Self and mutual inductance

Calibration and accurate measurements

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.

Calibration service

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.

• Inductance calibrations are operated in batch cycles twice per year, commencing April and November.
• Out-of-cycle calibrations are considered on request and may incur a surcharge.
• Calibration of self-inductance in the range 1 µH to 10 H.
• Calibration of fixed and variable mutual inductance in the range 1 µH to 500 mH
• The DC resistance of an inductor can also be measured, as an indication of the temperature of the instrument
• Inductors may be measured in the frequency range 20 Hz - 10 kHz depending on the type.
• Measurements are normally carried out at an ambient air temperature of 20 °C or 23 °C.
• Measurements can be made at a specified temperature within the range 18 °C to 25 °C on request.
• Non-sealed inductors are preconditioned at NPL in a humidity chamber, maintained within the range 50 ± 5  % RH prior to calibration.

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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