Which of the following is the CORRECT formula for permeance?

• A. B/H
• B. I/H
• C. φ/Fm
• D. Fm/L

Answer: (C)

Reluctance
Similar to the resistance characteristic in electricity, we have reluctance in magnetism which is the property of the substance that opposes the magnetic flux through it.
The reluctance of any part of a magnetic circuit may be defined as the ratio of the drop in magnetomotive force to the flux produced in that part of the circuit. It is measured in ampere-turns/Weber and is denoted by S.
Reluctance = m.m.f ⁄ flux
Reluctance is dependent on:
Nature of the substance.
Area of the cross-section of the material through which flux is passing (a).
Length of the magnetic path.
Permeance
The reciprocal of reluctance is called permeance and is a measure of the readiness with which the magnetic flux is developed. it is analogous to conductance in an electric circuit and is measured by Weber/ampere-turn.
Permeance = 1/Reluctance
∴ Permeance = flux ⁄ m.m.f