Astbury's modification of the Carey-Foster bridge is briefly described, with an account of the experience gained of its use since its installation in 1938. A more detailed description is then given of the precision Schering bridge in use at the National Physical Laboratory, in which particular attention has been paid to the screening of the components. The substandard air and mica capacitors, covering the range 200 μμF to 1 μF, which are used in conjunction with the Schering bridge, are described, with the methods used for their calibration. The tendency in recent years has been to use the Schering bridge for measurements of capacitance and power factor, and to use the Astbury bridge chiefly for the verification of the substandards. Errors do not exceed ± 1 part in 10 000 on capacitance and ± 0.00002 on power factor when testing any but the smallest capacitors.The Appendix gives an account of the method used to determine the inductance of the internal connections of capacitors.