Three designs of ac bridge circuits for use in carbon or germanium resistance thermometry at low temperature are presented. These circuits combine simplicity of design with high sensitivity and stability of calibration during a low‐temperature run and enable the performance of the bridge to be checked accurately. The designs are intended for use in applications in which the thermometer can be calibrated during each low‐temperature run. The combination of these designs as alternate modes of operation of a single, versatile, practical bridge circuit is described. The operation of this bridge with regard to sensitivity and noise is discussed.