Fall regions and electrode effects in atmospheric arcs of vanishing length

The cathode and anode fall of DC arcs at low current are found by fast oscillography of the voltage for vanishing arc length. The closing electrodes are activated by a spring accelerating a movable anode towards a fixed cathode. When the fall spaces vanish, steps appear in the voltage trace. Surface asperity changes give rise to voltage pulses. From the electrode speed the fall region thicknesses are estimated. The work is confined to arcs between solid copper and graphite electrodes in slow flowing argon or stagnant air. The cathode fall for copper electrodes is the same in argon as in air indicating that Cu vapour is the dominant medium. In argon the anode fall step is often absent. Since the arc voltage just before closure is higher than the cathode fall, this suggests that surface asperities may short-circuit the arc before its anode fall has been quenched.