Two vacuum interrupters equipped with 7 cm-diameter electrodes have been power tested with and without the presence of an axial magnetic field during arcing. The field strength was 2×10−5 teslaper arcing ampere, with a negligible field at a current zero. Observations with both interrupters at arc currents of 10–25 kA (r.m.s.) revealed that the field suppressed the peak arc voltage at electrode spacings of approximately 0.4 cm by about 30%. The current-interruption ability of one interrupter increased by at least 20%, and the magnetic field also increased the interruption probability of the second interrupter. These effects are attributed to plasma confinement by the field, which reduces the total input power to the interrupter and increases the threshold current associated with anode-spot formation. The plasma confinement also reduces the probability of arcing to the vapour-condensation shield.