Traceability for low level low frequency magnetic field measurements

In principle, the heart of a traceable calibration system for magnetic field strength and flux density instruments is a means of providing a uniform field of known value. This is best achieved using a Helmholtz pair of coils. The greatest uniformity of the field strength is achieved when, for circular coils, the spacing between the pair of identical coils is equal to their radius or, for square coils, the spacing is equal to √(A/π) where A is the area of the winding. The magnetic field strength to current constant of a Helmholtz coil system can be determined using a mutual inductance bridge and standard search coil at a frequency of approximately 20 Hz. Traceability is then through a calibrated mutual inductor to the quantum standard of resistance (determined from the quantised Hall effect) through a calibration chain within the LF & DC Section at NPL. The standard search coil is wound in a single layer on a fused quartz former, and the area is determined from metrological measurements of the mean coil diameter traceable to the standard of length. Precautions to be taken against significant sources of error are discussed