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Concentricity alignment between electromagnet yoke and suspended coil in NIM-2

The concentricity alignment error between the electromagnet yoke and the suspended coil is one of the key undetermined sources of Planck constant measured by Joule Balance NIM-2. Firstly, three capacitance sensors are mounted on the frame of the suspended coil with 120° equal angular distribution to measure the concentricity alignment errors. Secondly, the theoretical model of the concentricity alignment error is built. Finally, the concentricity alignment error is measured by experiments. Experimental and theoretical results show that the concentricity error is 17μm which meets the alignment requirement of Joule Balance NIM-2.