Glossaryof Magnet Terms
Air gap, is a low permeabilitygapin the flux path of a magnetic circuit. Often air, but inclusiveofother materials such as paint, aluminum, etc.
Anisotropy, Literally meanshavingdifferent properties depending onthe inspecteddirection. Magnets which are anisotropic,or have an easyaxis of magnetization, have their anisotropydeveloped by twomethods: Shape and Magnetocrystalline.
As Area of the airgap,or the cross sectional area of the air gapperpendicular to theflux path, is the average cross sectional areaof that portion ofthe air gap within which the applicationinteraction occurs. Areais measured in sq. cm. in a plane normal tothe central flux lineof the air gap.
Am Area of themagnet,is the cross sectional area of the magnetperpendicular to thecentral flux line, measured in sq. cm. at anypoint along itslength. In design, Am is usually considered the areaat the neutralsection of the magnet.
B Magnetic induction, isthemagnetic field induced by a field strength, H, at a given point.Itis the vector sum, at each point within the substance, ofthemagnetic field strength and resultant intrinsic induction.Magneticinduction is the flux per unit area normal to the directionof themagnetic path.
Bd Remnant induction,isany magnetic induction that remains in a magnetic materialafterremoval of an applied saturating magnetic field, Hs. (Bd isthemagnetic induction at any point on the demagnetizationcurve: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 asthepermeance coefficient, shear line, load line and unit permeance.
BdHdEnergyproduct, indicates the energy that a magnetic material cansupplyto an external magnetic circuit when operating at any pointon itsdemagnetization curve; measured in megagauss-oersteds.
(BH) max Maximumenergyproduct, is the maximum product of (BdHd) which canbeobtained on the demagnetization curve.
Bis, (or J) Saturationintrinsicinduction, is the maximum intrinsic inductionpossible ina material.
Bg, Magnetic induction in theairgap, is the average value of magnetic induction overthearea of the air gap, A; or it is the magnetic inductionmeasured ata specific point within the air gap; measured in gauss.
Bi (or J)Intrinsicinduction, is the contribution of the magneticmaterial tothe total magnetic induction, B. It is the vectordifferencebetween the magnetic induction in the material and themagneticinduction that would exist in a vacuum under the samefieldstrength, 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,isthe magnetic induction that remains in a magnetic materialaftermagnetizing and conditioning for final use; measured in gauss.
Bo, Magneticinduction,at the point of the maximum energy product(BH)max; measured ingauss.
Br, Residual induction (orfluxdensity), is the magnetic induction corresponding tozeromagnetizing force in a magnetic material after saturation inaclosed circuit; measured in gauss.
A closed circuit conditionexistswhen the external flux path of a permanent magnet is confinedwithhigh permeability material.
CurieTemperature, The transition temperatureabove which thealloy loses its magnetic properties. This isnot the maximumserviceable temperature, which is usually muchlower.
The demagnetization curve isthesecond (or fourth) quadrant of a major hysteresis loop. Pointsonthis curve are designated by the coordinates Bd and Hd.
Domains, Areas in a magnetic alloy whichhavethe same orientation. Themagneticdomains are regions where the atomic moments ofatomscooperate and allow for a common magnetic moment. It isthedomains which are rotated and manipulated by anexternalmagnetizing field to create a useful magnet which has anetmagnetic moment. In un-magnetized material the domainsareun-oriented and cancel each other out. In thisconditionthere is no net external field.
Eddy currents, arecirculatingelectrical currents that are induced in electricallyconductiveelements when exposed to changing magnetic fields,creating anopposing force to the magnetic flux. Eddy currents canbe harnessedto perform useful work (such as dampening of movement),or may beunwanted consequences of certain designs, which should beaccountedfor of minimized.
Electromagnet, is amagnet,consisting of solenoid with an iron core, which has amagneticfield existing only during the time of current flow throughthecoil.
f Reluctance factor, accountsforthe apparent magnetic circuit reluctance. This factor isrequireddue to the treatment of H, and H, as constants.
F Leakage factor, accounts forfluxleakage from the magnetic circuit. It is the ratio betweenthemagnetic flux at the magnet neutral section and the averagefluxpresent in the air gap. F=(B mA m)/(B, A g).
F Magneto motive force,(magneticpotential difference), is the line integral of the fieldstrength,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, isamaterial whose permeability is very much larger than 1 (from 60toseveral thousands times 1), and which exhibitshysteresisphenomena.
Flux, is the condition existingina medium subjected to a magnetizing force. This quantityischaracterized by the fact that an electromotive force is inducedina conductor surrounding the flux at any time the flux changesinmagnitude. The cgs unit of flux is the Maxwell.
A fluxmeter is an instrument thatmeasures the change offlux linkage with a search coil.
Fringing fields are leakagefluxparticularly associated with edge effects in a magneticcircuit.
The gauss is the unit ofmagneticinduction, B, in the cgs electromagnetic system. One gaussis equalto one maxwell per square centimeter.
A gaussmeter isan instrument that measures theinstantaneous value of magneticinduction, B. Its principle ofoperation is usually based on one ofthe following: the Hall effect,nuclear magnetic resonance (NMR),or the rotating coil principle.
The gilbert is the unit ofmagnetomotive force, F, in the cgs electromagnetic system.
H Magnetic fieldstrength,(magnetizing or demagnetizing force), is themeasure of the vectormagnetic quantity that determines the abilityof an electriccurrent, or a magnetic body, to induce a magneticfield at a givenpoint; measured in oersteds.
Hc Coercive force ofamaterial, is equal to the demagnetizing force requiredtoreduce residual induction, Br to zero in a magnetic fieldaftermagnetizing to saturation; measured in oersteds.
HciIntrinsiccoercive force of a material, <indicates itsresistance to demagnetization. It is equal to thedemagnetizingforce which reduces the intrinsic induction, Bi, inthe material tozero after magnetizing to saturation; measured inoersteds.
Hd, is that value ofHcorresponding to the remnant induction, Bd; on thedemagnetizationcurve, measured in oersteds.
Hmv, is that value ofHcorresponding to the recoil induction, B,; measured in oersteds.
Ho, is themagneticfield strength at the point of the maximum energy product(BH)max;measured in oersteds.
Hs, Neteffectivemagnetizing force, is the magnetizing force required inthematerial, to magnetize to saturation measured in oersteds.
A hysteresis loop is a closedcurveobtained for a material by plotting (usually torectangularcoordinates) corresponding values of magnetic induction,B, forordinates and magnetizing force, H, for abscissa when thematerialis passing through a complete cycle between definite limitsofeither magnetizing force, H, or magnetic induction. B.
Irreversible losses are definedaspartial demagnetization of the magnet, caused by exposure tohighor low temperatures external fields or other factors. Theselossesare recoverable by remagnetization. Magnets can bestabilizedagainst irreversible losses by partial demagnetizationinduced bytemperature cycles or by external magnetic fields.
J, see Bi Intrinsic induction.
Js, see Bis, Saturationintrinsicinduction.
A keeper is a piece (or pieces)ofsoft iron that is placed on or between the pole faces ofapermanent magnet to decrease the reluctance of the air gapandthereby reduce the flux leakage from the magnet. It also makesthemagnet less susceptible to demagnetizing influences.
Keepers, A keeper is a highpermeabilitymaterial, typically mild steel, which is installed on amagnet ormagnetic assembly to reduce the reluctance of themagneticcircuit. This reduces the overall leakage fieldsgenerated bythe magnet or magnetic assembly. Keepers aretypicallyinstalled to help the magnet or magnetic assemblyresistdemagnetization during handling, transportation, orstorage. Keepers are typically found on Alnico magnets andAlnico magneticassemblies.
Knee of the demagnetizationcurveis the point at which the B-H curve ceases to belinear. All magnetmaterials, even if their second quadrant curvesare straight lineat room temperature, develop a knee at sometemperature. Alnico 5exhibits a knee at room temperature. If theoperating point of amagnet falls below the knee, small changes in Hproduce largechanges in B, and the magnet will not be able torecover itsoriginal flux output without re-magnetization.
Leakage flux is flux, ø, whosepathis outside the useful or intended magnetic circuit; measuredinmaxwells.
lg Length of theairgap, is the length of the path of the central flux lineofthe air gap; measured in centimeters.
lm Length of themagnet,is the total length of magnet material traversedin one completerevolution of the centerline of the magneticcircuit; measured incentimeters.
lm/D Dimension ratio,isthe ratio of the length of a magnet to its diameter, or thediameterof a circle of equivalent cross-sectional area. Forsimplegeometries, such as bars and rods, the dimension ratio isrelatedto the slope of the operating line of the magnet, BdHa.
Load line is a line drawn fromtheorigin of the demagnetization curve with a slope of B/H,theintersection of which with the B-H curve represents theoperatingpoint of the magnet. Also see permeance coefficient.
Magnetic Assemblies, Acombinationof materials, magnetic and non-magnetic, which form aparticularsolution. Incorporates a permanent magnet as the fluxgenerator andusually relies on mild steel to conduct the flux tothe workface.Allows for better means of mounting-tapped holes,threads, pressfits, etc.
Magnetic circuit, anassemblyconsisting of some or all of the following: permanentmagnets,ferromagnetic conduction elements, air gaps, andelectricalcurrents.
Magnetic Length, The physicallengthof the magnet dimension which corresponds to thedirection themagnet is magnetized. This may or may not be themagnet'sorientation direction.
The major hysteresis loop ofamaterial is the closed loop obtained when the material iscycledbetween positive and negative saturation.
The maxwell is the unit ofmagneticflux in the cgs electromagnetic system. One maxwell is oneline ofmagnetic flux.
The neutral section of apermanentmagnet is defined by a plane passing through themagnetperpendicular to its central flux line at the point ofmaximumflux.
North pole, is the pole of amagnetwhich, when freely suspended, would point to the northmagnetic poleof 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 ofmagneticfield strength, H, in the cgs electromagnetic system. Oneoerstedequals a magneto motive force of one gilbert per centimeterof fluxpath.
An open circuit conditionexistswhen a magnetized magnet is by itself with no external fluxpath ofhigh permeability material.
The operating line for agivenpermanent magnet circuit is a straight line passing throughtheorigin of the demagnetization curve with a slope of negativeBd/Hd.(Also known as permeance coefficient line.)
The operating point of apermanentmagnet is that point on a demagnetization curve defined bythecoordinates (BdHd) or that point within the demagnetizationcurvedefined by the coordinates (BmHm).
Orientation direction, is thedirectionin which an anisotropic magnet should be magnetized inorder 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 agiven direction.
Paramagnetic material, isamaterial having a permeability slightly greater than 1.
A permeameter is an instrumentthatcan measure, and often record, the magnetic characteristics ofaspecimen.
P Permeance, is the reciprocalofthe reluctance, R, measured in maxwells per gilbert.
Pole pieces, areferromagneticmaterials placed on magnetic poles used to shape andalter theeffect of lines of flux.
R Reluctance, is somewhatanalogousto electrical resistance. It is the quantity thatdetermines themagnetic 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, areconductionelements in a magnetic circuit, which provide a lowreluctance pathfor the magnetic flux.
Saturation, A condition wheretheincrease in applied external field yields no increaseininduction. When this condition is met, all of theelementarymagnetic moments have the same alignment. Thiscondition isimportant in permanent magnet alloys and inFerromagneticalloys. Magnet alloys must always be magnetizedtosaturation. The magnet may not be used at this level,butbefore conditioning and stabilization the magnetmustalways first be magnetized to saturation. Usuallysaturation should not be exceeded in Ferromagneticalloyswhich comprise the yoke or return path elements of amagneticcircuit. If ferromagnetic elements are saturated therewill be flux leakage in the system and a redesignshould beconsidered.
A search coil is acoiledconductor, usually of known area and number of turns, that isusedwith a flux meter to measure the change of flux linkage withthecoil.
Sintered, A sintered magnetiscomprised of a compacted powder which is then subjected to aheattreat operation where the full density and magnetic orientationisachieved.
Stabilization, is exposure ofamagnet to demagnetizing influences expected to be encounteredinuse in order to prevent irreversible losses duringactualoperation. Demagnetizing influences can be caused by high orlowtemperatures, or by external magnetic fields.
Tc, Curietemperature,is the transition temperature above which a materialloses itsmagnet properties.
T max Maximum servicetemperature,is the maximum temperature to which the magnetmay be exposed withno significant long-range instability orstructural changes.
Reversible temperaturecoefficientsare changes in flux which occur withtemperature change. These arespontaneously regained when thetemperature is returned to itsoriginal point.Magnetic saturationof a material exists when anincrease in magnetizing force producesno increase in intrinsicinduction.
The temperature coefficient is a factorwhichdescribes the reversible change in a magnetic property withachange in temperature. The magnetic property spontaneouslyreturnswhen the temperature is cycled to its original point. Itusually isexpressed as the percentage change per unit oftemperature.
An unoriented (isotropic)materialhas equal magnetic properties in all directions.
Vg Air gap volume,isthe useful volume of air or nonmagnetic material betweenmagneticpoles; measured in cubic centimeters.
Weber, is the practical unitofmagnetic flux. It is the amount of magnetic flux which, whenlinkedat a uniform rate with a single-turn electric current duringaninterval of 1 second, will induce in this circuit anelectromotiveof force of 1 volt.
µ permeability, is the generaltermused to express various relationships between magneticinduction, B,and the field strength, H.
µre recoilpermeability,is the average slope of the recoilhysteresis loop. Also known asthe minor loop.
ø magnetic flux, is a contrivedbutmeasurable concept that has evolved in an attempt to describethe“flow” of a magnetic field. Mathematically, it is thesurfaceintegral 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 andisnormal to the area, A, the flux, ø = BA.