Column control in the magnetic deflection of a short arc

It has previously been found, over a wide range of conditions, that arcs moving in a transverse magnetic field, both in the retrograde and in the forward direction, were controlled by cathode fall processes; in particular by the transfer of emitting sites along the cathode surface. In the experiments described a short arc between ring-shaped electrodes moved repeatedly over the same surface. The emission site formation time was reduced, due possibly to unneutralized positive ions, to such an extent that processes in the arc column, rather than those at the cathode surface, determined the arc velocity. Sufficiently high arc velocities were attained to induce a transition from a diffuse to a cored arc at very low currents. At higher currents a further transition occurred with a sudden fall in velocity as the arc reverted to a diffuse type. Measurements in a number of gases, over a pressure range from about 1 torr to 14 atm, have given evidence of cathode control at the lower pressures giving way to column control above about atmospheric pressure.