Magnetic Terms And Definitions

Magnetic Terms And Definitions

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Air Gap: The portion of a magnetic circuit which contains no ferrous or magnetic material; this is the portion of the magnetic circuit where the desired magnetic action will occur.

Air Gap Length - lg: The length of the path of the central flux line of the air gap; measured in centimeters.

Air Gap Volume - Vg: The useful volume of air or non-magnetic material between magnetic poles; measured in cubic centimeters.

Air Gap Area - Ag: The cross-sectional area of the air gap perpendicular to the flux path is measured in square centimeters in a place normal to the central flux line.

Ampere-Turn/Meter - A/m: The SI unit of magnetizing force or magnetic field strength.

Anisotropic Material: A material having a preferred magnetization axis. This characteristic is produced during manufacture by crystalline structure orientation and/or magnetic field enhancement. Also referred to as "oriented material".

Closed Circuit: A condition that exists when the external flux path of a permanent magnet is confined with material having high permeability.

Coercive Force - Hc: The demagnetizing force required to reduce residual induction (Br) to zero in a magnetic field after magnetizing to saturation; measured in oersteds.

Coercivity: The property of magnetic material, which is a function of the maximum value of its coercive force. A high coercivity material is one, which exhibits a high coercive force.

Core Loss: The power expended (as heat) in a magnetic or ferrous material when it is subjected to a varying magnetizing force. Expressed in watts/pound of material.

Curie Temperature - Tc: The transition temperature above which a material loses its magnetic properties.

Demagnetization: The partial or complete removal of residual magnetism from a body.

Demagnetization Curve: The second or fourth quadrants of a major hysteresis loop. The coordinates Bd and Hd designate points on this curve.

Diamagnetic Material: A material having permeability less than that of a vacuum.

Dimension Ratio - lm/D: The ratio of the length of a magnet to its diameter, or the diameter of a circle of equivalent cross-sectional area. For simple geometrics such as bars and rods, the dimensional ratio is related to the slope of the operating line of the magnets (Bd/Hd).

Eddy Current Loss: The portion of the core loss in a material due to circulating currents resulting from emf induced by varying magnetic induction. A highly conductive material will sustain stronger eddy currents for a relatively long time, whereas the eddy currents in poor electrical conductors are weaker and tend to decay rapidly, if existant at all.

Electromagnet: A temporary magnet, typically formed from a coil of wire wound around a core made from a soft magnetic material (such as iron). An electromagnet will only produce a magnet field when current is passed through the coil.

Energy Product - BdHd: The amount of energy that a magnetic material can supply to an external magnetic circuit when operating at any point on its demagnetization curve; measured in megagauss-oersteds.

Energy Product Curve: The graphic representation of the external energy produced by a permanent magnet. It is a function of the flux density and the demagnetizing force shown in the demagnetization curve (BdHd) plotted against the induction Bd.

Ferromagnetic Material: A material which exhibits hysteresis phenomena and whose permeability is dependent upon an applied magnetizing force.

Fluxmeter: An instrument that uses a search coil to measure flux linkage changes.

Gamma: The CGS units of low-level flux density; 100,000 gamma equals one oersted.

Gauss: The CGS unit of magnetic induction and flux density. One gauss equals one maxwell per square centimeter.

Gaussmeter: An instrument that measures the instantaneous value of magnetic induction (B). Guassmeters usually use either the Hall effect, nuclear magnetic resonance (NMR), or the rotating coil principle for operation.

Gilbert: The CGS of magnetomotive force. One gilbert equals the amount of force required to produce one maxwell of flux in a magnetic circuit having unity reluctance.

Hard Magnetic Materials: Materials, which tend to resist demagnetization once they are magnetized. All permanent magnet material is considered "hard". Also referred to as "high energy-product" materials.

Hysteresis Loop: The graphic representation of the relation between values of magnetic induction (B) and magnetizing force (H) for a ferrous or magnetic material. Often referred to as a "B-H" loop.

Intrinsic Coercive Force - Hci: An indication of a material’s resistance to demagnetization. It is equal to the demagnetizing force, which reduces a saturated material’s intrinsic induction (Bi) to zero; measured in oersteds.

Intrinsic Induction - Bi or J: 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 relation is expressed by subtracting the value of H from the value of B.

Irreversible Losses: Partial demagnetization of a magnet caused by exposure to high or low temperatures, external fields, or other factors. These losses can be recovered by remagnetization. Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields.

Isotropic Material: A material that has equal magnetic properties in all direction. Also referred to as "unoriented material".

Joint Reluctance: The resistance to conduction of a magnetic field caused by joints or discontinuities in a magnetic circuit. In effect, an air gap, which causes some degradation in the flux density available in the working gap of the magnetic assembly.

Keeper: One or more magnetic conductors used to close a magnetic circuit. Generally used to protect permanent magnets of magnetic assemblies from demagnetizing influences when the assembly is not being used. It can also serve to minimize leakage.

Leakage Factor - F: Accounts for flux leakage from the magnetic circuit. It is the ratio between the magnetic flux at the magnet neutral section and the average flux present in the air gap.

Leakage Flux: The part of a magnet field that is outside the useful of intended magnetic circuit; measured in maxwells.

Low Energy Product Material: Ferromagnetic material having a low energy product and which is easily demagnetized. Also known as "soft" magnetic material.

Magnetic Area - Am: The cross-sectional area of the magnet perpendicular to the central flux line, measured in square centimeters at any point along its length. For design purposes, this is usually considered the area at the neutral section of the magnet.

Magnet Length - lm: The total length of magnet material traversed in one complete revolution of the centerline of the magnet circuit; measured in centimeters.

Magnet Treating: The act of magnetically reducing the residual induction in a magnetic assembly in order to achieve a desired flux density level in its working gap. This also serves to protect the assembly against external influences of predetermined levels, which would otherwise tend to reduce the magnet’s residual induction. (Also see "stabilization")

Magnetic Domains: The molecular-size magnetic dipoles, which determine the magnetic characteristics of a permanent magnet.

Magnetic Field Strength - H: A measure of the vector magnetic quantity that determines the ability of an electric current or magnetic body to induce a magnetic field at a given point; measured in oersteds.

Magnetic Flux - : The physical indication of a magnetic condition existing in a material or medium which is subject to a magnetizing influence. It may be likened to the current flow in an electrical circuit. The CGS unit for magnetic flux is the maxwell (M); the SI unit is the weber (Wb).

Magnetic Induction - B: The magnetic field induced in a magnet or ferrous material by a magnetizing force, H. The CGS unit for magnetic induction is the gauss; the SI unit is the telsa. The phenomenon of induction is a function of the permeability of the material and the applied magnetizing force. Magnetic induction is the vector sum (at each point within the material) of the magnetic field strength and the resultant intrinsic induction, and is expressed as the amount of flux per unit area normal to the direction of the magnet path. Also referred to as "magnetic flux density".

Magnetic Induction In The Air Gap - Bg: The average magnetic induction value over the area of the air gap, of the magnetic induction measured at a specific point within the air gap; measured in gauss.

Magnetic Saturation: A condition that exists when an increase in magnetizing force (H) does not cause an increase in intrinsic magnetic induction (B). When this condition exists, the magnet is fully charged.

Magnetomotive Force - F: The line integral of the field strength (H) between any two points. This is the force, which tends to produce a magnetic field, expressed in gilberts in the CGS system and in ampere-turns in the SI.

Major Hysteresis Loop: The closed loop obtained when a magnetic material is cycled between positive and negative saturation.

Maximum Energy Product - (BH) max: The maximum product of (BdHd) which can be obtained on the demagnetization curve.

Maximum Field Strength - Ho: The amount of magnetic field strength corresponding to the maximum energy product (BH) max; measured in oersteds.

Maximum Magnetic Induction - Bo: The magnetic induction at the point of the maximum energy product (BH)max; measured in gauss.

Maximum Service Temperature - Tmax: The maximum temperature to which a magnet can be exposed without permanent changes to its stability or structural integrity.

Maxwell: The CGS unit of magnetic flux. One maxwell equals one line of magnetic flux.

Net Effective Magnetizing Force - Hs: The magnetizing force required to saturate a magnetic material; measured in oersteds.

Neutral Section: The portion of a permanent magnet defined by a plane passing through the magnet perpendicular to its central line at the point of maximum flux.

Oersted: The CGS unit of magnetic field strength (H). One oersted equals a magnetomotive force of one gilbert per centimeter of flux path.

Open Circuit: A condition that exists when a magnetized object is by itself, with no external flux path of high-permeability material.

Operating Line: A straight line passing through the origin of a permanent magnet demagnetization curve, with a slope of negative Bd/Hd; also called a "permanence coefficient line".

Operating Point: The point on a permanent magnet demagnetization curve defined by the coordinates Bd and Hd, or the point within the demagnetization curve defined by the coordinates Bm and Hm.

Permeability - u: A general term used to express various relationship between magnetic induction (B) and the field strength (H).

Permeameter: An instrument that can measure (and often record) the magnetic characteristics of a specimen.

Permeance - P: The reciprocal of reluctance (R), measured in maxwells per gilbert.

Recoil Force - Hm: The amount of magnetic field strength corresponding to the recoil induction (Bm); measured in oersteds.

Recoil Induction - Bm: The magnetic induction that remains in a magnetic material after magnetizing and treating for final use; measured in gauss.

Reluctance - R: A quantity that determines the amount of magnetic flux that is produced by a given magnetomotive force; it is somewhat analogous to electrical resistance. The reciprocal of permeance, it is expressed by the equation R = F/0 and measured in gilberts-per-maxwell in the CGS systems and ampere-turns-per-weber in the SI.

Reluctance Factor - f: Accounts for apparent magnetic circuit reluctance. Required due to the treatment of Hm and Hg as constants.

Remanence - Bd: The magnetic induction remaining in a magnetic circuit after the removal of an applied saturating magnetic field. If the circuit incorporates an air gap, the remanence will be less than the residual induction. Also referred to as "remanent induction".

Remanent Force - Hd: The amount of magnetic field strength corresponding to the remanent induction (Bd); measured in oersteds.

Residual Induction - Br: The magnetic induction corresponding to zero magnetizing force in a magnetic material after saturation in a closed circuit, measured in gauss. Also referred to as "flux density".

Retentivity: A material’s ability or inability to retain magnetism after the magnetizing force is removed.

Reversible Temperature Coefficients: Temporary changes in flux, which occur as ambient temperature varies. When the ambient temperature returns to normal, the flux will return to its original value.

Saturation Intrinsic Induction - Bis or Js: The maximum intrinsic induction possible in a material.

Search Coil: A coiled conductor used as a sensing element for a fluxmeter. The coil area and number of turns are usually known; this information is used to measure flux linkage changes.

Soft Magnetic Material: Ferromagnetic material, which is very easily, demagnetized; materials having low coercivity, such as relay steels.

Stabilization: The treatment of a magnetic material in order to obtain a desired magnetic level and /or to achieve some degree of permanence of that level. Stabilization may consist of treating the magnet assembly with and external magnetic field of physical shock and vibration or temperature extremes or any combination of these factors. (also see "magnet treating".)

Temperature Coefficient: A factor, which describes the reversible change in a magnetic property with respect to ambient temperature. The magnetic property will return to its original value when the ambient temperature returns to normal. Temperature coefficients are usually expressed as the amount of change per unit pf temperature.

Weber - Wb: The SI unit of magnetic flux.

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