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The slow change in output of a Moll-type thermopile when exposed to radiation has been shown to depend upon the relative rate of rise in temperature of the thermojunction housing and the hot junction, and not as usually stated, upon the ambient temperature. By surrounding the housing with a water jacket, stable readings have been obtained. The sensitivity of the thermopile has been derived in terms of the dimensions and thermal properties of the thermojunction metals. Two instruments built for work at intensities of radiation up to 12.5 watts cm^-2 (3 cal cm^-2sec^-1) have stable readings almost independent of the quality of the radiation. Two later instruments are suitable for measurements up to 40 watts cm^-2 (10 cal cm^-2sec^-1) and 200 watts cm^-2 (50 cal cm^-2sec^-1) respectively, but the stability of their calibrations is not yet known.