Abstract
A glow tube with thorium electrodes can be used to measure the intensity of ultraviolet light. The simple circuit used for this purpose does not give reliable results for photo-currents below 10−10 amp., and the exposure time to light may become several hours. The writer has devised certain improved methods which can measure small photo-currents of the order of 10−13 amp., requiring an exposure time of only a few minutes. If the exposure time is increased to several hours, then it is possible to measure currents even below 10−14 amp., provided the components are carefully insulated.

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