Low temperature melting curve of He³

Measurements of the melting curve of He³ have been made from 3 m° K to 40 m° K, after cooling the liquid-solid He³ mixture by the adiabatic solidification method. The slope of the melting curve shows that at the lowest temperature there is a substantial departure of the solid He³ entropy from R ln 2. This effect is due to nuclear spin ordering. Temperatures below 3 m° K were obtained during compression. By theoretically estimating the melting curve below 3 m° K it is possible to estimate temperatures from pressure measurements. By such means it is estimated that Kelvin temperatures of 1.5 to 2 m° K were actually achieved.