Determination of the magnetic field in open magnetic samples is a very difficult problem. Two basic approaches exist - to calculate the field from the applied current or to measure it near the sample surface. Both depend on the quality of sample-yoke contacts that affect the circuit reluctance and the field gradient above the sample. In this work, a method of the surface field determination by extrapolation from the field measurements at different distances from the surface is investigated. The field behavior above the sample is studied for a single C-yoke system numerically and experimentally on magnetically different samples. The influence of the driving coil position, yoke size, air gap, and the sample material on the field gradient is determined. The main factors affecting the resulting accuracy are discussed. The applicability of the method was checked by comparing derivatives of the results, i.e., differential permeability curves measured for different air gaps. (c) 2005 American Institute of Physics.
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