The Positive Column of High- and Low- Current Arcs

Abstract

TRANSITIONS from a diffuse to a concentrated positive column have been observed to occur in electrical discharges in hydrogen as the current is increased¹, and a similar transition occurs from the ‘low-current’ to the ‘high-current’ carbon arc in air^2a,d. As a result of a wide investigation of arcs in these and other gases, a purely thermal theory of these transitions has been developed, in contrast to the suggested explanation based on the magnetic pinch effect^2a,3. A development and numerical integration of the equation of energy balance for the positive column of the arc after the manner of Brinkman⁴ has enabled the radial temperature distribution for different central temperatures, and the variation of central temperature with current, to be calculated. In this the characteristic relationship between the temperature and the thermal conductivity, including the contributions of the classical kinetic theory and of the diffusion of ionized and dissociated pairs, which has been studied by a number of authors^2b,4–6 is of paramount importance. The data for nitrogen are illustrated in Fig. 1. The two maxima of the thermal conductivity at about 7,000° K. and 14,000° K. result from the dissociation and ionization of the gas respectively. In diatomic gases, the ‘low-current’ arc exists with central temperatures corresponding to the positive slope to the first maximum of the thermal conductivity–temperature curve, and the development of the concentrated core of the ‘high-current’ arc occurs in the region of negative slope, that is, between 7,000° and 9,000° K., for the arc in air or nitrogen.