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PHYSICS: ST. JOHN AND BABCOCK
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and 51 cm. long, the ends of'which were covered with w'ndows of clear rock salt. This tube was evacuated with an oil pump and then filled with air which had been passed through phosphorous pentoxide. It therefore contained carbon dioxide which causes a small amount of
absorption.
The transmission was determined by noting a series of galvanometer deflections caused by black body radiation (800° C.) which was passed through the evacuated tube and focused upon a linear thermopile of bismuth-silver. Immediately thereafter a stopcock was opened and either dried or undried air was permitted to enter, under atmospheric pressure.
Using air containing 9.95 grams of water per cubic meter the absorption amounted to 0.9 %. Using dry air, the average value of the absorption (3 series of measurements) was 0.09%; which is the magnitude of the errors of observation. In view of the fact that in the measurements of the radiation constant, the column of (dry) air was less than 50 cm. if any correction was to be applied-it could hardly be greater than 0.1%. Millikan's calculations of o, on the basis of recent determinations of Planck's element of action, h, from photoelectric measurements, and from his redetermination of the electron, e, is = 5.72 X 10-'2 0.034 watt cm. -2 deg. -4, which is exactly the result I obtained by direct measurement. The only misgiving is that this coincidence is accidental. In conclusion it may therefore be stated that further experiments are in progress in which the whole radiometric apparatus is in a vacuum, the radiator being an enclosure surrounded by molten metal. 1Coblentz, W. W., Washington, Bul. Bur. Standards, 12, 1916, (533). Millikan, R. A., these PROCEEDINGS, 3, 1916, (231). Coblentz, W. W., Washington, Bull. Bur. Standards, 9, 1913, (283). -
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THE DEVELOPMENT OF A SOURCE FOR STANDAR