A simple system is described for maintaining controlled experimental temperatures between the normal boiling points of helium and nitrogen. No special apparatus is involved other than that normally required for the handling of liquid helium and the measurement of low temperatures. The technique has been used to control the temperature of a resonant cavity to within ±0.2°K, forT>20°K, using a copper‐Constantan thermocouple and a variable range recording potentiometer to measure temperature.