This study is to look at the distribution of current densities in proton exchange membrane fuel cells (PEMFC) to enable optimisation of fuel cell performance. The feasibility of using a new measurement technique of the local magnetic field, in the conductive plates of the cell was studied. The magnetic field is measured throughout the cell using the Maxwell equations and the current densities calculated. The measurement system and its validation are outlined in the first section. The next section outlines the experimental current density distribution within the cell, operating under standard conditions and special configurations, such as the partially active membrane electrodes assembly (MEA). Using a Matlab or Femlab model of the cell, (which is briefly outlined) our experiments are compared and an attempt made to explain the distribution of the current densities. Finally recent developments of the device are described, which will be used in several tests of PEMFC small stacks.