${\displaystyle {\alpha }_{\mu }=\frac{{\mu }_{\theta }-{\mu }_{\text{ref}}}{{\mu }_{\text{ref}}\left(\theta -{\theta }_{\text{ref}}\right)}}$

where µ_θ and µ_ref are the permeabilities at temperatures θ and θ_ref respectively

${\displaystyle {\alpha }_{\mu }=\frac{{\mu }_{\theta }-{\mu }_{\text{ref}}}{{\mu }_{\text{ref}}\left(\theta -{\theta }_{\text{ref}}\right)}}$

où µ_θ et µ_ref sont respectivement les perméabilités aux températures θ et θ_ref