A pyrometer is described which measures with a thermopile and galvanometer the emission of energy in an isolated narrow band of wave-lengths centered at 8.8μ. These radiations are isolated by successive reflections from quartz crystals. The residual-ray apparatus used to isolate the 8.8μ band is arranged in a manner to make it compact, to avoid aberrations and, owing to the polarization of the near infra-red radiations, it is more effective than earlier models of residual-ray apparatus in eliminating the undesired short wave-length infra-red energy. Because the band lies in the infra-red the pyrometer can be used to determine low as well as high temperatures. The instrument is particularly suited for work in the temperatures range 0°C±100°C. Ordinary temperatures are easily determined to 0.1°C. The band of radiations used by the instrument falls in a region of the infra-red spectrum where the atmosphere is very transparent. Accordingly, in most applications (meteorological and astronomical applications excepted) there is practically no absorption in the optical path. The procedure of making temperature measurements is described. A new temperature scale is employed. Tables are supplied for reducing the observations to the ordinary centigrade scale. The instrument measures surface temperatures (when the emissivity of the surface is known) without disturbing radiation transfer or convective heating and cooling at the surface. Many applications of the instrument are concerned with surfaces which radiate as blackbodies.
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