Reluctance

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Magnetic reluctance is the resistance of a material to a magnetic field. It is defined as the ratio of magnetomotive force to magnetic flux. The term was coined in May 1888 by Oliver Heaviside.

The definition can be expressed as:

[S=\frac]

where

S is the reluctance in ampere-turns per weber (a unit that is equivalent to turns per henry)

F is the magnetomotive force in ampere-turns

Φ is the magnetic flux in webers

The reluctance of a uniform magnetic circuit can be calculated as:

[S = \frac]

where

l is the length of the circuit in metres

[\mu_0] is the permeability of free space, equal to [4 \pi \times 10^] henry per metre

[\mu_r] is the relative magnetic permeability of the material (dimensionless)

A is the cross-sectional area of the circuit in square metres

The inverse of reluctance is called permeance.

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. One of the major applications is in magnetic pickups for electric guitars.#redirect

Increasing the reluctance of a magnetic circuit enables it to store more energy before core saturation. This effect is used in the flyback transformer.

References

Chambers Science and Technology Dictionary, "permeance".

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