The paper presents the results of an experimental investigation into the effect of a perpendicular magnetic field on the conduction current in transformer oil. Direct voltage tests on samples of Voltesso 35 oil show that the presence of a relatively weak magnetic field can cause a reduction of more than 65% in the conduction current under typical voltage stresses. The effect of impurities in the form of magnetite particles is also investigated by measuring the conduction current with and without an applied magnetic field. Three theoretical models are examined for possible explanation of the observed results in plain-oil samples.