A non-magnetic discontinuity in a magnetic circuit
(i.e. the distance between two magnetic poles). This gap often includes
other materials such as brass, aluminium or paint.
A magnet which has a preferred direction
of orientation so that the magnetic characteristics are optimum in one
This exists when the flux path external to
the permanent magnet is confined within high permeability materials
which contain the magnet circuit.
Coercive Force, Hc
The demagnetising force necessary to
reduce observed induction B to zero after the magnet has been brought
to saturation. Coercive force is measured in Oersteds or more recently
A/m and kA/m.
Curie Temperature, Tc
The temperature at which a material loses its permanent magnetic properties completely and is no longer able to hold magnetism.
The second/left quadrant of the
hysteresis loop, generally describing the behaviour of magnetic
characteristics in actual use. Also known as the B-H curve.
A material whose permeability is
very much larger than one, and which exhibits hysteresis magnetising
and demagnetising characteristics. The greater the flux carrying
potential, the bigger the value i.e. one to several thousands.
Magnetic flux is the condition existing in a medium
subjected to a magnetising force. This value is quantified by E.M.F.
(electromotive force). This measurement of force in cgs units is a
Leakage flux particularly associated with edge effects and leakage patterns in a magnetic circuit.
Lines of magnetic flux per square centimetre. Gauss
is measured in cgs units, Maxwell lines and Webers per square metre or
Tesla in the Si system.
A closed curve calculated by plotting
corresponding values of magnetic induction: B on the abscissa against
magnetising force H.
This is the magnetic flux per unit area of section in the applied magnetic direction of flux. This is measured in Gauss.
Intrinsic Coercive Force
This is a measure of the
resistance of the magnet material to a demagnetising force. Permanent
magnets with high intrinsic coercivity values are usually classified as
'hard' permanent magnets. Intrinsic coercive force indicates magnetic
stability at high temperatures. Also see stabilisation.
This is the partial demagnetisation of a
magnet material when introduced to external factors such as high/low
temperatures and demagnetising fields. Losses can only by rectified by
remagnetisation. However, magnets can be stabilised to prevent the
variation of performance caused by irreversible losses.
A magnetic material which does not have a
preferred direction of magnetic orientation and therefore can be
magnetised in any direction without the loss of magnetic
Knee of the Demagnetisation Curve
The point at which the
B-H curve ceases to be linear. If the operating point of the magnet
falls below the knee, the magnet will not be able to recover full
magnetic potential without the application of a magnetising force.
This is the loss of magnetic flux which occurs
through leakage caused by saturation or air gaps introduced into the
magnetic circuit. This induces a loss of efficiency in the circuit
which cannot be recovered.
Length of Air Gap, Lg
Indicates the length of the central flux path across an air gap.
A line drawn from the origin of the
Demagnetisation Curve with a slope. The intersection of the -B/H curve
and slope represents the operating point of the magnet. Also see
Permeance Coefficient, Pc
An assembly consisting of some or all of
the following: permanent magnets, ferromagnetic conduction elements,
air gaps, electrical currents.
The total magnetic induction over a given area.
Magnetising Force, H
The magnetomotive force per unit length at any point in a magnetic circuit. This is measured in Oersteds.
Magnetomotive Force, F
This is the potential magnetic difference between any two points.
Maximum Energy Product, BH max.
There is a point at the
Hysteresis Loop at which the product of magnetising force H and
induction B reaches a maximum. This maximum value is called the Maximum
Energy Product and is measured in Mega Gauss Oersted, MGOe.
A unit measure of magnetising force (cgs). This is equivalent to Ampere Turns per Inch (S.I.). Permeance
The inverse of reluctance.
Permeance Coefficient, Pc
Ratio of the magnetic induction
to self demagnetising force. This is also known as the 'load line' or
operating point of the magnet.
Usually illustrated in graph format, these curves
are a representation of the relationship between the attractive force
exerted by a magnet on a soft magnetic workpiece and the distance
between them. Pull Gap curve diagrams are useful when selecting a
magnet for a particular tractive or holding application.
Reluctance is the resistance in a magnetic
circuit and is related to the magnetomotive force, F and magnetic flux
(R =F/ magnetic flux) where F is the magnetomotive force.
Remenance is the magnetic induction which remains
in a magnetic circuit after the removal of an applied magnetising
force. If there is an air gap in the circuit, the remenance will be
less than the residual induction Br.
Residual Induction Br
This represents the maximum flux
output from a given magnet material measured at the point where the
Hysteresis Loop crosses the B axis at zero magnetising force.
A magnetic circuit which provides a low
reluctance path for the magnetic flux. Reversible Temperature
Coefficient A measure of the reversible changes in flux caused by
This is the condition whereby a magnet or
ferromagnetic material has reached a maximum value and an increase in
the appliance of magnetising force produces no increase in induction
i.e. saturation flux densities for steels range from 16,000 to 20,000
The process where a magnet is exposed to
demagnetising influences expected to be encountered in operation. The
exposure to these demagnetising influences such as high or low
temperatures or external magnetic fields prevents irreversible losses
during actual operation.