An a.c. Mueller bridge set for resistance thermometry at cryogenic temperature is described. A commercial tuned null detector is used at an operating frequency of 1025 c/s. The set includes a high stability oscillator, line reject filter, phase shifter, Q multiplier and selector box. The latter permits the dissipation in the thermometers not being measured to be maintained at the operating level with direct current. A temperature change of the order of 10 μdegK can be detected with 10-8 W applied to the thermometer.