W. E. Forsythe

Power Losses from Heated Objects in Still Air

The various factors affecting conduction and convection are discussed. Ordinarily, the power carried away from a heated body by these processes cannot be calculated accurately. Two ways of measuring it are described: (1) by boiling water in a container and subtracting from the power input the losses due to evaporation and radiation; and (2) by silvering the bulb of a 500-watt tungsten-filament lamp on the outside, and then making the power input such that the average bulb temperature is the same as that of the unsilvered lamp at normal input. A similar test, but with the bulb silvered on the inside, combined with the results of test (2), gave the power radiated by the bulb. Thus, we find 8 percent of the input to a regular 500-watt lamp carried off by conduction and convection, 11 percent by bulb radiation, the rest by direct filament radiation.Test (1) showed a conduction and convection loss of about 0.05 watt/cm2 at 100°C. This exceeded the radiation loss for every surface tested except a radiator enamel (total emissivity 0.77).