Magnetic reluctance is the resistance of a material to a magnetic field. It is defined as the ratio of magnetomotive force (SI unit: ampere-turns) to magnetic flux (SI unit: weber).
The reluctance R of a uniform magnetic circuit can be calculated as:
where
R is the reluctance, measured in ampere-turns per weber. This unit is equivalent to the reciprocal henry (H - 1) multiplied by the number of turns
l is the length of the circuit, measured in metres
μ is the magnetic permeability of the material, measured in henry per metre (H.m - 1)
A is the cross-sectional area of the circuit, measured in square metres
Applications
The variation in the reluctance of a magnetic circuit, in which part of the circuit is moving, is used in measuring instruments called reluctance sensors.
Increasing the reluctance of a magnetic circuit enables it to store more energy. This effect is used in the flyback transformer.