Uncertainty analysis of the BIPM susceptometer

Abstract

A magnetic susceptometer developed at the Bureau International des Poids et Mesures (BIPM) is used by many national laboratories to determine the magnetic properties of stainless-steel mass standards. This device relies on a small, cylindrical magnet to simulate a magnetic point dipole. According to simple theoretical considerations, (a) the uncertainty in the magnetic moment contributes weakly to the combined uncertainty of any measured susceptibility provided that unknowns are measured relative to a standard of similar size; (b) a cylindrical magnet with a height-to-diameter ratio of 0.87 best approximates a point dipole; and (c) the magnetic moment of the cylindrical magnet used can be determined by the susceptometer itself. Experimental tests of these assertions are presented. The properties of the cylindrical magnet generally become more important when determining the susceptibility of "small"-sized samples relative to a "large" standard. Small samples provide additional challenges, which are discussed in the context of a more general uncertainty analysis.

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