The Kibble balance is expected to become an important instrument in the near future for realizing the unit of mass, the kilogram, in the revised international system of units (SI). The Kibble balance assumes an equality of two magnetic profiles measured in the weighing and velocity phases. A recent study conducted in the Kibble balance group at the Bureau International des Poids et Mesures (BIPM) showed that the coil current could significantly affect the magnetic profile, which should be carefully taken into account in the Kibble balance experiment. This paper gives a deeper understanding and investigation of the effect, and discusses the magnetic profile change due to the coil current, for both the classical twomode and the one-mode Kibble balances. The coil current effect has been theoretically and experimentally investigated based on a typical magnet design with an air gap. One important conclusion found in the one-mode Kibble balance is that the magnetic profile change measured in the velocity phase is twice the change in the weighing phase. A compensation suggestion, to minimize the profile change due to the coil current in a BIPM-type magnet, is presented.

Note that the electrical power UI is proportional to the Planck constant h via electrical quantum standards [12], and therefore the test mass m can be linked and in the future it can be traced to the Planck constant by a Kibble balance. Currently equation (3) is used for determining the Planck constant, but in the future it will be employed for realizing the mass unit, i.e. the kilogram, in a revised international system of units [13].Two measurement phases are generally separated during the operation of a Kibble balance. In the BIPM Kibble balance, a single measurement scheme has been proposed in Metrologia

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After several years of development and continuous improvement of the BIPM Kibble balance, mass measurement of a 1 kg Pt/Ir artefact in terms of the Planck constant has been carried out. The paper describes the apparatus and its alignment, and explains the measurement and data processing. The balance operates in a one-mode and two measurement phase scheme using a bifilar coil. The mass value of the artefact was measured in vacuum with a relative standard uncertainty of 49 µg. This value differs from the value deduced from the calibration following the dissemination process after the redefinition of the kilogram by 15 µg.