The Electrodeless Discharge in Mercury Vapor

A method is described for studying the characteristics of the components of the electrodeless discharge in mercury vapor. A Hartley oscillator is used to excite the discharge, oscillating currents up to 45 amps. being obtained at frequencies from 5×${10}^6$ to 12×${10}^6$ oscillations per second. Shield experiments have been performed which clearly show the distinction between the dull glow discharge which is considered electric in its origin and the bright glow discharge which is shown to be electromagnetic in its origin; the dependence of the bright glow on the dull glow or on the effect of some ionizing agent is proved. The variation of the current necessary to start the discharge with the pressure of the mercury vapor has been investigated fully over the pressure range 0.002 to 0.2 mm of mercury. Curves are drawn and analyzed in terms of J. J. Thomson's electromagnetic theory of the discharge and Brasefield's electric theory. It is shown that neither theory is sufficient to explain the experimental facts; the predictions seem to hold for the bright glow with fair agreement but fail markedly with the dull glow.