yoke and poles of the magnet
shown act as a shunt to the yoke, taking some lines of force therefrom.
