In an electric circuit, current drift is opposed by the resistance of the material; in the same way there is opposition by the material to the flow of flux which is called reluctance.
It is well-defined as the resistance offered by the material to the movement of magnetic flux through it. It is denoted by 'S'. It is directly proportional to the length of the magnetic circuit while inversely proportional to the area of cross-section.
S ∝ (l/a)
Where “l” in meter and “a” in square meter
Where K = Constant of proportionality
= Reciprocal of absolute permeability of material = 1/µ
S = (l / µa)
S = (l / µ0µra) A/Wb
Where µ = µ0µr
It is measure in amperes per weber (A/Wb).
The reluctance can also express as the ration of magneto motive force to the flux produced.
i.e.. Reluctance = m.m.f / flux
∴ S= (NI/ф) AT/Wb or A/Wb
The permeance of the magnetic circuit is defined as the reciprocal of the reluctance.
It is defined as the property of the magnetic circuit due to which it allows flow of the magnetic flux through it.
Permeance = 1/Reluctance
It is measure in weber per amperes (Wb/A)