.sup.3 He-.sup.4 He dilution refrigerating machine

A .sup.3 He-.sup.4 He dilution refrigerator operable with a supply of gaseous helium, which comprises pump means for compressing and circulating said gaseous helium, means for cooling and condensing the compressed helium, a heat exchanger for further cooling the condensed helium, a mixing chamber for receiving the cooled condensed helium for separation into a liquid concentrated .sup.3 He phase and a liquid diluted .sup.3 He phase, a supply duct for flowing the condensed helium from said cooling and condensing means through said heat exchanger to said mixing chamber, a separating reservoir for receiving the liquid diluted .sup.3 He phase for separation into gaseous .sup.3 He and liquid .sup.4 He, a return duct for flowing said liquid diluted .sup.3 He phase from said mixing chamber through said heat exchanger in heat-exchange relation with said supply duct to said separating reservoir, a suction duct for flowing said gaseous .sup.3 He from said separating reservoir to said pump means, and a variable-flow restriction member positioned in said supply duct between said cooling and condensing means and said heat exchanger, said variable-flow restriction member being operated by the flow of condensed helium and, at a pressure difference exceeding a threshold value, allowing said flow of condensed helium to pass substantially independently of said pressure difference.