Abstract

A carbon-coated thermopile always gave smaller readings on ultraviolet lamps when the thermopile was evacuated than when it was filled with air or other gases. The transparency of the carbon coating for long wave-length radiations and its opacity to short wave-lengths, requiring the presence of gas molecules to transport part of the ultraviolet energy from the front surface to the thermal junctions explains the observed phenomenon. A thermopile having a coating of platinum black gave the same values whether evacuated or air filled because a thin coating of platinum black is an excellent absorber for all wave-lengths.

© 1941 Optical Society of America

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