Investigation of Freezing Temperatures of National Bureau of Standards Aluminum Standards

The design of a high-precision furnace for investigating the freezing points of metals up to 700 °C or higher is described. The freezing points of aluminum samples of nominally 99.999 percent purity from two batches were compared in terms of the ratio R(Al)/R(TP), the ratio of the resistance of the platinum resistance thermometer at the aluminum freezing point to that at the triple point of water. The average standard deviation of measurements of the ratio R(Al)/R(TP) obtained on six specimens corresponds to ±0.40 mK, while the average standard deviations of R(Al) and R(TP) correspond to ±0.17 mK and ±0.14 mK, respectively. (The variations in the measurements of R(TP) are amplified by 3.4 in the ratio R(Al)/R(TP).) The spread of the mean R(Al)/R(TP) obtained for five out of the six specimens corresponds to 0.51 mK; the deviation of the mean R(Al)/R(TP) of the sixth specimen from the mean R(Al)/R(TP) of the five specimens corresponds to -1.31 mK. (The sixth specimen may have been contaminated during the assembly of the freezing-point cell or the original sample bar was inhomogeneous.) The results show that aluminum can provide a freezing point (near 660 °C) that is at least as reproducible as the freezing point of antimony (near 631 °C).