Knowledge Base

Term

Description

**Anisotropic**

Material that have a "preferred" magnetization direction. These materials are typically manufactured in the influence of strong magnetic fields, and can only be magnetized through the preferred axis.

**Area of the air gap**

The cross sectional area of the air gap perpendicular to the flux path is the average cross sectional area of that portion of the air gap within which the application interaction occurs. Area is measured in sq. cm. in a plane normal to the central flux line of the air gap.

**Area of the magnet**

The cross-sectional area of the magnet perpendicular to the central flux line, measured in sq. cm. at any point along its length. In design, A*m* is usually considered the area at the neutral section of the magnet.

**BH Curve**

See Demagnetization Curve.

**CSM**

**C**ustomer **S**upplied **M**aterial. Referenced on quotations, sales orders, and invoices. Eg: Assemble 71399 Rev A to CSM. Here part# 71399 has been assembled to a customer supplied component.

**Closed circuit condition**

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

**Coercive force**

Of a material, is equal to the demagnetizing force required to reduce residual induction, Br, to zero in a magnetic field after magnetizing to saturation; measured in oersteds.

**Demagnetization curve**

Is the second (or fourth) quadrant of a major hysteresis loop. Points on this curve are designated by the coordinates Bd and Hd. This curve is also referred to as the Second Quadrant Curve, BH Curve, or abbreviated to "curve".

**Energy product**

Indicates the 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.

**FOD**

Foreign Object Damage - this is any material that may be introduced into a magnet assembly that was not designed to be there. Examples are coins, food crumbs, parts of a fixture or tool that was used to manufacture/assemble the component that was left behind inadvertently. FOD can cause disastrous problems and therefore technicians must be aware of this to prevent such contamination.

**Fluxmeter**

Is an instrument that measures the change of flux linkage with a search coil or Helmholtz coil. A fluxmeter is basically a voltage integrator, which is an integrating device totalizing the voltage output with respect to time.

**Gauss**

Unit of magnetic induction, B, in the cgs electromagnetic system. One gauss is equal to one maxwell per square centimeter.

**Gaussmeter**

Is an instrument that measures the instantaneous value of magnetic induction, B. Its principle of operation is usually based on one of the following; the Hall-effect, nuclear magnetic resonance (NMR), or the rotating coil principle.

**Gilbert**

The unit of magnetomotive force, F, in the cgs electromagnetic system.

**Hci Intrinsic coercive force**

Of a material indicates its resistance to demagnetization. It is equal to the demagnetizing force which reduces the intrinsic induction, Bi, in the material to zero after magnetizing to saturation; measured in oersteds.

**Hd**

Is that value of H corresponding to the remanent induction, Bd; measured in oersteds.

**Hk Coercive force - k**

The value of Hci at 0.9Br. This value gives an indication of the "squareness" of the intrinsic curve. The more square the intrinsic curve, the closer the material is to being ideal. Hk values that approach the Hci values are considered extremely good materials.

**Hm**

Is that value of H corresponding to the recoil induction, Bm; measured in oersteds.

**Ho**

Is the magnetic field strength at the point of the maximum energy product (BH)max; measured in oersteds.

**Hs Net effective magnetizing force**

The minimum magnetizing force required in the material to magnetize it to saturation, measured in oersteds.

**Hysteresis Loop**

The closed curve obtained for a material by plotting (usually to rectangular coordinates) corresponding values of magnetic induction, B, for ordinates and magnetizing force, H, for abscissa when the material is passing through a complete cycle between definite limits of either magnetizing force, H, or magnetic induction, B.

**Hysteresis Loop, Major**

Of a material is the closed loop obtained when the material is cycled between positive and negative saturation.

**Intrinsic induction**

Bi = B - H

where: Bi = intrinsic induction in gauss; B = magnetic induction in gauss; H = field strength in oersteds.

**Irreversible Losses**

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

**Isotropic**

A magnetic material that has the "same" properties in all directions. Such a material may be magnetized in any direction since it does not have a "preferred" alignment direction. Also known as "unoriented" material.

**J**

See Bi Intrinsic induction.

**Js**

See Bis Saturation intrinsic induction.

**Keeper**

Is a piece (or pieces) of soft iron that is placed on or between the pole faces of a permanent magnet to decrease the reluctance of the air gap and thereby reduce the flux leakage from the magnet. It also makes the magnet less susceptible to demagnetizing influences.

**Leakage Flux**

Is that flux, Ų, whose path is outside the useful or intended magnetic circuit; measured in maxwells.

**Leakage factor**

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. F = (Bm Am)(Bg Ag)

**Lg Length of the air gap**

Is the length of the path of the central flux line of the air gap; measured in centimeters.

**Lm Length of the magnet**

The total length of magnet material (magnetic length) traversed in one complete revolution of the centerline of the magnetic circuit. An easy way to think of this is the distance between the magnetic poles. (Measured in centimeters when using the cgs system for calculations, but typically inches are used when communicating with manufacturers).

**Lm/D Dimension ratio**

The ratio of the length of a magnet to its diameter, or the diameter of a circle of equivalent cross-sectional area. For simple geometries, such as bars and rods, the dimension ratio is related to the slope of the operating line of the magnet, Bd/Hd.

**Magnetic field strength**

Magnetizing or demagnetizing force, is the measure of the vector magnetic quantity that determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point; measured in oersteds.

**Magnetic induction**

The magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and the resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.

**Magnetic induction**

Magnetic induction at the point of the maximum energy product (BH)max; measured in gauss in the cgs system.

**Magnetic induction in the air gap**

Is the average value of magnetic induction over the area of the air gap, Ag; or it is the magnetic induction measured at a specific point within the air gap; measured in gauss.

**Magnetic saturation**

Of a material exists when an increase in magnetizing force, H, does not cause an increase in the intrinsic magnetic induction, B, of the material.

**Magnetomotive force, (magnetic potential difference)**

Is the line integral of the field strength, H, between any two points, p1 and p2.

p2

*F* = ∫ H dl

p1

*F* = magnetomotive force in gilberts

H = field strength in oersteds

dl = an element of length between the two points, in centimeters.

**Maximum energy product**

Is the maximum product of (BdHd) which can be obtained on the demagnetization curve.

**Maxwell**

Unit of magnetic flux in the cgs electromagnetic system. One maxwell is one line of magnetic flux.

**Neutral Section**

Of a permanent magnet is defined by a plane passing through the magnet perpendicular to its central flux line at the point of maximum flux.

**Oersted**

The unit of magnetic field strength, H, in the cgs electromagnetic system. One oersted equals a magnetomotive force of one gilbert per centimeter of flux path.

**Operating Line**

For a given permanent magnet circuit is a straight line passing through the origin of the demagnetization curve with a slope of negative Bd/H. (Also known as permeance coefficient line.)

**Operating Point**

Of a permanent magnet is that point on a demagnetization curve defined by the coordinates (BdHd) or that point within the demagnetization curve defined by the coordinates (BmHm)

**Oriented Material**

An anisotropic material. Material with a preferred direction of magnetization. This type of material should be magnetized only through this preferred direction. Trying to magnetize through the other directions will result in substantial losses in magnetic properties, and the data provided will not be valid.

**P Permeance**

Is the reciprocal of the reluctance, R, measured in maxwells per gilbert.

**Permeameter**

Is an instrument that can measure, and often record, the magnetic characteristics of a specimen.

**Polarity**

The characteristic of a particular pole at a particular location of a permanent magnet. Differentiates the North from the South Pole.

**R Reluctance**

Is somewhat analogous to electrical resistance. It is the quantity that determines the magnetic flux, Ų,resulting from a given magnetomotive force, F.where: R = F/Ų

R = reluctance, in gilberts per maxwell

F = magnetomotive force, in gilberts

Ų = flux, in maxwells

**Recoil induction, Bm**

The magnetic induction that remains in a magnetic material after magnetizing and conditioning for final use; measured in gauss.

**Reluctance factor**

Accounts for the apparent magnetic circuit reluctance. This factor is required due to the treatment of Hm and Hg as constants.

**Remanent induction**

Is any magnetic induction that remains in a magnetic material after removal of an applied saturating magnetic field, Hs. (Bd is the magnetic induction at any point on the demagnetization curve; measured in gauss).

Residual induction

(or flux density), is the magnetic induction corresponding to zero magnetizing force in a magnetic material after saturation in a closed circuit; measured in gauss, shown in data as "Br".

**Reversible temperature coefficient**

Are changes in flux, which occur with temperature change. These are spontaneously regained when the temperature is returned to its original point.

**Saturation intrinsic induction**

Is the maximum intrinsic induction possible in a material.

**Search Coil**

A coiled conductor, usually of known area and number of turns, that is used with a fluxmeter to measure the change of flux linkage with the coil.

**Slope of the operating line**

Is the ratio of the remanent induction, Bd, to a demagnetizing force, Hd. It is also referred to as the permeance coefficient, shear line, load line and unit permeance.

**Tc Curie temperature**

Is the transition temperature above which a material loses its magnet properties.

**Temperature coefficient of Br**

A factor which describes the reversible change in a magnetic property with a change in temperature. The magnetic property spontaneously returns when the temperature is cycled to its original point. It usually is expressed as the percentage change per unit of temperature. See article Stabilization and Calibration in the Knowledge Base for more information.

**Tmax Maximum Operating Temperature**

The maximum operating temperature, also known as maximum service temperature, is the temperature at which the magnet may be exposed to continuously with no significant long-range instability or structural changes. Note that this temperature is a function of the operating point of the magnet, and not an absolute value. See Temperature effects on magnets for more information.

**Unoriented**

See isotropic

**Vg Air gap volume**

Is the useful volume of air or non-magnetic material between magnetic pole; measured in cubic centimeters.

**open circuit condition**

Exists when a magnetized magnet is by itself with no external flux path of high permeability material.

**permeance coefficient line**

See operating line.

**µ Permeability**

Is the general term used to express various relationships between magnetic induction, B, and the field strength, H.

**µre Recoil Permeability**

The average slope of the recoil hysteresis loop. Also known as a minor loop.

**Ų Magnetic Flux**

Is a contrived but measurable concept that has evolved in an attempt to describe the “flow” of a magnetic field. Mathematically, it is the surface integral of the normal component of the magnetic induction, B, over an area, A.

Ų = ∫∫ B **·** dA

where:

Ų = 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 is normal to the area, A, the flux, Ų = BA.