Encyclopedia of Electrical Engineering

Magnetic reluctance which is also known as reluctance, magnetic insulator, or a magnetic resistance is defined as the resistance provide by a magnetic circuit to the flow or production of magnetic flux (magnetic field lines). It is the property of the material that opposes the creation of magnetic flux in a magnetic circuit.
The concept of magnetic reluctance is similar to the resistance of a material to the flow of charge (current) ) is determined for electric circuits by the equation:
$$R = \rho {L \over A} \quad \text{(ohm, Ω)}$$
Where the reluctance of a material to the setting up of magnetic flux lines
in the material is determined by the following equation:
$$S = R_m = {l \over \mu A} \quad \text{(rels, A.t/wb)}$$
where S is the reluctance, l is the length of the magnetic path, and A is
the cross-sectional area. The t in the units $At/Wb$ is the number of turns
of the applied winding. More is said about ampere-turns (At) in the next
section.

Obviously, therefore, materials with high permeability, such as the ferromagnetic, have very small reluctances and will result in an increased measure of flux through the core. There is no widely accepted unit for reluctance, although the rel and the At/Wb are usually applied.
Permeability
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Ohms Law for Magnetic Circuits