Mimetic discretizations of Maxwell’s equations (e.g. by FIT) use a primal and dual grid. The discrete temperatures are located at primary nodes, the line-integrated electric field and the magnetic vector potentials are defined at primary edges and finally the facet-integrated source current density at dual surfaces. The material laws relate the quantities on both grids, e.g., the conductivity connects by Ohm’s Law the currents and voltages, i.e. . The figure shows the case for a current in x-direction.