A bridge for maintenance of inductance standard

A bridge for maintenance of primary 10 mH inductance standard is described. The bridge simplifies and speeds up the maintenance procedure of group inductance standard and enables also the calibration of an external standard in relation to any member of the group. It can measure directly the inductance and series resistance differences of compared inductors at frequency 1 kHz and at operating current less than 70 mA. The measurement range is within ±10 μH and ±200 mΩ of the nominal value, the resolution ±1 nH and ±20 μΩ, and type A standard uncertainties are smaller than ±2 nH, and ±1 mΩ, respectively. The inductance measurement uncertainty is approximately 20 times smaller than observed one-year drift of a single standard inductor. The bridge is provided with a serial PC interface, and a manual switch for fast choice of compared inductors. The paper presents the concept of the circuit and measurement procedure, enhancing the result accuracy. The influence of connecting cables and problems with switching of compared inductors are also discussed