THE REPRODUCIBILITY OF THE SULPHUR POINT

Abstract

The reproducibility of the normal boiling point of sulphur, a fixed calibration point on the International Temperature Scale, has been investigated using a closed manometer-boiler system. Measurements embracing several sources of sulphur and a number of changes in the operating conditions have shown that the sulphur point can be reproduced with a standard deviation of about 0.001 °C with our apparatus.Tests were made on eight samples of sulphur from three different sources in an attempt to resolve the uncertainty in the time the sulphur takes to reach temperature equilibrium after it has been brought to the boiling point. The results indicate that pure sulphur will reach equilibrium almost immediately but that an impurity content of as little as 0.01% can delay equilibrium up to 10 days. The temperature–time dependence can be ascribed to the effect of impurities on the time required for allotropic equilibrium to be attained. This hypothesis is discussed in detail and it is shown that it gives a consistent interpretation of the results presented here and those of previous investigations. The merits of replacing the sulphur point with the freezing point of zinc on the International Temperature Scale are also examined.The long-term stability of the coefficients of a Meyers platinum resistance thermometer is determined and a method of improving this stability for prolonged use at high temperatures is outlined.