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The unit of inductance (henry) is derived from the farad. NPL maintains two primary self-inductors whose values are established from capacitance standards using two special transfer inductance standards. The primary inductors are used to establish a range of secondary inductors, which are used to calibrate customer's standards from 1 µH to 10 H.
An inductance bridge is used to intercompare the NPL secondary inductors and customer’s standards. It is a simple shielded four-arm bridge consisting of a calibrated inductive voltage divider (IVD) and two inductors, one being of known value. A variable resistance box whose residual inductance has been calibrated is in series with one inductor and is used with shields maintained at the detector potential.
The inductance calibration service at NPL has experience in the calibration of at least 25 different types of inductor, from more than 9 leading manufacturers. The service is UKAS accredited.
The best measurement capability, expressed as an expanded uncertainty (k = 2) is 70 ppm of nominal value, which applies to a wide range of instruments and test conditions.
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