The distribution of temperature along a thin rod electrically heated in vacuo. III. Experimental

Abstract

The present part is concerned with a detailed experimental verification of the main results obtained theoretically in the earlier parts. Thin rods of Acheson graphite were used for this purpose, and were found very suitable. In addition to the experimental data for the temperature distribution along rods of different lengths, and heated by alternating currents of different densities, one also needs for such a verification, data for the thermal and the electrical conductivities, and the spectral and the total emissivities of the surface, at different temperatures. These measurements also have been made. The observed temperature variation along the rod is itself used to determine the thermal conductivity and its temperature variation. The thermal conductivity is found to decrease and tend to a constant value at high temperatures, as it should. The other physical constants are determined directly. At low temperatures the spectral emissivity is found to be much larger than the total emissivity, and they tend to approach each other at high temperatures. Further, the spectral emissivity is found to decrease rapidly with increase of wave-length, as in the case of metals. Coming back to the temperature distribution, the main results that are verified here relate to the range of validity of the logarithmic formula in the B-region of a long rod, the constants involved in the formula, the deviation from the formula as one moves further into the B-region, the effect of the temperature coefficients, the parabolic law of variation near the centre, and the nature of the deviation from the parabolic law in long and short rods as one moves further away from the centre.