Electrical conductance decay of interrupted arc columns

The decays of the electrical conductance of interrupted cylindrically symmetric arc columns have been determined for 12-26 A arcs in atmospheric air. The measurements were obtained using peripheral electrostatic probes and an understanding of many of the fundamental properties and limitations of these probes has been obtained. An electron collection technique substantially improves the accuracy of the measurements, so that transient potential differences can be measured accurately up to values of 10 7 v cm -1 sec -1 and transient arc column resistance values with an accuracy to ±4%. The results show that during the first 250 μsec after interruption an initial fast decay occurs followed by a slower decay which is of exponential form with a time constant of approximately 95 μsec. The initial decay, if approximated by an exponential form, yields time constants of approximately 12 μsec. Theoretical considerations indicate that the time constant values are consistent with an arc column decaying by radial thermal conduction to the tube walls. The larger time constant is associated with the decay of the full arc column filling the tube, whereas the initial small time constant is most probably associated with the collapse of a high-temperature core within the arc column.