magnetglossary ||

Air Gap:

Alow permeability gap in the flux path of amagnetic circuit. Oftenair, but inclusive of other materials suchas paint, aluminum,etc.

AnisotropicMagnet:

Amagnet having a preferred direction of magneticorientation, sothat the magnetic characteristics are optimum in onepreferreddirection.

ClosedCircuit:

Thisexists when the flux path external to apermanent magnet isconfined within high permeability materials thatcompose the magnetcircuit.

CoerciveForce,H_c:

Thedemagnetizing force, measured in Oersteds,necessary to reduceobserved induction, B, to zero after the magnethas previously beenbrought to saturation.

CurieTemperature,T_c:

Thetemperature at which the parallel alignment ofelementary magneticmoments completely disappears, and the materialis no longer ableto hold magnetization.

DemagnetizationCurve:

Thesecond quadrant of the hysteresis loop,generally describing thebehavior of magnetic characteristics inactual use. Also known asthe B-H Curve.

EddyCurrents:

Circulating electrical currents that are inducedinelectrically conductive elements when exposed to changingmagneticfields, creating an opposing force to the magnetic flux.Eddycurrents can be harnessed to perform useful work (such asdampingof movement), or may be unwanted consequences of certaindesigns,which should be accounted for or minimized.

Electromagnet:

Amagnet, consisting of a solenoid with an ironcore, which has amagnetic field existing only during the time ofcurrent flowthrough the coil.

EnergyProduct:

Indicates the energy that a magnetic material cansupply to anexternal magnetic circuit when operating at any pointon itsdemagnetization curve. Calculated as B_d xH_d,and measured in Mega Gauss Oersteds, MGOe.

FerromagneticMaterial:

Amaterial whose permeability is very much largerthan 1 (from 60 toseveral thousand times 1), and which exhibitshysteresisphenomena.

Flux

The condition existing in a mediumsubjected to amagnetizing force. This quantity is characterized bythe fact thatan electromotive force is induced in a conductorsurrounding theflux at any time the flux changes in magnitude. Thecgs unit offlux is the Maxwell.

fluxmeter:

Aninstrument that measures the change of fluxlinkage with a searchcoil.

FringingFields:

Leakage flux particularly associated with edgeeffects in amagnetic circuit.

Gauss:

Lines of magnetic flux per square centimeter, cgsunit of fluxdensity, equivalent to lines per square inch in theEnglish system,and Webers per square meter or Tesla in the SIsystem.

gaussmeter:

Aninstrument that measures the instantaneousvalue of magneticinduction, B. Its principle of operation isusually based on one ofthe following: the Hall effect, nuclearmagnetic resonance (NMR),or the rotating coil principle.

HysteresisLoop:

Aclosed curve obtained for a material by plottingcorrespondingvalues of magnetic induction, B, (on the abscissa)againstmagnetizing force, H, (on the ordinate).

Induction,B:

Themagnetic flux per unit area of a sectionnormal to the direction offlux. Measured in Gauss, in the cgssystem of units.

Intrinsic CoerciveForce,Hci:

Measured in Oersteds in the cgs system, this is ameasure ofthe materialيs inherent ability to resistdemagnetization. It isthe demagnetization force corresponding tozero intrinsic inductionin the magnetic material after saturation.Practical consequencesof high Hci values are seen in greatertemperature stability for agiven class of material, and greaterstability in dynamic operatingconditions.

Intrinsic Induction,Bi:

Thecontribution of the magnetic material to thetotal magneticinduction, B. It is the vector difference between themagneticinduction in the material and the magnetic induction thatwouldexist in a vacuum under the same field strength, H.Thisrelationship is expressed as: Bi = B-H.

IrreversibleLoss:

Defined as the partial demagnetization of amagnet caused byexternal fields or other factors. These losses areonly recoverableby re-magnetization. Magnets can be stabilized toprevent thevariation of performance caused by irreversiblelosses.

IsotropicMagnet:

Amagnet material whose magnetic properties arethe same in anydirection, and which can therefore be magnetized inany directionwithout loss of magnetic characteristics.

Keeper:

Apiece of soft iron that is placed on or betweenthe poles of amagnet, decreasing the reluctance of the air gap andtherebyreducing the flux leakage from the magnet.

Knee of theDemagnetizationCurve:

Thepoint 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:

Thatportion of the magnetic flux that is lostthrough leakage in themagnetic circuit due to saturation orair-gaps, and is thereforeunable to be used.

Length ofair-gap,L_g:

Thelength of the path of the central flux line inthe air-gap.

Load Line:

Aline drawn from the origin of theDemagnetization Curve with aslope of -B/H, the intersection ofwhich with the B-H curverepresents the operating point of themagnet. Also see PermeanceCoefficient.

Magnetic Circuit:

Anassembly consisting of some or all of thefollowing: permanentmagnets, ferromagnetic conduction elements, airgaps, electricalcurrents.

MagneticFlux:

Thetotal magnetic induction over a given area.When the magneticinduction, B, is uniformly distributed over anarea A, MagneticFlux = BA.

MagnetizingForce,H:

Themagnetomotive force per unit length at anypoint in a magneticcircuit. Measured in Oersteds in the cgssystem.

Magnetomotive Force,F:

Analogous to voltage in electrical circuits, thisis themagnetic potential difference between any two points.

Maximum EnergyProduct,BHmax:

Thepoint on the Demagnetization Curve where theproduct of B and H isa maximum and the required volume of magnetmaterial required toproject a given energy into its surroundings isa minimum. Measuredin Mega Gauss Oersteds, MGOe.

NorthPole:

Thatpole of a magnet which, when freelysuspended, would point to thenorth magnetic pole of the earth. Thedefinition of polarity can bea confusing issue, and it is oftenbest to clarify by using "northseeking pole" instead of "northpole" in specifications.

Oersted,Oe:

Acgs unit of measure used to describe magnetizingforce. The Englishsystem equivalent is Ampere Turns per Inch, andthe SI systemيs isAmpere Turns per Meter.

OrientationDirection:

Thedirection in which an anisotropic magnetshould be magnetized inorder to achieve optimum magneticproperties. Also known as the"axis", "easy axis", or "angle ofinclination".

ParamagneticMaterial:

Amaterial having a permeability slightly greaterthan 1.

Permeance:

Theinverse of reluctance, analogous toconductance in electricalcircuits.

PermeanceCoefficient,Pc:

Ratio of the magnetic induction, Bd, to its selfdemagnetizingforce, Hd. Pc = Bd / Hd. This is also known as the"load line","slope of the operating line", or operating point ofthe magnet,and is useful in estimating the flux output of themagnet invarious conditions. As a first order approximation, Bd /Hd =Lm/Lg, where Lm is the length of the magnet, and Lg is thelengthof an air gap that the magnet is subjected to. Pc istherefore afunction of the geometry of the magneticcircuit.

PolePieces:

Ferromagnetic materials placed on magnetic polesused to shapeand alter the effect of lines of flux.

RelativePermeability:

Theratio of permeability of a medium to that of avacuum. In the cgssystem, the permeability is equal to 1 in avacuum by definition.The permeability of air is also for allpractical purposes equal to1 in the cgs system.

Reluctance,R:

Analogous to resistance in an electrical circuit,reluctance isrelated to the magnetomotive force, F, and themagnetic flux by theequation R = F/(Magnetic Flux), parallelingOhm's Law where F isthe magnetomotive force (in cgs units).

Remanence,Bd:

Themagnetic induction that remains in a magneticcircuit after theremoval of an applied magnetizing force. If thereis an air gap inthe circuit, the remanence will be less than theresidualinduction, Br.

ResidualInduction,Br_:

Thisis the point at which the hysteresis loopcrosses the B axis atzero magnetizing force, and represents themaximum flux output fromthe given magnet material. By definition,this point occurs at zeroair gap, and therefore cannot be seen inpractical use of magnetmaterials.

ReturnPath:

Conduction elements in a magnetic circuit whichprovide a lowreluctance path for the magnetic flux.

ReversibleTemperatureCoefficient:

Ameasure of the reversible changes in flux causedby temperaturevariations.

Saturation:

Thecondition under which all elementary magneticmoments have becomeoriented in one direction. A ferromagneticmaterial is saturatedwhen an increase in the applied magnetizingforce produces noincrease in induction. Saturation flux densitiesfor steels are inthe range of 16,000 to 20,000 Gauss.

SearchCoil:

Acoil conductor, usually of known area and numberof turns that isused with a fluxmeter to measure the change of fluxlinkage withthe coil.

Stabilization:

Exposure of a magnet to demagnetizing influencesexpected to beencountered in use in order to prevent irreversiblelosses duringactual operation. Demagnetizing influences can becaused by high orlow temperatures, or by external magneticfields.

TemperatureCoefficient:

Afactor, which describes the change in a magneticproperty withchange in temperature. Expressed as percent change perunit oftemperature.

Weber:

Thepractical unit of magnetic flux. It is theamount of magnetic fluxwhich, when linked at a uniform rate with asingle-turn electriccircuit during an interval of 1 second, willinduce in this circuitan electromotive force of 1volt.