Design of the KRISS watt balance

Abstract

We report the design of the KRISS watt balance, which includes a magnet, a guiding stage and a coil position measurement system. The KRISS watt balance incorporates a closed-type cylindrical permanent magnet and a motion guiding stage. For the magnet, a flux shunt is used to reduce flux changes due to temperature variations. A piston gauge is used to achieve linearity in the motion guiding stage. In the weighing mode, the residual force between the weight of the test mass and the Lorentz force generated in a coil is measured by a commercial weighing cell. In the dynamic mode, a linear motor in the motion guiding stage vertically translates the coil and the weighing cell. The in-plane motion of the coil is measured by position sensors, and the out-of-plane motion is measured by single-pass homodyne interferometers.

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

“…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. 23 The National Institute of Metrology (NIM) of China proposed a Joule balance, in which the derivative of the flux with respect to z is calibrated using a mutual inductance method instead of moving the coil in the magnetic flux.…”

A discussion ofBlconservation on a two dimensional magnetic field plane in watt balances

“…The analysis in this article is based on one of the most popular watt balance magnets employed [4,5,15,16]. Figure 1 shows the construction of the magnet with a symmetrical structure in both the r and z axes.…”

“…A similar watt balance magnet is employed in many NMI watt balances, e.g. the BIPM watt balance [5], METAS-II watt balance [4], NIST-4 watt balance [15], KRISS watt balance [16].…”

Field representation of a watt balance magnet by partial profile measurements

“…As a result, the Planck constant h can be related to the test mass m by comparing the electrical power to the mechanical power in conjunction with the quantum Hall effect [3] and the Josephson effect [4]. Since the proposal in 1975, the watt balance has been widely spread and persuaded at many national metrology institutes (NMIs) [5][6][7][8][9][10][11][12][13]. The detailed principle and recent progress of watt balance experiments at NMIs can be found in several review papers, e.g., [14][15][16].…”