Black Bodies in the Extreme Infra-Red

Absorbing power of various materials of wave-lengths greater than 50μ—For blackening a receiver of a thermocouple for radiation for obtaining pure rotation band spectra, it is desirable to use a material that is an efficient absorber of the extreme infra-red but a poor absorber of radiation of shorter wave-lengths. The absorbing power of 25 materials were tested for radiation of wave-lengths near 4μ and greater than 50μ. Of all the materials tested, white lead, litharge, red phosphorus, powdered glass, copper sulphide, and celestite were found the best for covering a receiver to absorb the extreme infa-red radiation. The 25 materials were tested by painting them on a receiver of a thermocouple which was exposed to extreme infra-red radiation. The same receiver and thermocouple were used throughout the experiment. A D'Arsonval type galvanometer was improved by using a quartz suspension and replacing each conducting lead with silver leaf or gold plated silver ribbon 1μ thick wound in a spiral.

Glass as a source of extreme infra-red radiation.—Hot glass made a good source of extreme infra-red radiation when used with a cold thin glass shutter.