Reluctance (S):
This is the property of a material which opposes the passage of magnetic flux in it.
Reluctance S = l /μ0 μr A AT/Wb
From Eq.
Flux = m.m.f / reluctance
or,
φ= (NI/ S) Wb
Permeance
This is the reciprocal of reluctance and its unit is AT/Wb.