The physical basis of the residual vacuum characteristic of a thermionic valve

The physical basis of the form of the residual vacuum factor characteristic of a thermionic valve is investigated over the anode voltage range 50-250 V. The characteristic is found to be almost wholly due to soft X-ray radiation from the bombarded anode surface. The paper describes a method of separating X-ray photoelectric and ionic effects, and uses this separation to undertake low pressure measurements in the range 10 -6 -10 -9 mm of mercury.