- A very low-noise, variable temperature insert for a standard liquid-helium cryostat has been developed. The design provides a 10–100 K operating range, with temperature stability better than 20 mK in the range 25–100 K, improving to 2 mK below 25 K. This temperature stability figure is obtained by balancing the cooling power due to the thermal coupling of the sample to liquid helium, with the heating power of a stream of room-temperature He gas flowing through a heat exchanger. The heating power is continuously regulated within a feedback loop by throttling the gas flow with a proportional valve, using the error signal from the temperature controller. The system provides good temperature stability without the need for low-resistance leads in the vicinity of the sample holder as those needed to feed an electrical heater. This is relevant to experiments where electromagnetic interference must be minimized.