Abstract
Experimental work is described in which pressure fluctuations as great as two orders of magnitude as recorded by a new type of cold-cathode vacuum gauge were undetected by a Bayard-Alpert ionization gauge. It is shown that the Bayard-Alpert gauge measures the pressure inside its electron-accelerating grid only and not that within the remainder of the evacuated system. Operation of the heated filament of this gauge within a glass tube or near glass parts gives rise to sodium, potassium, and other low ionization-potential materials throughout the vacuum system but in spite of being a major source of contamination, the Bayard-Alpert gauge does not detect the presence of these substances. This is because such low ionization-potential materials are photoionized by the light from the heated filament and cannot penetrate the positively charged electron-accelerating grid in order to be detected. Some other effects related to the phenomenon are discussed.
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