Alignment of the NRC watt balance: considerations, uncertainties and techniques

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This paper presents a comprehensive analysis of the alignment uncertainties in the NRC watt balance. We show that, due to the use of a beam balance in both phases of the experiment, some of the alignment errors of the weighing phase are correlated with errors in the moving phase, which makes our watt balance insensitive to some forms of misalignment. These correlations become inexact in the presence of parasitic forces acting on the coil former. We present evidence of such forces and estimate their effect and contribution to the various uncertainty components. Finally we describe the techniques used to align the apparatus and we give an estimate of all relevant alignment uncertainty components.