Emissivities of Copper and Aluminum

The total hemispherical emissivities of copper and aluminum were determined over the approximate range of temperatures from 100°C to 400°C. The material under test was formed into a hollow sphere 4.5 cm in diameter which was mounted concentrically in an evacuated spherical glass flask 28 cm in diameter. The metal sample was heated electrically from within and the flask was kept at a constant temperature by the circulation of ice water over its outer surface. Temperatures of the specimens were measured on the inner surfaces with copper-constantan thermocouples. Heat losses through the thermocouple and power leads were evaluated by a substitution method. Polishing was done mechanically using a modified metallographic procedure with levigated alumina. The emissivity of copper was found to be practically constant at 0.03 but for aluminum the value was considerably higher and showed a temperature dependence. Comparison of the results with those predicted by the theory of Davisson and Weeks shows moderate agreement for aluminum and disagreement for copper.