Progress on the BIPM watt balance

Abstract

Since the beginning of the development in spring 2005, considerable progress has been made on the BIPM watt balance. We have continued the development of a room temperature experiment to test the feasibility of simultaneous force and velocity measurements. We are now able to simultaneously acquire the velocity and voltage of the moving coil and to deduce their ratio. This article briefly recalls the main ideas of the BIPM watt balance and reports the progress to date and the preliminary results.

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 nonlinearity in permanent-magnet systems used in watt balances

“…Recently, yoke-based permanent-magnet systems seem to be the preferred choice in watt balances [9,11,12,13,15]. Compared to electro magnets, these systems benefit from a stronger magnetic field, lower operating cost, and better magnetic self-shielding.…”

“…Figure 1 shows two typical designs for such magnet systems. The two-permanent-magnet, onecoil structure as shown in figure 1(a) is employed by the BIPM watt balance [11] at the Bureau International des Poids et Mesures, METAS-2 [12] at the Federal Institute of Metrology, Switzerland, and NIST-4 [9]. The one-permanent-magnet, two-coil structure as shown in figure 1(b) is built into the NPL-NRC watt balance at the National Physical Laboratory, UK and the National Research Council, Canada [13,14].…”