On Discharges from Points in Gases, with Special Regard to So-Called Dark Discharges

Low current discharges from points.—(1) Variation of luminosity with current for air and HCl vapor. A study was undertaken to determine whether socalled dark discharges are really distinct forms of discharge and whether they are altogether devoid of light. Discharges from points were chosen because the discharge is so concentrated at a point that luminosity should most easily be seen if present. Observations were made through a microscope and show (1) that the light does not disappear suddenly but fades out gradually as the current is reduced and (2) that there is no abrupt change of current with voltage in this initial region of the discharge. The limiting current to give a visible glow increased from 2×${10}^{-9\mathrm{}}$ to 8×${10}^{-8\mathrm{}}$ amp. for negative discharges in air at atmospheric pressure as the diameter of the point was increased from 0.01 to 1.14 mm. For positive discharges the limiting current was roughly twice as large. With pure HCl vapor, higher voltages were necessary but the behavior was similar. (2) Initial stages in discharges from points in air were studied by increasing by means of alpha rays the ions normally present until the current due to them alone was measurable on a galvanometer. Typical results are as follows. In the case of a certain point, as the voltage was increased, the positive current began to increase gradually at about 2,000 volts below the normal starting voltage 5,000 volts, whereas the negative current was constant to over 4,500 volts and then increased very suddenly. This behavior is explained in terms of known properties of the ions. (3) Luminous effects of point discharges in air and in helium. Microphotographs are reproduced showing the thin glow covering the surface of the positive electrode and the fan shaped brush spreading from a small luminous area of the negative electrode. Changes in the luminous effects with changes of pressure are noted.