Absolute scale of thermoelectricity III

Abstract

Abstract We have measured the Thomson coefficient μ of platinum and tungsten from 900 to 1600 K and calculated the absolute thermopower S of each. We describe a numerical technique for evaluating the temperature profile of a wire which is undergoing self-heating by direct current and discuss the experimental design problems associated with measuring μ at high temperatures. We develop a method in which the direction of the current through the specimen is reversed about once per second and observe the rates of heating and cooling at different places with an optical pyrometer. By changing the magnitude of the current at the same time as its reversal the change in the Joule heating is balanced with the change in the Thomson heating. We describe a technique for calibrating the pyrometer, report measurements of μPL and μW, calculate S Pt and S w, and test the results according to Kelvin's relation, S = f μ/T dT. The difference between our S Pt, and that given by Cusack and Kendall (1958) varies from −0·7μV K−1 at 900K to −0·4 μV K−1 at 1600K, whereas our estimated uncertainty varies from ± 0·05 to ± 0·2 μ V K−1 over the same range.