An instrument that measures small absolute humidity changes by the photoelectric examination of the 9,440 Ångstrom-units absorption band of water vapor is described. The instrument consists of a small source of light which sends its radiation over an air path of less than one and a half meters to a dispersing system. The resulting spectrum then is allowed to fall on two vacuum phototubes; one centered in the 9,400 Ångstrom-units absorption band of water vapor, the other located at 8,000 ngstrom units where no water vapor absorption bands exist. As the absolute humidity in the air path is varied, the phototube in the region of the band is affected; whereas the reference phototube is not. The phototubes are arranged in an amplifying circuit so as to magnify the effect of varying humidity. The instrument uses a portable microammeter instead of the sensitive galvanometer of all previous spectral hygrometers. Humidity changes of 2 to 8 × 10−5 centimeter of precipitable water path over 143 centimeters of air path can be measured. An investigation of the small sensitive range of the instrument was carried out and the results indicate that the device is confined to use over a small humidity range with equipment available at the present time.

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* This work was begun in 1943 and completed in 1944. It was supported by the N.A.C.A. from 1943 under Contract NAW-2247.

** Now National Research Fellow at California Institute of Technology.

*** Now at Los Alamos.