Glossary of Magnet Terms
Air gap, is a low permeability gapin the flux path of a magnetic circuit. Often air, but inclusive ofother materials such as paint, aluminum, etc.
Anisotropy, Literally means havingdifferent properties depending on the inspecteddirection. Magnets which are anisotropic, or have an easyaxis of magnetization, have their anisotropy developed by twomethods: Shape and Magnetocrystalline.
As Area of the air gap,or the cross sectional area of the air gap perpendicular to theflux path, is the average cross sectional area of that portion ofthe air gap within which the application interaction occurs. Areais measured in sq. cm. in a plane normal to the central flux lineof the air gap.
Am Area of the magnet,is the cross sectional area of the magnet perpendicular to thecentral flux line, measured in sq. cm. at any point along itslength. In design, Am is usually considered the area at the neutralsection of the magnet.
B Magnetic induction, is themagnetic field induced by a field strength, H, at a given point. Itis the vector sum, at each point within the substance, of themagnetic field strength and resultant intrinsic induction. Magneticinduction is the flux per unit area normal to the direction of themagnetic path.
Bd Remnant induction,is any magnetic induction that remains in a magnetic material afterremoval of an applied saturating magnetic field, Hs. (Bd is themagnetic induction at any point on the demagnetization curve:measured in gauss.)
Bd/Hd, Slopeof the operating line, is the ratio of the remnant induction, Bd,to a demagnetizing force, Hd. It is also referred to as thepermeance coefficient, shear line, load line and unit permeance.
BdHd Energyproduct, indicates the energy that a magnetic material can supplyto an external magnetic circuit when operating at any point on itsdemagnetization curve; measured in megagauss-oersteds.
(BH) max Maximum energyproduct, is the maximum product of (BdHd) which can beobtained on the demagnetization curve.
Bis, (or J) Saturation intrinsicinduction, is the maximum intrinsic induction possible ina material.
Bg, Magnetic induction in theair gap, is the average value of magnetic induction overthe area of the air gap, A; or it is the magnetic inductionmeasured at a specific point within the air gap; measured in gauss.
Bi (or J) Intrinsicinduction, is the contribution of the magnetic material tothe total magnetic induction, B. It is the vector differencebetween the magnetic induction in the material and the magneticinduction that would exist in a vacuum under the same fieldstrength, H. This relation is expressed by the equation:
Bi=B-H where: Bi = intrinsic induction in gauss; B = magneticinduction in gauss; H = field strength in oersteds .
Bm, Recoil induction,is the magnetic induction that remains in a magnetic material aftermagnetizing and conditioning for final use; measured in gauss.
Bo, Magnetic induction,at the point of the maximum energy product (BH)max; measured ingauss.
Br, Residual induction (or fluxdensity), is the magnetic induction corresponding to zeromagnetizing force in a magnetic material after saturation in aclosed circuit; measured in gauss.
A closed circuit condition existswhen the external flux path of a permanent magnet is confined withhigh permeability material.
CurieTemperature, The transition temperature above which thealloy loses its magnetic properties. This is not the maximumserviceable temperature, which is usually much lower.
The demagnetization curve is thesecond (or fourth) quadrant of a major hysteresis loop. Points onthis curve are designated by the coordinates Bd and Hd.
Domains, Areas in a magnetic alloy whichhave the same orientation. The magneticdomains are regions where the atomic moments of atomscooperate and allow for a common magnetic moment. It is thedomains which are rotated and manipulated by an externalmagnetizing field to create a useful magnet which has a netmagnetic moment. In un-magnetized material the domains areun-oriented and cancel each other out. In this conditionthere is no net external field.
Eddy currents, are circulatingelectrical currents that are induced in electrically conductiveelements when exposed to changing magnetic fields, creating anopposing force to the magnetic flux. Eddy currents can be harnessedto perform useful work (such as dampening of movement), or may beunwanted consequences of certain designs, which should be accountedfor of minimized.
Electromagnet, is a magnet,consisting of solenoid with an iron core, which has a magneticfield existing only during the time of current flow through thecoil.
f Reluctance factor, accounts forthe apparent magnetic circuit reluctance. This factor is requireddue to the treatment of H, and H, as constants.
F Leakage factor, accounts forflux leakage from the magnetic circuit. It is the ratio between themagnetic flux at the magnet neutral section and the average fluxpresent in the air gap. F=(B mA m)/(B, A g).
F Magneto motive force, (magneticpotential difference), is the line integral of the field strength,H, between any two points, p1 and p2.
p2
F=? H dl
p1
F = magneto motive force in gilberts
H = field strength in oersteds
dl = an element of length between the two points, in centimeters.
Ferromagnetic material, is amaterial whose permeability is very much larger than 1 (from 60 toseveral thousands times 1), and which exhibits hysteresisphenomena.
Flux, is the condition existing ina medium subjected to a magnetizing force. This quantity ischaracterized by the fact that an electromotive force is induced ina conductor surrounding the flux at any time the flux changes inmagnitude. The cgs unit of flux is the Maxwell.
A fluxmeter is an instrument that measures the change offlux linkage with a search coil.
Fringing fields are leakage fluxparticularly associated with edge effects in a magnetic circuit.
The gauss is the unit of magneticinduction, B, in the cgs electromagnetic system. One gauss is equalto one maxwell per square centimeter.
A gauss meter isan instrument that measures the instantaneous value of magneticinduction, B. Its principle of operation is usually based on one ofthe following: the Hall effect, nuclear magnetic resonance (NMR),or the rotating coil principle.
The gilbert is the unit of magnetomotive force, F, in the cgs electromagnetic system.
H Magnetic field strength,(magnetizing or demagnetizing force), is the measure of the vectormagnetic quantity that determines the ability of an electriccurrent, or a magnetic body, to induce a magnetic field at a givenpoint; measured in oersteds.
Hc Coercive force of amaterial, is equal to the demagnetizing force required toreduce residual induction, Br to zero in a magnetic field aftermagnetizing to saturation; measured in oersteds.
Hci Intrinsiccoercive force of a material, < indicates itsresistance to demagnetization. It is equal to the demagnetizingforce which reduces the intrinsic induction, Bi, in the material tozero after magnetizing to saturation; measured in oersteds.
Hd, is that value of Hcorresponding to the remnant induction, Bd; on the demagnetizationcurve, measured in oersteds.
Hmv, is that value of Hcorresponding to the recoil induction, B,; measured in oersteds.
Ho, is the magneticfield strength at the point of the maximum energy product (BH)max;measured in oersteds.
Hs, Net effectivemagnetizing force, is the magnetizing force required in thematerial, to magnetize to saturation measured in oersteds.
A hysteresis loop is a closedcurve obtained for a material by plotting (usually to rectangularcoordinates) corresponding values of magnetic induction, B, forordinates and magnetizing force, H, for abscissa when the materialis passing through a complete cycle between definite limits ofeither magnetizing force, H, or magnetic induction. B.
Irreversible losses are defined aspartial demagnetization of the magnet, caused by exposure to highor low temperatures external fields or other factors. These lossesare recoverable by remagnetization. Magnets can be stabilizedagainst irreversible losses by partial demagnetization induced bytemperature cycles or by external magnetic fields.
J, see Bi Intrinsic induction.
Js, see Bis, Saturation intrinsicinduction.
A keeper is a piece (or pieces) ofsoft iron that is placed on or between the pole faces of apermanent magnet to decrease the reluctance of the air gap andthereby reduce the flux leakage from the magnet. It also makes themagnet less susceptible to demagnetizing influences.
Keepers, A keeper is a high permeabilitymaterial, typically mild steel, which is installed on a magnet ormagnetic assembly to reduce the reluctance of the magneticcircuit. This reduces the overall leakage fields generated bythe magnet or magnetic assembly. Keepers are typicallyinstalled to help the magnet or magnetic assembly resistdemagnetization during handling, transportation, or storage. Keepers are typically found on Alnico magnets and Alnico magneticassemblies.
Knee of the demagnetization curveis the point at which the B-H curve ceases to be linear. All magnetmaterials, even if their second quadrant curves are straight lineat room temperature, develop a knee at some temperature. Alnico 5exhibits a knee at room temperature. If the operating point of amagnet falls below the knee, small changes in H produce largechanges in B, and the magnet will not be able to recover itsoriginal flux output without re-magnetization.
Leakage flux is flux, ø, whosepath is outside the useful or intended magnetic circuit; measuredin maxwells.
lg Length of the airgap, is the length of the path of the central flux line ofthe air gap; measured in centimeters.
lm Length of themagnet, is the total length of magnet material traversedin one complete revolution of the centerline of the magneticcircuit; measured in centimeters.
lm/D Dimension ratio,is the ratio of the length of a magnet to its diameter, or thediameter of a circle of equivalent cross-sectional area. For simplegeometries, such as bars and rods, the dimension ratio is relatedto the slope of the operating line of the magnet, BdHa.
Load line is a line drawn from theorigin of the demagnetization curve with a slope of B/H, theintersection of which with the B-H curve represents the operatingpoint of the magnet. Also see permeance coefficient.
Magnetic Assemblies, A combinationof materials, magnetic and non-magnetic, which form a particularsolution. Incorporates a permanent magnet as the flux generator andusually relies on mild steel to conduct the flux to the workface.Allows for better means of mounting-tapped holes, threads, pressfits, etc.
Magnetic circuit, an assemblyconsisting of some or all of the following: permanent magnets,ferromagnetic conduction elements, air gaps, and electricalcurrents.
Magnetic Length, The physical lengthof the magnet dimension which corresponds to the direction themagnet is magnetized. This may or may not be the magnet'sorientation direction.
The major hysteresis loop of amaterial is the closed loop obtained when the material is cycledbetween positive and negative saturation.
The maxwell is the unit ofmagnetic flux in the cgs electromagnetic system. One maxwell is oneline of magnetic flux.
The neutral section of a permanentmagnet is defined by a plane passing through the magnetperpendicular to its central flux line at the point of maximumflux.
North pole, is the pole of amagnet which, when freely suspended, would point to the northmagnetic pole of the earth. The definition of polarity can be aconfusing issue, and it is often the best to clarify by using“north seeking pole” instead of “north pole” in specifications.
The oersted is the unit ofmagnetic field strength, H, in the cgs electromagnetic system. Oneoersted equals a magneto motive force of one gilbert per centimeterof flux path.
An open circuit condition existswhen a magnetized magnet is by itself with no external flux path ofhigh permeability material.
The operating line for a givenpermanent magnet circuit is a straight line passing through theorigin of the demagnetization curve with a slope of negative Bd/Hd.(Also known as permeance coefficient line.)
The operating point of a permanentmagnet is that point on a demagnetization curve defined by thecoordinates (BdHd) or that point within the demagnetization curvedefined by the coordinates (BmHm).
Orientation direction, is the directionin which an anisotropic magnet should be magnetized in order toachieve optimum magnetic properties. Also known as the “axis,”“easy axis,” or “angle of inclination.”
An oriented (anisotropic) materialis one that has better magnetic properties in a given direction.
Paramagnetic material, is amaterial having a permeability slightly greater than 1.
A permeameter is an instrumentthat can measure, and often record, the magnetic characteristics ofa specimen.
P Permeance, is the reciprocal ofthe reluctance, R, measured in maxwells per gilbert.
Pole pieces, are ferromagneticmaterials placed on magnetic poles used to shape and alter theeffect of lines of flux.
R Reluctance, is somewhatanalogous to electrical resistance. It is the quantity thatdetermines the magnetic flux, ø, resulting from a given magnetomotive force, F.
Where: R=F/ø
R=reluctance, in gilberts per maxwell
F=magneto motive force, I gilberts
Ø=flux, in maxwells.
Return path, are conductionelements in a magnetic circuit, which provide a low reluctance pathfor the magnetic flux.
Saturation, A condition where theincrease in applied external field yields no increase ininduction. When this condition is met, all of the elementarymagnetic moments have the same alignment. This condition isimportant in permanent magnet alloys and in Ferromagneticalloys. Magnet alloys must always be magnetized tosaturation. The magnet may not be used at this level, butbefore conditioning and stabilization the magnet mustalways first be magnetized to saturation. Usuallysaturation should not be exceeded in Ferromagnetic alloyswhich comprise the yoke or return path elements of a magneticcircuit. If ferromagnetic elements are saturat ed therewill be flux leakage in the system and a redesign should beconsidered.
A search coil is a coiledconductor, usually of known area and number of turns, that is usedwith a flux meter to measure the change of flux linkage with thecoil.
Sintered, A sintered magnet iscomprised of a compacted powder which is then subjected to a heattreat operation where the full density and magnetic orientation isachieved.
Stabilization, is exposure of amagnet to demagnetizing influences expected to be encountered inuse in order to prevent irreversible losses during actualoperation. Demagnetizing influences can be caused by high or lowtemperatures, or by external magnetic fields.
Tc, Curie temperature,is the transition temperature above which a material loses itsmagnet properties.
T max Maximum service temperature,is the maximum temperature to which the magnet may be exposed withno significant long-range instability or structural changes.
Reversible temperaturecoefficients are changes in flux which occur withtemperature change. These are spontaneously regained when thetemperature is returned to its original point.Magnetic saturationof a material exists when an increase in magnetizing force producesno increase in intrinsic induction.
The temperature coefficient is a factor whichdescribes the reversible change in a magnetic property with achange in temperature. The magnetic property spontaneously returnswhen the temperature is cycled to its original point. It usually isexpressed as the percentage change per unit of temperature.
An unoriented (isotropic) materialhas equal magnetic properties in all directions.
Vg Air gap volume, isthe useful volume of air or nonmagnetic material between magneticpoles; measured in cubic centimeters.
Weber, is the practical unit ofmagnetic flux. It is the amount of magnetic flux which, when linkedat a uniform rate with a single-turn electric current during aninterval of 1 second, will induce in this circuit an electromotiveof force of 1 volt.
µ permeability, is the generalterm used to express various relationships between magneticinduction, B, and the field strength, H.
µre recoilpermeability, is the average slope of the recoilhysteresis loop. Also known as the minor loop.
ø magnetic flux, is a contrivedbut measurable concept that has evolved in an attempt to describethe “flow” of a magnetic field. Mathematically, it is the surfaceintegral of the normal component of the magnetic induction, B, overan area, A.
ø = ??B • dA
where:
ø = magnetic flux, in maxwells
B = magnetic induction, in gauss
dA = an element of area, in square centimeters
When the magnetic induction, B, is uniformly distributed and isnormal to the area, A, the flux, ø = BA.