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Glossary
Glossary
of Magnetic Terms
A | B | C
| D | E | F
| G | H | I | J | K
| L | M | N
| O | P | Q | R
| S | T | U | V | W
| X | Y | Z
Air
Gap: A
low permeability gap in the flux path of a magnetic circuit.
Often air, but inclusive of other materials such as paint,
aluminum, etc.
Anisotropic
Magnet: A
magnet having a preferred direction of magnetic orientation,
so that the magnetic characteristics are optimum in one
preferred direction.
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Closed Circuit:
This
exists when the flux path external to a permanent magnet
is confined within high permeability materials that compose
the magnet circuit.
Coercive
Force, Hc: The
demagnetizing force, measured in Oersteds, necessary to
reduce observed induction, B, to zero after the magnet
has previously been brought to saturation.
Curie
Temperature, Tc: The
temperature at which the parallel alignment of elementary
magnetic moments completely disappears, and the material
is no longer able to hold magnetization.
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Demagnetization
Curve: The
second quadrant of the hysteresis loop, generally describing
the behavior of magnetic characteristics in actual use.
Also known as the B-H Curve.
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Eddy
Currents: Circulating
electrical currents that are induced in electrically conductive
elements when exposed to changing magnetic fields, creating
an opposing force to the magnetic flux. Eddy currents
can be harnessed to perform useful work (such as damping
of movement), or may be unwanted consequences of certain
designs, which should be accounted for or minimized.
Electromagnet:
A magnet, consisting of a solenoid
with an iron core, which has a magnetic field existing
only during the time of current flow through the coil.
Energy
Product:
Indicates
the energy that a magnetic material can supply to an external
magnetic circuit when operating at any point on its demagnetization
curve. Calculated as Bd x Hd, and
measured in Mega Gauss Oersteds, MGOe.
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Ferromagnetic
Material:
A
material whose permeability is very much larger than 1
(from 60 to several thousand times 1), and which exhibits
hysteresis phenomena.
Flux:
The condition existing
in a medium subjected to a magnetizing force. This quantity
is characterized by the fact that an electromotive force
is induced in a conductor surrounding the flux at any
time the flux changes in magnitude. The cgs unit of flux
is the Maxwell.
Fluxmeter: An
instrument that measures the change of flux linkage with
a search coil.
Fringing
Fields: Leakage
flux particularly associated with edge effects in a magnetic
circuit.
Gauss:
Lines
of magnetic flux per square centimeter, cgs unit of flux
density, equivalent to lines per square inch in the English
system, and Webers per square meter or Tesla in the SI
system.
Gaussmeter: An
instrument that measures the instantaneous value of magnetic
induction, B. Its principle of operation is usually based
on one of the following: the Hall effect, nuclear magnetic
resonance (NMR), or the rotating coil principle.
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Hysteresis
Loop: A closed curve obtained
for a material by plotting corresponding values of magnetic
induction, B, (on the abscissa) against magnetizing force,
H, (on the ordinate).
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Induction,
B: The magnetic flux per
unit area of a section normal to the direction of flux.
Measured in Gauss, in the cgs system of units.
Intrinsic
Coercive Force, Hci: Measured
in Oersteds in the cgs system, this is a measure of the
material's inherent ability to resist demagnetization.
It is the demagnetization force corresponding to zero
intrinsic induction in the magnetic material after saturation.
Practical consequences of high Hci values are seen in
greater temperature stability for a given class of material,
and greater stability in dynamic operating conditions.
Intrinsic
Induction, Bi: The contribution
of the magnetic material to the total magnetic induction,
B. It is the vector difference between the magnetic induction
in the material and the magnetic induction that would
exist in a vacuum under the same field strength, H. This
relationship is expressed as: BI = B-H.
Irreversible
Loss: Defined as the partial
demagnetization of a magnet caused by external fields
or other factors. These losses are only recoverable by
re-magnetization. Magnets can be stabilized to prevent
the variation of performance caused by irreversible losses.
Isotropic
Magnet: A magnet material
whose magnetic properties are the same in any direction,
and which can therefore be magnetized in any direction
without loss of magnetic characteristics.
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Keeper:
A piece of soft iron that is
placed on or between the poles of a magnet, decreasing
the reluctance of the air gap and thereby reducing the
flux leakage from the magnet.
Knee
of the Demagnetization Curve: The
point at which the B-H curve ceases to be linear. All
magnet materials, even if their second quadrant curves
are straight line at room temperature, develop a knee
at some temperature. Alnico 5 exhibits a knee at room
temperature. If the operating point of a magnet falls
below the knee, small changes in H produce large changes
in B, and the magnet will not be able to recover its original
flux output without re-magnetization.
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Leakage Flux:
That portion of the magnetic
flux that is lost through leakage in the magnetic circuit
due to saturation or air-gaps, and is therefore unable
to be used.
Length
of air-gap, Lg: The
length of the path of the central flux line in the air-gap.
Load
Line: A line drawn from
the origin of the Demagnetization Curve with a slope of
-B/H, the intersection of which with the B-H curve represents
the operating point of the magnet. Also see Permeance
Coefficient.
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Magnetic
Circuit: An assembly consisting
of some or all of the following: permanent magnets, ferromagnetic
conduction elements, air gaps, electrical currents.
Magnetic
Flux: The total magnetic
induction over a given area. When the magnetic induction,
B, is uniformly distributed over an area A, Magnetic Flux
= BA.
Magnetizing
Force, H: The magnetomotive
force per unit length at any point in a magnetic circuit.
Measured in Oersteds in the cgs system.
Magnetomotive
Force, F: Analogous to
voltage in electrical circuits, this is the magnetic potential
difference between any two points.
Maximum
Energy Product, BHmax: The
point on the Demagnetization Curve where the product of
B and H is a maximum and the required volume of magnet
material required to project a given energy into its surroundings
is a minimum. Measured in Mega Gauss Oersteds, MGOe.
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North
Pole: That pole of a magnet
which, when freely suspended, would point to the north
magnetic pole of the earth. The definition of polarity
can be a confusing issue, and it is often best to clarify
by using "north seeking pole" instead of "north
pole" in specifications.
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Oersted,
Oe: A cgs unit of measure
used to describe magnetizing force. The English system
equivalent is Ampere Turns per Inch, and the SI system's
is Ampere Turns per Meter.
Orientation
Direction: The direction
in which an anisotropic magnet should be magnetized in
order to achieve optimum magnetic properties. Also known
as the "axis", "easy axis", or "angle
of inclination".
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Paramagnetic
Material: A material having
a permeability slightly greater than 1.
Permeance:
The inverse of reluctance,
analogous to conductance in electrical circuits.
Permeance
Coefficient,Pc: Ratio
of the magnetic induction, BD, to its self demagnetizing
force, HD PC = BD / HD This is also known as the "load
line", "slope of the operating line", or
operating point of the magnet, and is useful in estimating
the flux output of the magnet in various conditions. As
a first order approximation, BD / HD = Lm/Lg, where Lm
is the length of the magnet, and Lg is the length of an
air gap that the magnet is subjected to. PC is therefore
a function of the geometry of the magnetic circuit.
Pole Pieces: Ferromagnetic
materials placed on magnetic poles used to shape and alter
the effect of lines of flux.
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Relative Permeability:
The ratio of permeability of
a medium to that of a vacuum. In the cgs system,
the permeability is equal to 1 in a vacuum by definition.
The permeability of air is also for all practical purposes
equal to 1 in the cgs system.
Reluctance,
R: Analogous
to resistance in an electrical circuit, reluctance is
related to the magnetomotive force, F, and the magnetic
flux by the equation R = F/(Magnetic Flux), paralleling
Ohm's Law where F is the magnetomotive force (in cgs units).
Remanence, BD: The
magnetic induction that remains in a magnetic circuit
after the removal of an applied magnetizing force. If
there is an air gap in the circuit, the remanence will
be less than the residual induction, Br.
Residual Induction, Br:
This is the point at which
the hysteresis loop crosses the B axis at zero magnetizing
force, and represents the maximum flux output from the
given magnet material. By definition, this point occurs
at zero air gap, and therefore cannot be seen in practical
use of magnet materials.
Return
Path: Conduction elements
in a magnetic circuit which provide a low reluctance path
for the magnetic flux.
Reversible
Temperature Coefficient: A
measure of the reversible changes in flux caused by temperature
variations.
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Saturation:
The
condition under which all elementary magnetic moments
have become oriented in one direction. A ferromagnetic
material is saturated when an increase in the applied
magnetizing force produces no increase in induction. Saturation
flux densities for steels are in the range of 16,000 to
20,000 Gauss.
Search Coil: A
coil conductor, usually of known area and number of turns
that is used with a fluxmeter to measure the change of
flux linkage with the coil.
Stabilization:
Exposure
of a magnet to demagnetizing influences expected to be
encountered in use in order to prevent irreversible losses
during actual operation. Demagnetizing influences can
be caused by high or low temperatures, or by external
magnetic fields.
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Temperature
Coefficient: A factor,
which describes the change in a magnetic property with
change in temperature. Expressed as percent change per
unit of temperature.
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Weber:
The practical unit of magnetic
flux. It is the amount of magnetic flux which, when linked
at a uniform rate with a single-turn electric circuit
during an interval of 1 second, will induce in this circuit
an electromotive force of 1 volt.
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DESIGN
GUIDES
The following section will provide
a brief insight into the design and application of permanent
magnets… |
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FAQ
A collection of answers to questions. |
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