Magnetic materials such as iron and ferrite generally
have a high value of permeability, i.e., a coil of a given number of turns
will have much more inductance than the same core in air.
However, a wound component with a high-permeability core
has a very steep B-H curve and, therefore, can tolerate only a very small
DC bias current or the core will saturate.
If the core saturates, the inductance will fall to a very
In order to make a coil that will operate with higher
values of dc bias current, it is necessary to reduce the permeability of
This is done by introducing air gaps in the magnetic
circuit, either by using a physical spacing or by using a core made of a
composite of magnetic and non-magnetic materials (providing the effect of
Cores with air gaps have a much lower overall
permeability and can tolerate much larger dc currents before saturating: