### Glossary

**Air gap:** Space between the poles of a magnet in which there exists a useable magnetic field.

**Anisotropic Magnet: **A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in one preferred direction.

**Closed Circuit:** This exists when the flux path external to a permanent magnet is confined within high permeability materials that compose the magnet circuit.

**Coercive Force, Hc: **The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, B, to Zero after the magnet has prviously been brought to saturation.

**Curie Temperature, Tc:** The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the material is no longer able to hold magnetization.

**Demagnetization Curve:** The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use, Also known as the B-H curve.

**Columnar crystalline materials: **Especially AlNiCo alloys where an orientation of the crystals is formed by a controlled solidification of the molten material. The material AlNiCo 700 shows a very distinct anisotropy, in contrast to types where an anisotropy is produced only by applying a magnetic field during heat treatment.

**Dipole field: **First approximation of the field of a magnet at a large distance. The dipole field is defined only by orientation and amount of the magnetic moment and decreases according to 1/r3 with increasing distance r.

**Dipole moment: **see moment (magnetic).

**Flux density B: **No. of field lines per unit of surface area. Unit: 1 Tesla = 1 VS/m2 = 10-4 VS/cm2 (= 10-4Gau?)

Flux, magnetic: When a magnetic field is represented by field lines, the total number of lines through a given surface is known as the

magnetic flux: measured as an electrical impulse in a coil surrounding this surface on a change to this flux. Unit: 1 Weber (Wb) = 1 Vs (= 108 Maxwell).

**Helmholtz coil: **Classically, a double coil to produce extremely homogeneous fields. The distance between the two coils is equivalent to their radius. The coil is used for measuring magnetic moments.

Hysteresis Loop: A closed curve obtained for a material by plotting corresponding values of magnetic induction , B , (on the abscissa) against magnetizing force, H (on the ordinate).

**Induction, B: **The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the cgs system of units. Intrinsic Coercive force, Hci: Measured in Oersteds in the cgs system, this is a measure of the material’s inherent ability to resist demagnetization. It is the demagnetization force correponding to zero intrinsic induction in the magnetic material after saturation. Practical consequences of high Hci values are seen in greater temperature stability for a given class of material, and greater stability in dynamic operating conditions.

**Irreversible Loss:** Defined as the partial demagnetization of a magnet caused by external fields of other factors. These losses are only recoverable by remagnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.Isotropy: Equality of physical properties in all directions.

**J: **Character used to represent magnetic polarization: Unit: 1 T = 1 Vs/m2 .

**Load Line: **A line drawn from the origin of the Demagnetization Curve with a slope of B/H, the intersection of which with the B-H curve represents the operating point of the magnet. Also see Permeance Coeffcient.

**Magnetizing Force, H:** The magnetomotive force per unit length at any point in a magnetic circuits. Measured in oersteds in the cgs system.

**Maximum Energy Product, BHmax: **The point on the Demagnetization Curve where the product of B and H is a maximum and the required volume of magnet material required to project a given energy into its surroundings is a minimum. Measured in Mega Gauss Oersteds, MGOe.

**Moment, magnetic:** (also dipole moment) Product of polarization J and volume V of a homogeneously magnetized magnet. The moment, expressed in terms of Vsm, corresponds to the mechanical torque in Nm experienced by the magnet in a magnetic field H of 1 A/m perpendicular to the magnetization (Coulomb’s magnetic moment mCoul). The magnetic moment is measured directly in a Helmholtz coil in combination with a fluxmeter. (see also SI units) Formerly the Ampere magnetic moment mAmp was commonly used as the product of the magnetization M and volume V of a solid, where mCoul=μ0 mAmp.

**Permeance Coefficient, Pc:** Ratio of the magnetic induction, Bd, to its self demagnetizing force, Hd. Pc=Bd/ Hd. his is also known as the “load line” or operating point of the magnet, and is useful inestimating the flux output of the magnet, and is useful in estimating the flux output of the magnet in various conditions. As a first order approximation, Bd/Hd=Lm/Lg, where Lm is the length of the magnet, and Lg is the length of an air gap that the magnet is subjected to. Pc is therefore a function of the geometry of the magnetic circuit.

**Radial magnetization:** Magnetizing a ring magnet between two coils carrying currents of opposite directions leads to a radial magnetization. One pole is then located on the inner circumference of the magnet and the other pole on the outer circumference.

**Remenance, Bd: **The magnetic induction which emains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remanence will be less than the residual induction, Br.

**Residual Induction, Br:** This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum

flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.