A Low Temperature Adiabatic Calorimeter for Condensed Substances. Thermodynamic Properties of Argon

Abstract

An adiabatic calorimeter has been constructed for the study of properties of condensed substances in the temperature range 2° to 300°K. Two resistance thermometers were used, one of platinum for T>11°K and one of carbon for T<11°K. The carbon thermometer was calibrated on the thermodynamic temperature scale by using the calorimeter vessel as a gas thermometer; subsequent absolute shifts of its resistance on cycling between room temperature and liquid helium temperatures were taken into account by calibrations against the platinum thermometer. The heat capacities of solid and liquid argon have been measured between 2° and 86°K with estimated accuracies to ±2% at the lowest temperatures, increasing to ±0.2% for T>20°K but decreasing to ±0.5% at the highest temperatures. In conjunction with these measurements, vapour pressures were measured in the temperature range 66° to 86°K. The heat of fusion was found to be 284.5±0.4 cal mole^-1, the triple point temperature 83.810°K and the triple point pressure 516.86±0.02 mm Hg. The amount of impurity in the specimen of argon was estimated to be 3 parts per million from a study of the melting range. Direct measurement of the heat of vaporization of the liquid gave the result 1563.6±4.6 cal mole ^-1 at 85.67°K, which corresponds to 1555.0±4.6 cal mole^-1 at the normal boiling point.