Reluctance:

Opposition to the production of flux in a material is known as reluctance that corresponds to resistance. A symbol for reluctance is R, and it has the units of ampere-turns per weber (At/wb).

Reluctance is associated to magnetomotive force, and flux, mmf, Φ, through the relationship display in equation (1-15).

R = mmf/Φ                                 (1-15)

Reluctance is inversely proportional to permeability (µ). An Iron cores have high permeability and, thus, low reluctance.  Air has a low permeability and, thus, a high reluctance.

Commonly, different kinds of materials have various values of reluctance that was shown in the Figure. Air gap is the air space among two poles of a magnet.  Because air has an extremely high reluctance, the size of the air gap affects the value of reluctance:  the shorter the air gap, the stronger the field in the gap. Air is nonmagnetic and will not concentrate magnetic lines. A larger air gap only gives space for the magnetic lines to spread out.