Abstract
The thermal conductivity K is derived in terms of the difference in temperature which, in the steady state, is set up between the axis and the surface of a rod or tube of the material when the latter is electrically heated in an evacuated enclosure. The electrical conductivity σ is derived in the course of the same experiment from measurements of the current flowing in, and the potential-difference across, a length of the rod located near the centre. Solutions of the heat-flow equations are obtained for the case in which K and the electrical resistivity ρ are not constant but vary with temperature in a linear manner.
Experimental results are given for a variety of carbon consisting of 80 per cent of petroleum coke combined with 20 per cent of lampblack at temperatures up to about 2000° c. and for Acheson graphite up to about 2700° c. Evidence for the graphitization of carbon is obtained in that the results for this material at the highest temperatures tend to conform with those for graphite.
At normal temperatures the Lorenz functions, Kρ/T of carbon and graphite are respectively about 17 and 200 times as great as the value normally obtained for metals. The values, however, decrease rapidly with increase in temperature, and at 1700° c. the Lorenz functions of both materials are only about 4 times as great as those of metals.