Osmotic Pressure ofHe3in LiquidHe4, with Proposals for a Refrigerator to Work below 1°K

An experimental study has been carried out of the osmotic pressure of solutions of the isotope ${\mathrm{He}}^3$ in liquid ${\mathrm{He}}^4$ at low temperatures, between 0.8 and 1.2°K. A superleak, a tube packed with a fine powder, acted as a semipermeable membrane which allowed only the superfluid ${\mathrm{He}}^4$ to pass. The conclusion from these experiments was that the measured osmotic pressures were in reasonable agreement with values expected from the thermodynamic relations with other equilibrium properties of the mixtures, notably their vapor pressures. Thermodynamic equilibrium therefore seemed to have been attained under the conditions of the experiments. The second half of this paper concerns a study of the cooling which must take place during the adiabatic dilution of ${\mathrm{He}}^3$ by ${\mathrm{He}}^4$. If the dilution is carried out at low temperatures where the solutions separate into two phases, the absorption of heat is estimated to be usefully large. After dilution the solution can be distilled, condensed and recirculated so as to make a continuously acting refrigerator. It should be possible to operate at temperatures of 0.1°K or below.