The effective drag width of short moving arcs in argon

Previous experiments have shown that short arcs rotating between ringshaped electrodes in a radial magnetic field move at a speed which is controlled by aerodynamic drag on the arc column when the pressure is above 1-2 atm. For these conditions it is possible to calculate an effective average arc size (which includes a drag coefficient) transverse to the direction of motion. Measurements have been made which enable the change in this effective arc size due to (a) arc speed, (b) arc current, (c) inter-electrode spacing and (d) gas pressure to be estimated for arcs moving in argon at a pressure of 3 atm. These changes are shown for arc speeds up to 80 m sec-1, arc currents up to 700 A and for spacings up to 1 cm, and it is suggested that electrode material introduced by vapour jets may be a significant factor.