A.C.-arc-column and current-zero properties

Highly reproducible electrical-conductance measurements are reported for a.c. cylindrical-wall-stabilised air and argon columns for currents (r.m.s. value) of 4¿85 A and in the frequency range 25¿1500 Hz. The tube diameters were 4, 5, 6 and 7 mm. The general features of the results are as expected for thermal arc columns. However, one experimental feature, e.g. the instant of conductance minimum being independent of the rate of current approaching zero for a fixed tube diameter, is not predicted by analysis previously available. A constant-pressure and local-thermal equilibrium theory is presented, and numerical solutions, taking into consideration the full nonlinearities of transport coefficients, have been obtained for arcs burning in air. Good agreement is obtained between the computed data and the experimental results. It is found that the self-generated radial convection plays an important role in the energy transfer around current zero in an air arc column.