The reproducibility of the platinum thermocouple at the freezing points of gold, silver and antimony

The investigation deals with the reproducibility of the standard platinum couple at the three points, viz. the freezing point of gold, silver and antimony, at which it is calibrated for the realization of the international temperature scale from 660° to 1063° C. The factors limiting reproducibility are considered under three heads, namely, electrical measurement, temperature conditions in the furnaces containing the ingots, and homogeneity of the thermocouple wires. The electrical measuring unit is found to be adequate for an accuracy of the order of ±0.01° C., whilst in connexion with the other two factors improvements have been made which render such an increased accuracy just attainable as an upper limit. In conclusion it is considered that the quality of the thermocouple wires, particularly in relation to their state of strain, constitutes the main obstacle to an increase in reproducibility beyond the normal limit of ±0.1° C.