A determination of the Planck constant by the generalized joule balance method with a permanent-magnet system at NIM

Abstract

The joule balance experiment has been carried out at the National Institute of Metrology, China (NIM) since 2007. By the end of 2013 the first generation of the joule balance (NIM-1) achieved a measurement uncertainty of 7.2 × 10 −6 (k = 1). To reduce the measurement uncertainty further, the next generation of the joule balance apparatus (NIM-2) system is under construction. A new coil system using ferromagnetic material is being adopted in NIM-2 to reduce self-heating in the coils. However, the effects on the measurement of the mutual inductance from the nonlinearity and hysteresis of the ferromagnetic material will bring a considerable measurement uncertainty. Inspired by the watt balance, the measurement of the mutual inductance is replaced by an equivalent measurement of the magnetic flux linkage difference. The nonlinearity and hysteresis will not be a problem in the measurement of the magnetic flux linkage difference. This technique comes from the watt balance method. It is called the generalized joule balance method, which is actually a modification of the watt balance method. However, it still represents a valid change that can reduce the difficulty of dynamic measurement experienced using the watt balance. Permanent magnets can also be adopted in the generalized joule balance. To check the feasibility of the generalized joule balance method, some preliminary experiments have been performed on NIM-1. A yokeless permanent magnet system has been designed and used to replace the exciting coils in NIM-1. In this paper, the structure of the yokeless permanent magnet system is introduced. Furthermore, a determination of the Planck constant with the permanent magnet system is presented. The value of the Planck constant h we obtained is 6.626 069(17) × 10 −34 J s with a relative standard uncertainty of 2.6 × 10 −6 .

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

“…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. 24 The principle of the watt balance experiment is presented in the simplified scheme of Fig. 1.…”

The irony of the magnet system for Kibble balances -- a review

“…The left shows the construction of the system, and the right presents the designed (green) and achieved (red) magnetic profiles. Reproduced from [43]. The labeled components in both systems are: 1 -upper main solenoid/magnet ring, 2 -lower main solenoid/magnet ring, 3 -upper trim solenoid/magnet ring, 4 -lower trim solenoid/magnet ring, 5 -main movable coil, 6 -upper fixed compensation coil, 7 -lower fixed compensation coil.…”

“…A second attempt to improve the field strength and reduce the ohmic heating for the coil system was undertaken by researchers at the National Institute of Metrology (NIM, China) for the Joule balance experiment [43]. The idea is to replace the field generating coils with permanent magnets yielding two advantages: 1) the ohmic heating of the field generating coils is removed, and 2) a stronger magnetic field is created.…”

“…The stronger magnet can also be weakened by adding a magnetic shunt in parallel to the permanent magnet. As shown in figure 11 and equation (43), the magnetic shunt returns part of the magnetic flux, which is then no longer available for the gap. The NIM-2 magnet system used both shimming and a magnetic shunt to remove the B r (z) slope [68].…”

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

“…(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.…”