Ohm’s law for Magnetic Circuit –

It can be stated as "The MMF applied across a magnetic circuit is directly proportional to the flux set up in the magnetic circuit".

Thus, mathematically,

$MMF \propto\phi$

$MMF = S\cdot\phi$

The constant $S$ is reluctance of the magnetic circuit which represents the opposition to the setting of $\phi$. Thus $\dfrac {MMF}{\phi} =S$ is required Ohm’s law for magnetic circuit.