A magnet system for the MSL watt balance

Abstract

A novel design for the magnet system of a watt balance is presented. While primarily intended for the watt balance being developed by MSL, this design could be used for other watt balances. It has a radial magnetic field in an annular gap that is screened from external fields. The magnetic field is generated by a single ring-shaped permanent magnet and the geometry is such that the field induced in the permanent magnet by the current in the coil in the weighing mode is zero on average. Hence the magnetic field in the gap is expected to be the same for both the weighing and dynamic modes of a watt balance. From finite element modelling, the magnetic field strength can be uniform to within 40 parts in 10 6 over more than 50 mm (50% of the gap height) and the usable working range for the coil is about 40 mm. Further, the diameter of the annular gap can be relatively small, allowing the coil and its support structure to be kept relatively stiff and light.

Invited Article: A precise instrument to determine the Planck constant, and the future kilogram

“…This is achieved by using, directly or indirectly, five standards: mass, voltage, resistance, time, and length. In addition to the efforts at NIST [11][12][13][14] and NPL, 15 20 the Measurement Standards Laboratory of New Zealand (MSL), 21 and the Korea Research Institute of Standards and Science (KRISS). 22 The National Research Council (NRC) of Canada purchased the NPL apparatus in 2011 and made improvements that lowered the overall uncertainty.…”

Contributions of precision engineering to the revision of the SI

“…The uncertainty of a Kibble balance experiment depends on numerous factors and is influenced by myriad design and manufacturing choices. Results from five very different balances can be found in the literature [163,166,55,58,181], and in anticipation of a redefined unit of mass several new instruments are in various stages of completion [78,13,94,176,194] as illustrated in Fig. 16.…”

A Five-Freedom Active Damping and Alignment Device Used in the Joule Balance

“…Several national metrology institutes (NMIs) across the world are working hard on redefining one of the seven SI base units, the kilogram (kg), in terms of the Planck constant h, by means of either the watt/joule balance [1][2][3][4][5][6][7][8][9] or the X-ray crystal density (XRCD) method [10]. The watt balance, which was proposed by Kibble in 1975 [11], has been adopted by the majority of NMIs.…”

“…(a) The overall structure of the damping device with the top cover 1 open. (b) The permanent magnets(5,6,7,11,12,13) and the inner yokes(8,9,10). (c) Combination of auxiliary coils 14 and 15.…”