TRACEABLE MAGNETIC-FIELD STRENGTH MEASUREMENTS TO 120KHZ

With the advent of a series of reports on studies of the effects of ma gnetic fields on human health there has been a demand for traceable ca librations of the plethora of instruments produced for this type of me asurement. To meet this and ongoing requirements, a facility has been established at the National Physical Laboratory (NPL) for low AC magne tic field calibrations up to a frequency of 120 kHz and for improved D C calibrations, made possible by an ambient field cancellation system. A prime consideration for low magnetic field calibrations is the leve l of the ambient field and the field gradient. The paper describes the surveys carried out which highlight the significant effect of transie nt disturbances from the electric train service, choice of materials f or fitting out the laboratory, and the means of cancelling the ambient magnetic field using a triaxial Helmholtz coil and servo system. Deta ils of the design, construction and calibration of a Helmholtz coil sy stem for producing magnetic field strengths at high frequencies are gi ven together with the requirements of the key circuit elements (power supply, current measurement system). An outline of the design, constru ction and calibration of a NPL triaxial Helmholtz coil system is also given.