NI = Phi * R

Where N is the number of turns of the source wire, I is the current flowing through that wire, Phi is the magnetic flux, and R is the reluctance of the magnetic circuit element. Note that "R" is usually written in script to distinguish relucatance from resistance.

This is not truely Ohm's Law, but follows the same form, and thusly is often referred to as such. Note these similarities: NI, the magnetomotive force, corresponds to V, the electromotive force; Phi, the magnetic flux, corresponds to I, the current; and R, the reluctance, corresponds to R, the resistance.