Magnetothermodynamics of Single‐Crystal CuSO4·5H2O. II. Fields ‖ to the α‐Magnetic Axis. Thermodynamic Temperature and Heat Capacity without Heat Introduction below 0.4°K. Calorimetric Heat Capacity from 0.4°–4.2°K with Fields of 1 and 3 kG

The heat capacity of a 3.700‐cm‐diam single crystal of CuSO4·5H2O has been measured by means of heat introduction over the range 0.4°–4.2°K with magnetic fields of 1 and 3 kG parallel to the α magnetic axis. These data supplement the previous observations of Anderson and Giauque (A and G) over the range 0–90 KG. A series of 29 adiabatics has been used to evaluate the temperature, heat capacity, and other magnetothermodynamic properties without heat introduction in the range 0.021°–0.4°K. The magnetic irreversibility study reported by A and G on copper sulfate pentahydrate has been extended and indicates that reversible thermodynamic methods may be used when the electronic entropy is as low as 0.05 gibbs/mole provided the field has not been reduced below 1 kG. When the entropy is about 0.1 gibbs/mole or more the field may be reduced to zero reversibly.