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.