Glossary of Magnetic Terms
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. |
