The pressure effect and other phenomena in gaseous discharges

Abstract In the positive column of the mercury are the electrons have velocities distributed in accord with Maxwell's law, and the directions of motion are distributed nearly at random. The average velocity of the electrons is approximately independent of the arc current but depends on the pressure. The average kinetic energy of the electrons, expressed in terms of the corresponding voltage, is 3.9 volts at 1 bar pressure, 2.9 at 4.4 bars and 1.7 at 33 bars. Isolated electrodes, or glass surfaces, in the uniformly ionized gas of the arc, become charged 5 to 15 volts negatively with respect to the gas. The wall of the tube thus absorbs all positive ions moving towards it, but repels all but a minute fraction of the electrons, so that these act largely as if specularly reflected from the wall without loss of energy or momentum. The impact of the postive ions on the wall causes a heating effect on the wall which ranges from 15 to 50 per cent. of the total energy in the positive column. The momentum delivered to the wall by the ions is the probable cause of the pressure effect observed by Hamburger and Skaupy by which the pressure of the gas near the anode is greater than at the cathode. Equations are derived by which this pressure effect may be calculated. The results are in rough agreement with the meager experimental data available. Two methods are described for measuring the space potential in ionized gases. The first consists in the use of a hot sounding electrode whose potential is adjusted until the electrons emitted by it are just able to escape. The second is based upon a kink observed in a semi-logarithmic plot of the current voltage curve of a cold sounding electrode, which occurs when the electrode reaches the same potential as the surrounding space.