Effect of Surface Finish and Wall Thickness on the Operating Temperature of Graphite Radio-Tube Anodes

The effect of rough and smooth surfaces and variable wall thicknesses on the operating temperature of radio-tube graphite anodes has been determined. Simultaneous measurements of internal and external temperatures were made with two optical pyrometers on rough and polished hollow graphite cylinders with variable wall thicknesses. Because of the ideal black-body conditions inside the graphite tubes, true internal temperatures were obtained. The optical pyrometer sighted on the outside of the cylinder measured the radiation at a wave length of 6530 angstrom units in terms of brightness temperature. From these data, calculations were made showing the true temperature differences and differences in emissivity and in radiation of the various graphite cylinders. Observations made show that varying the wall thickness of these graphite cylinders from 0.07 to 0.10 inch gave a difference of not more than 20 degrees centigrade in operating temperature. Polishing the surface of the graphite reduced the emissivity by 8 per cent, and increased the surface temperature by 25 degrees centigrade over that obtained for the rough-surface cylinder at 1000 degrees centigrade. Radiation from these surfaces indicates that a roughened anode surface at 1000 degrees centigrade has a radiation capacity increased over that of the polished anode by 1.1 watts per square centimeter.