Definition
In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field.
Basics
In electromagnetism, permeability is the degree of magnetization of a material that responds linearly to an applied magnetic field. In other words: just as electricity travels through some materials better than others, magnetism travels with ease through some materials, and has more difficulty traveling through others.
Magnetic permeability is represented by the Greek letter μ. The term was coined in September, 1885 by Oliver Heaviside.
In SI units, permeability is measured in henries per metre, or newtons per ampere squared. The constant value μ0 is known as the magnetic constant or the permeability of vacuum, and has the exact or defined value μ0 = 4π×10−7 N·A−2.
Ferromagnets
Some materials, called ferromagnetic or ferromagnets, are highly magnetic
by nature, relative to most materials. They are composed of a large
number of very small magnetic units working together called domains.
Domains are not always aligned, and they often act against each other
to reduce the strength of the net magnetic field.
If the ferromagnetic material is put into an externally applied
magnetic field, the domains tend to line up, so that the sum of the
fields from the ferromagnet and the applied magnetic field is higher in
magnitude than the applied magnetic field alone.
Permeability in linear materials owes its existence to the approximation:
where 
where
- μ is the material's permeability, measured in henries per meter.
- B is the magnetic field (also called the magnetic flux density or the magnetic induction) in the material, measured in teslas
- H is the auxiliary magnetic field, measured in amperes per metre
- M is the magnetic moment per unit of volume or magnetization, measured in teslas
The permittivity of free space (the vacuum permittivity) and the magnetic constant are related to the speed of light (c) by the formula: 
Relative permeability
Relative permeability, sometimes denoted by the symbol μr, is the ratio of the permeability of a specific medium to the permeability of free space μ0:
- , and that the definition that the speed of light
is exactly 299,792,458 meters/second. The agreed upon international
definitions and best determinations of the values of the fundamental physical constants in SI are given by the CODATA database supported on the web by NIST
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)
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