A High Accuracy Self-Calibrating Bridge with Coupled Inductive Ratio Arms Used for Standard Inductors Comparison

A bridge with inductively coupled ratio arms based on a seven decade inductive voltage divider is described. It is used for mutual comparisons between the standard inductors which form the IEN reference bank of inductance. Provided some auxiliary balances are made, this bridge is able to measure its own residual inductances, both the inductances of the standard-to-bridge connections (about 2 ¿H), made constant by the rigidity of the system geometry within a few nanohenries, and those of the variable resistance elements, also known within a few nanohenries. The self-calibrations are sufficiently accurate because the bridge has a high sensitivity even in comparisons of 1-¿H inductors and in the use of ratios up to 1:100. Three variable resistance components are used to balance the bridge: a commercial box of 1-¿ steps, with calibrated inductance increments; a two-material helicoidal-motion wire resistor (0 to 1 ¿), suitably designed with inductance variations less than ±1 nH; and a constant-geometry three-decade Kelvin-Varley divider, also designed for this specific purpose, with its total and partial inductance measured, and its total inductance kept constant within 1 nH. Where possible, connections between bridge components are realized by rigid tetra-axial conductors (4 coaxial tubular conductors). To avoid inductive coupling between the standards and the third tubular conductor, it is made of coated wires aligned along the generatrices of a cylinder. Provided the phase errors of the inductive voltage divider are known only within 1 ¿rad, the bridge has an estimated ratio accuracy of ±[2.