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Glossary of Magnetic Terms

 

Air Gap

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.

Anisotropic Magnet

A magnet which has a preferred direction of orientation so that the magnetic characteristics are optimum in one preferred direction.

Closed Circuit

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.

Demagnetisation Curve

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.

Ferromagnetic Material

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.

Flux

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 Maxwell.

Fringing Fields

Leakage flux particularly associated with edge effects and leakage patterns in a magnetic circuit.

Gauss

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.

Hysteresis Loop

A closed curve calculated by plotting corresponding values of magnetic induction: B on the abscissa against magnetising force H.

Induction, B

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.

Irreversible Loss

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.

Isotropic Magnet

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 characteristics.

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.

Leakage Flux

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.

Load Line

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.

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.

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.

Oersted, Oe

A unit measure of magnetising force (cgs). This is equivalent to Ampere Turns per Inch (S.I.).

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.

Pull Gap

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, R

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

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.

Return Path

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 temperature variations.

Saturation

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 Gauss.

Stabilisation

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.