Absolute Radiometry Based on a Change in Electrical Resistance

A blackbody and an electrical method of absolute radiometry employing direct current instruments for the measurement of current and resistance are described. The blackbody, designed for performance near 1000°C, is an electrically heated tubular furnace containing an oxidized steel chamber as the radiating source. The radiometer consists, essentially, of a coil of platinum wire wound spirally and bifilar-wise on a blackened, thin, silver disk from which it is electrically insulated but in good thermal contact. The change in resistance of the coil is a measure of the radiation absorbed by the coated disk. By heating the coil electrically, the energy required to produce this change is readily calculated; thus, the deviation of the radiant energy of the source from the corresponding computed perfect blackbody radiation can be determined. This method indicated that the radiant flux emitted by the above source is within 1% of that from such an ideal blackbody at a temperature between 800°C and 1100°C. Included is a description of the apparatus used in the assemblage, as well as of the procedure followed in obtaining data, with a discussion of the probable sources of error.