A helium cryostat which is cooled by the isentropic expansion of helium without the aid of liquid air or other refrigerant provides a cold chamber for experiments at any temperature down to 2°K. Any gas can be liquefied and delivered to external receivers as desired. Three expansion devices are described. One is a diaphragm engine having a mechanically supported diaphragm and unusual valves which require no valve gear. The others are piston-cylinder-crankshaft assemblies in which leakage is controlled by extreme close fitting of piston to cylinder, and in which wear of unlubricated surfaces is avoided by constructing the piston and cylinder of nitrided alloy steel and by mounting the piston on a thin flexible piston rod which operates under tension. The piston thereby has enough freedom to permit perfect alignment within the cylinder.