Measurements of Submarine Daylight

Abstract

IN the course of the last nine years Poole and Atkins¹ have developed a very ingenious method for measuring the intensity of the daylight penetrating into the sea by means of alkali-metal photo-cells, using a delicate balance method for measuring the very weak photo-currents. The many vagaries of the photo-cells make measurements with this contrivance rather difficult to any but trained experimenters. In order to find a simpler and less expensive method of measuring submarine daylight we have used the novel ‘Sperrschicht’ selenite photo-cells due to Dr. B. Lange. As these cells give a photo-current several hundred times more intense than the most sensitive of the alkali cells, it is possible to use an ordinary pointer galvanometer, or some similar instrument, for the observations, while at the same time they are simpler and more easily manipulated by relatively untrained observers. The instrument used for our measurements gave a deflection of one scale-unit for 3.76 × 10^-7 amperes and could be read to within 0.1 scale-unit. Another advantage of the Lange cell is its broad maximum of sensibility between 4500 and 5500 A., that is, with its centre near the minimum of light-extinction found by Knudsen for coastal water at 5100 A. The narrow region of maximum sensibility characteristic of the potassium cell is situated much nearer the violet end of the spectrum.