The reluctance of a uniform magnetic circuit can be calculated as : or where l is the length of the circuit in metres is the permeability of vacuum , equal to 4π × 10 − 7 henry per metre is the relative magnetic permeability of the material ( dimensionless ) is the permeability of the material ( ) A is the cross-sectional area of the circuit in square metres The inverse of reluctance is called permeance .
The reluctance of a uniform magnetic circuit can be calculated as : or where l is the length of the circuit in metres is the permeability of free space , equal to henry per metre is the relative magnetic permeability of the material ( dimensionless ) is the permeability of the material ( ) A is the cross-sectional area of the circuit in square metres The inverse of reluctance is called permeance .