Abstract
The triple point of equilibrium hydrogen is realized calorimetrically using a closed cycle refrigerator (Gifford-McMahon type refrigerator) instead of conventional liquid helium baths. The usage of the closed cycle refrigerator with closed fixed-point cells is found not only to be the easiest and the simplest way in handling and in operation, but to be one of the most accurate realizing methods of the low temperature fixed points. The standard uncertainty in calibrating standard platinum resistance thermometers at the triple point of equilibrium hydrogen is estimated to be 0.07mK, which is better than the average values of the conventional realization. By the high reproducibility of the calorimetric measurement is observed an abnormality in the heat capacity of solid hydrogen at temperatures just below the triple point when the catalyst of ferric hydroxide is used for the ortho-para conversion of hydrogen molecular modifications, but no abnormality is observed in the case of a cell without catalyst.
