Glossary of Magnetic Terms
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.
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.
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 be rectified by remagnetisation. However, magnets can be stabilised to prevent the variation of performance caused by irreversible losses. Isotropic Magnet A magnet 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 and electric currents.
Magnetic Flux The total magnetic induction over a given area.
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