Low-Pressure Electrical Discharges

Abstract

SOME experiments awaiting publication show how remarkably easy it is to start high-frequency electrodeless discharges at pressures of the order of 1/100 or even 1/1,000 mm. of mercury in a uniform electric field. For a bulb of given size, the wave-lengths must be less than a certain value which is directly proportional to the diameter of the bulb. A rough working rule is that the maximum permissible wavelength (measured in metres) is four times the diameter of the bulb (measured in centimetres). If the pressure inside the bulb is increased from the above small value, this 'cut off' wave-length is also increased, and at a still higher pressure the 'cut off' disappears. At wave-lengths beyond the 'cut off' enormous voltages are required to get a discharge—if one can begot at all. The discharge starts most easily for wavelengths from about one half to three quarters of the 'cut off' wave-length.