A spherical bulb containing mercury vapor at 25°C and air at various pressures was excited to luminosity when placed in a fixed helical coil carrying damped high frequency currents developing a predominant electrostatic field. Range of frequency of excitation 1800 to 555 k., of pressure 3 to 60 microns, of current 0.8 to 1.6 amperes. Visual intensity measured by a flicker photometer.
No simple relations were found to exist between visual intensity of the electrodeless discharge and air pressure, current or frequency, but consistent irregularities were observed.
The visual intensity was a maximum at a pressure of about 3 microns for all values of the exciting current but the minimum measurable intensity shifted from higher to lower pressures as the exciting current was decreased. Periodic breaks were found to exist in curves showing visual intensity as a function of exciting current. Constant current and pressure maximum values of visual intensity were found for wave lengths of 252, 358, and 490 meters, corresponding to frequencies of n, n+k, and n+2k.