Traceable magnetic field strength measurements to 120 kHz

With the advent of a series of reports on studies of the effects of magnetic fields on human health there has been a demand for traceable calibrations of the plethora of instruments produced for this type of measurement. To meet this and ongoing requirements, a facility has been established at the National Physical Laboratory (NPL) for low AC magnetic field calibrations up to a frequency of 120 kHz and for improved DC calibrations, made possible by an ambient field cancellation system. A prime consideration for low magnetic field calibrations is the level of the ambient field and the field gradient. The paper describes the surveys carried out which highlight the significant effect of transient disturbances from the electric train service, choice of materials for fitting out the laboratory, and the means of cancelling the ambient magnetic field using a triaxial Helmholtz coil and servo system. Details of the design, construction and calibration of a Helmholtz coil system for producing magnetic field strengths at high frequencies are given together with the requirements of the key circuit elements (power supply, current measurement system). An outline of the design, construction and calibration of a NPL triaxial Helmholtz coil system is also given