Critical Pressures of the Positive Corona Discharge between Concentric Cylinders in Air

Abstract

If, in a positive corona discharge in air, the voltage is raised at constant gas pressure over a range of pressures beyond atmospheric, the sparkover voltage is found to exhibit a maximum and then decline, eventually to coincide with the corona-starting voltage. The pressure at this point of intersection, beyond which sparkover alone obtains, without antecedent corona, is termed the critical pressure, and the associated gas density the critical density. The critical characteristics of dry air at room temperature are here determined in terms of electrode dimensions for a coaxial wire-tube system. The critical relative density δcr (air density relative to atmospheric conditions) is shown to fit the empirical relation δcr=k[(1/r)+c],where r is the wire radius, k an increasing function of tube radius, and c is a constant. The sparkover voltage Vcr corresponding to the critical density is given empirically by Vcr=ln(R/r){Ar+B[k(1+cr)]1/2},where R is the tube radius and A and B are constants of the gas. The unattainability of positive corona above the critical density is shown not to be absolute as hitherto supposed. Substantial corona currents are possible behond δcr provided that a suitable path in voltage-current-pressure space is followed in circumventing the critical density.