LIX.Langmuir probe measurements in the normal copper arc

Abstract

Abstract Langmuir's theory of a collector in an ionized gas is applied to the normal arc in air, the collector being a fine platinum wire which is made to swing horizontally through a section of the arc. The volt-ampere probe characteristic is obtained by interposing between the cathode and probe a small alternating potential superimposed on a direct potential of suitable magnitude, and recording the current on an oscillograph used in conjunction with an amplifier. The complete characteristic is thus obtained on a single oscillogram. It is found that the density of the drift current is of the order of 100 times that of the random current, which leads to the conclusion that the temperature motion of the electrons is relatively unimportant. This complicates the interpretation of the probe characteristic according to the collector theory, so that, although satisfactory measurements of space potentials in the arc are secured, it is found possible to obtain only rough approximations to the electron density and energy. The results indicate a density of the order of 7 × 1011 cm.-3 and an average energy of 2 volts.