Cryogenic cooling apparatus includes a tubular heat exchanger comprising a gas supply pipe wound around a former within a Dewar vessel. In use pressurized refrigerant gas is supplied through the pipe to a Joule-Thomson expansion nozzle to liquefy a portion of the gas in the Dewar vessel and low pressure gas then returns between the former and the Dewar vessel. A valve member cooperates with the nozzle to vary its area for automatically varying the flow of the refrigerant gas. The nozzle is fixed and the valve member is movable and connected to a first guide member which affords a first guide surface cooperating with a fixed second guide surface. One of the guide surfaces is tapered in the direction of relative movement of the nozzle and valve member whereby when the valve member and the nozzle are in the relatively open position the two guide surfaces are in contact and as the valve member and the nozzle progressively move into the relatively closed position the guide surface become progressively spaced apart.