Measurements have been made, using electrometer methods, to determine the sign and the decay rate of charges left on the anode and cathode tracks by an arc which has been driven along electrodes by a transverse magnetic field. Electrodes of brass, copper, aluminium and platinum have been used with arc currents in the range 10 to 50 A and arc velocities of 5-10 msec^-1 and some tests have been made on brass electrodes after arcs of 1800 A moved at 100 msec^-1. Pulses which appear to be due to negative charges have been detected on the cathode and anode tracks of all electrodes within a few milliseconds after the passage of the arc. These charges, after an initial rapid decay, have been found to exist in appreciable amounts for some weeks. These negative charges are thought to be due to a dipole layer associated with adsorbed gas and oxides on the metal surface although electron emission may also contribute somewhat in the initial stages.