Four-Terminal-Pair Inductance Comparison Between INRIM and CTU

For 40 years, it has been known that a four-terminal-pair (4TP) definition achieves the highest accuracy in impedance measurement. While, for ac resistors and capacitors, a 4TP standard definition is commonplace, and 4TP commercial measurement systems are available on the market, for inductors, a two- or three-terminal definition is still the most common. This paper describes the first international intercomparison of inductance performed with 4TP measurement systems and a 4TP travelling standard. At the Istituto Nazionale di Ricerca Metrologica (INRIM), the measurement system is based on a 4TP implementation of the three-voltage method, and traceability is to calibrated ac resistors. At Czech Technical University (CTU), Prague, a 4TP double-balance transformer bridge is employed, and traceability is via calibrated capacitors, ac resistance, and ac resistance ratio. The comparison has been performed in the context of EUROMET Project 607, with a 4TP 100-mH temperature-controlled travelling standard constructed for the purpose. Despite the big differences in the measurement methods and traceability chains, the measurements reported by INRIM and CTU are fully compatible, within their relative expanded uncertainties of 7 times 10^-6 and 6.2 times 10^-5, respectively.