Abstract: The present invention provides a method and apparatus for determining the current densities in an alumina reduction cell by the measuring of magnetic fields without contact with the anodes or cathodes. The current density is modelled by determining the currents in the anodes and/or cathodes by measuring the magnetic field produced by the anodes or cathodes, or the conductors feeding them, and electronically correcting for ambient effects. The apparatus consists of Hall Effect devices to measure the magnetic field and electronics to correct, display, log and analyze the data.