The Transmission of Waves Through an Ionized Gas

A discharge in mercury vapor at approximately 0.001 mm pressure maintained in a 9 cm spherical bulb between a hot filament and a nearby disk anode was found to become oscillatory at a value of the filament current which depended on the anode potential and was quite definite for a given voltage. By means of a movable probe, the energy of the electrons was determined at various points in the nonoscillating and in the oscillating discharge. When oscillations set in, the electron temperature increased markedly. The frequencies present seemed to consist of a fundamental and a long series of harmonics ranging from about 2×${10}^4$ to ${10}^6$ cycles per sec. The fundamental was constant over a wide range of anode currents and voltages and its frequency was in good agreement with that calculated on the basis of Sir. J. J. Thomson's theory. The overtone that was most intense changed discontinuously with the anode current as though the discharge favored one mode of oscillation at one time as does an organ pipe. It was concluded that the glow was vibrating as a whole in a manner similar to the air in a Helmholtz resonator. A movable probe maintained about ten volts positive with respect to the filament collected currents that showed variations with position and had two or three marked maxima and minima along a diameter of the spherical bulb. In a long cylindrical tube, the currents collected by a probe moved along the tube axis showed no maxima or minima, but those collected by a probe moved along a diameter varied in a manner similar to those in the spherical bulb.