The Uniform Positive Column of an Electric Discharge in Mercury Vapor

Abstract

Uniform positive column of an electric discharge in mercury vapor. The uniform positive column was studied in a long circular cylindrical tube by means of the Langmuir probe electrode method. Measurements were made of the space potentials, random electron current densities, and the electron temperatures along the axis and across a diameter of the tube at vapor pressures ranging from 0.27 baryes to 7.13 baryes. The mobility of the electrons was obtained and the results interpreted in terms of the mean free path of the electrons by Langevin's expression for mobility. ($\mu =\frac{0.75e\lambda }{m\overline{v}}$) These mean free paths were in fair agreement with those obtained in angular scattering experiments by other investigators. The rate of generation of positive ions per electron was obtained from measurements of the positive ion current to the walls and the total number of electrons per unit length of tube. With the probabilities of ionization for electrons of various velocities obtained by other observers, it is found that at vapor pressures above 1.4 baryes the ionization is accounted for by the direct impacts with neutral atoms of electrons whose velocity distribution is Maxwellian. At lower vapor pressures it seems necessary to assume the presence of a larger number of higher speed electrons than is present in a Maxwellian distribution. The ratio of the concentrations of electrons at any two points on a diameter satisfies the Boltzmann equation. As the temperature of the electrons is lowered from 38,000°K to 19,900°K by raising the pressure of the mercury vapor the energy delivered to the walls by the recombination of positive ions and electrons decreases from 48 percent to 14 percent of the total input. Since less than 0.03 percent is lost through elastic collisions of electrons and atoms the remainder of the energy goes into excitation.