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Magnetic permeability (μ) is the ability of a magnetic material to support magnetic field development. In other words, magnetic permeability is the constant in the proportionality between magnetic induction and magnetic field intensity. The greater the magnetic permeability of the material, the greater the conductivity for magnetic lines of force, and vice versa. The magnetic permeability of a material indicates the ease with which an external magnetic field can create a higher magnetic force of attraction in the material. The SI unit of magnetic permeability is Henry per meter.
A material with good magnetic permeability either gets magnetized in the direction of the magnetic field or in opposition to the magnetic field. The magnetic permeability of a vacuum or air is considered the poorest. Magnetic permeability is not a constant and is influenced by many factors such as temperature, the medium used, humidity and strength of the magnetic field applied. As a result, in engineering applications, magnetic permeability is usually expressed in relative terms rather than in absolute terms. Purified iron and many magnetic alloys have maximum relative magnetic permeability.
Based on magnetic permeability, materials are classified as diamagnetic, paramagnetic or ferromagnetic. Diamagnetic materials provide opposition to external magnetic fields, paramagnetic materials are weakly attracted by external magnetic fields and ferromagnetic materials are strongly attracted by external magnetic fields. Magnetic permeability of ferromagnetic materials is significantly greater than a vacuum, and as a result, they are used in construction of magnetic cores in magnetic circuits.