In electrical circuits, Ohm's law is an empirical relation between the EMF E applied across an element and the current I it generates through that element. It is written as: E = I R . where R is the electrical resistance of that material. There is a counterpart to Ohm's law used in magnetic circuits. This law is often called Hopkinson's law, after John Hopkinson, but was actually formulated earlier by Henry Augustus Rowland in 1873. It states that F = Φ Φ R . where F is the magnetomotive force (MMF) across a magnetic element, Φ Φ is the magnetic flux through the magnetic element, and R 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 = μ H, 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.