Part of Quality Specification Table for Magnets

Air Gap : A low permeability gap in the flux path of a magnetic circuit. Often air, but inclusive of other materials such as paint, aluminum, etc.

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

Saturation : The condition under which all elementary magnetic moments have become oriented in one direction. Neodymium mangets is saturated when an increase in the applied magnetizing force produces no increase in induction. Saturation flux densities for steels are in the range of 16,000 to 20,000 Gauss.

Temperature Coefficient : A factor, which describes the change in a magnetic property with change in temperature. Expressed as percent change per unit of temperature.

Weber : The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.

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.

Eddy Currents : Circulating electrical currents that are induced in electrically conductive elements when exposed to changing magnetic fields, creating an opposing force to the magnetic flux. Eddy currents can be harnessed to perform useful work (such as damping of movement), or may be unwanted consequences of certain designs, which should be accounted for or minimized.

Electromagnet : A magnet, consisting of a solenoid with an iron core, which has a magnetic field existing only during the time of current flow through the coil.

Fringing Fields : Leakage flux particularly associated with edge effects in a magnetic circuit.

Gaussmeter : 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.

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 Induction, Bi : The contribution of the magnetic material to the total magnetic induction, B. It is the vector difference between the magnetic induction in the material and the magnetic induction that would exist in a vacuum under the same field strength, H. This relationship is expressed as: BI = B-H.

Isotropic Magnet : A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetized in any direction without loss of magnetic characteristics.

Leakage Flux : That portion of the magnetic flux that is lost through leakage in the magnetic circuit due to saturation or air-gaps, and is therefore unable to be used.

Length of air-gap, Lg : The length of the path of the central flux line in the air-gap.

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 Coefficient.

Magnetic Circuit : An assembly consisting of some or all of the following: permanent magnets, ferromagnetic conduction elements, air gaps, electrical currents.

Magnetic Flux : The total magnetic induction over a given area. When the magnetic induction, B, is uniformly distributed over an area A, Magnetic Flux = BA.

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.

North Pole : That pole of a magnet which, when freely suspended, would point to the north magnetic pole of the earth. The definition of polarity can be a confusing issue, and it is often best to clarify by using "north seeking pole" instead of "north pole" in specifications.

Paramagnetic Material : A material having a permeability slightly greater than 1.

Permeance : The inverse of reluctance, analogous to conductance in electrical circuits.

Permeance Coefficient,Pc : Ratio of the magnetic induction, BD, to its self demagnetizing force, HD PC = BD / HD This is also known as the "load line", "slope of the operating line", or operating point 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.

Pole Pieces : Ferromagnetic materials placed on magnetic poles used to shape and alter the effect of lines of flux.

Relative Permeability : The ratio of permeability of a medium to that of a vacuum. In the cgs system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the cgs system.

Reversible Temperature Coefficient: A measure of the reversible changes in flux caused by temperature variations.

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