Vacuum Factor of the Oxide-Cathode Valve

The vacuum factor k of an oxide-cathode triode valve is defined as the positive ion current measured in a negative control-grid per milliampere of ionizing electron current to the anode. Measurements on a variety of new valves show wide variations of k , but these values fall, after a period of operation, to an approximately constant value k 0 in the range 300 to 900 μμA/mA. The characteristics of this residual vacuum factor k 0 are examined in some detail and a theory proposed to explain the anomalous form of variation of k 0 with anode voltage. This theory has a bearing on the interpretation of ionization gauge measurements at pressures below 1 × 10 -6 mm. of mercury.