Subordinate Lines Research Articles

Theory and Experiments Relating to the Striated Glow Discharge in Mercury Vapor

Theory of the glow discharge in a monatomic gas.---For the case of parallel plane electrodes with a hot cathode as source of electrons, the potential distribution and ion concentration in the Crookes dark space, negative glow, Faraday dark space and positive column are shown to be predictable from considerations of space charge and of ionization and excitation of the gas. While with weak currents there is a negative space charge throughout, sufficiently intense ionization is shown to lead to a cathode drop, followed by a region of reversed electric field in which positive ions and electrons both move toward the anode by diffusion, owing to their large concentration gradient. Still farther from the cathode the field changes to its normal direction and increases up to the positive column. In the positive column the field and concentration are uniform unless atoms excited by electron impacts in certain layers are prevented from diffusing between the layers, when striations may be obtained with periodic changes of field and of concentration. The cathode edge of each striation has a positive space charge. The theory of the arc discharge is essentially the same, the arc being simply the negative glow of the longer glow discharge.Glow discharge in mercury vapor.---Various predictions of the above theory were verified by experiments with Hg vapor in vacuum tubes provided with hot cathodes. (1) Potential distribution and ion concentration were investigated by Langmuir's modified probe method and found to agree with the theoretical deductions, except that the concentration of positive ions in the positive column comes out too large. This result indicates the presence of negative mercury ions. (2) The distribution of velocities of electrons is Maxwellian except between striations. (3) The emission of light seems associated more with excitation by electron collision than with ionization and recombination. (4) Conditions for existence of striations. Striations are not found in pure Hg vapor unless the current is small or some substance like ${\mathrm{H}}_{2}$ is introduced to remove excited atoms. (5) The presence of atomic hydrogen which should be produced in the process of removing excited Hg atoms was proved by use of tungsten oxide. (6) Introduction of He, which cannot remove excited atoms, does not tend to produce striations. (7) The relative concentrations of excited atoms was determined from the optical absorption of subordinate series Hg lines. It was found that excited atoms exist in striations but not in the regions between, and are more numerous if the amount of ${\mathrm{H}}_{2}$ impurity is reduced.Band spectrum of HgH seems associated with the action of excited Hg atoms on hydrogen, and is emitted as a result of inelastic collisions in striations.