where formula_1 is the magnetomotive force (MMF) across a magnetic element, formula_11 is the magnetic flux through the magnetic element, and formula_12 is the magnetic reluctance of that element. (It will be shown later that this relationship is due to the empirical relationship between the H-field and the magnetic field B, B="μH", where "μ" is the permeability of the material). Like Ohm's law, Hopkinson's law can be interpreted either as an empirical equation that works for some materials, or it may serve as a definition of reluctance.