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A photoelectric detector is used in conjunction with a modulated beam of light for the m easurement by triangulation of the height of clouds during the daytime. An a-c operated mercury-arc lamp is used to obtain the modulated beam. An electronic " synchronous switch" is used to eliminate the effect of the varying background brightness of the clouds. The shot noise of the phototube, resulting from the relatively high brightness of clouds during the daytime, limits the detection. Dark overcast clouds at an elevation of 9,000 feet have been detected.

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THE MEASURING EQUIPMENT