NOTE 1 – Where the different materials are in series along the magnetic path and the permeability can be assumed constant over any cross-section, the following equation is applicable:

${\displaystyle \frac{1}{{\mu }_{\text{e}}}\sum \frac{l}{A}=\sum \frac{l}{{\mu }_{\text{r}}A}}$

where l is the length, measured along the magnetic path, of each part of the core of uniform cross-section A and homogeneous permeability µ_r.

NOTE 2 – The effective permeability is used particularly in relation to cores having (air) gaps and it is usually restricted to cases where the leakage flux is relatively small.