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

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The first determination of the Planck constant with the second generation of the joule balance, NIM-2, was completed in 2017 with an uncertainty of 2.4 × 10 −7 . Due to the movement of the magnet during the measurement process, the effect of the external field is a critical problem in NIM-2. At present, the electromagnet system is used in NIM-2. By taking the average of the results with positive and negative exciting currents, the uncertainty from the external field is reduced to 1.7 × 10 −7 , which is still the largest source in the uncertainty budget as all the other items are less than 1 × 10 −7 . In the near future, a permanent magnet system will be applied in NIM-2 and the main field cannot be reversed. Although the coupling of the external magnetic field in the permanent magnet system is about 40 times less than that in the electromagnet system, further reduction of this effect is still required in the permanent magnet system. In this paper, the effect of the external field is analyzed in both an electromagnet system and a permanent magnet system based on simulations and experiments. Then, the methods of magnetic shielding and compensation coils are proposed and simulated in the permanent magnet system. The results show that it may be possible to reduce the uncertainty of the external field to less than 2 × 10 −8 in the permanent magnet system by employing the two methods.

Section: The Actual External Field Sources In Nim-2mentioning

confidence: 99%

Research on the effect of the external magnetic field in the joule balance at NIM

Xu¹,

You²,

Li³

et al. 2018

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“…The magnetic susceptibility of the mass is about 6 × 10 −4 ; the field at the location of the mass is measured by a gauss meter [18]. Although the measured field is comparable to the earth's magnetic field, the field gradient is much larger, about 1.3 µT mm −1 .…”

Section: Mass Calibrationmentioning

confidence: 99%

“…Although the measured field is comparable to the earth's magnetic field, the field gradient is much larger, about 1.3 µT mm −1 . Hence, the relative uncertainty caused by the interaction between the magnetic field and the mass is about 2 × 10 −8 [18]. During the measurement, the pressure in the vacuum chamber is less than 1 Pa.…”

Section: Mass Calibrationmentioning

confidence: 99%

“…vertical integral displacement is about 40 mm. With the damping and alignment device described in [18], the variations of the horizontal displacement x, y and the three rotation angles θ x , θ y , θ z can be reduced to 6 µm, 6 µm, 4 µrad, 4 µrad, 25 µrad respectively. The suspended system of the joule balance is similar to that of the LNE Kibble balance.…”

Section: Alignmentmentioning

confidence: 99%

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The first determination of the Planck constant with the joule balance NIM-2

Zhang

et al. 2017

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The National Institute of Metrology (NIM, China) proposed a joule balance method to measure the Planck constant in 2006, and built the first prototype NIM-1 to verify its principle with a relative uncertainty of 8.9 × 10 -6 by 2013. Since 2013, a new joule balance NIM-2 has been designed, with a series of improvements to reduce the measurement uncertainty. By April 2017, NIM-2 has been constructed and can be employed to measure the Planck constant in vacuum. A first measurement on NIM-2 yields a determination of the Planck constant is 6.626 069 2(16) × 10 -34 Js with a relative uncertainty of 2.4 × 10 -7 . The determination differs in relative terms by −1.27 × 10 -7 from the CODATA 2014 value. Further improvement of NIM-2 is still in progress towards 10 -8 level uncertainty in the future.

“…The average magnetic flux density is about 0.1 mT and the gradient is about 1.2 mT m −1 . The large gradient of the magnetic field is caused by the three damping devices which is about 160 mm far from the mass [19]. The field of the permanent magnets in the damping devices is leaked from the holes of the outer yokes.…”

Section: Miscellaneousmentioning

confidence: 99%

The upgrade of NIM-2 joule balance since 2017

Bai

et al. 2020

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In 2017, we published the first measurement result with the NIM-2 joule balance and the relative standard uncertainty was 2.4 × 10 −7 . Since then, a series of improvements have been carried out to upgrade the NIM-2 joule balance towards the 10 -8 level relative uncertainty. The main improvements include a permanent magnet, as well as a two-layer magnetic shield to decrease the main uncertainty contribution from the external magnetic field, a parasitic displacement measurement system and a new suspension for improving alignments, and a new mass translation stage constructed with all non-magnetic materials. In September 2019, when the NIM-2 joule balance was used to calibrate a 1 kg mass, the type A relative standard uncertainty had been reduced to 3 × 10 −8 (k = 1) and the combined relative standard uncertainty was less than 1 × 10 −7 (k = 1). In October and November 2019, a 1 kg Pt/Ir mass and a 1 kg stainless steel mass were measured with the NIM-2 joule balance, and the relative standard uncertainties were 5.2 × 10 −8 and 6.5 × 10 −8 , respectively. The relative difference between the measured values and the calibration values obtained from the mass group were 4 × 10 −9 and 3 × 10 −8 , respectively.