Abstract
A number of ex-services RCA 931–A photomultipliers have been examined, and their characteristics are compared with those of the 1P21. The reduction of dark-current is discussed. A method of examining local cathode sensitivity reveals that some tubes have markedly uneven sensitivity and may be unsuitable for astronomical applications. Types of stabilized power supply are discussed, and the effect of geomagnetism on photomultiplier sensitivity is verified.

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