Thermoelectric measurements on tungsten at ultra low temperatures

Measurements are reported of the thermoelectric ratio G for three zone-refined tungsten samples at temperatures down to 45 mK. For the purest sample $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000)}$, G behaves simply and in accord with expectations. For two less pure samples $\text{(}\text{R}{}_{\mathrm{300K}}\text{R}{}_{\mathrm{0K}}\text{= 44 000 and 22 000)}$, G behaves anomalously, becoming increasingly more negative with decreasing temperature down to at least 45 mK. This anomalous behavior is tentatively attributed to trace amounts of iron.