The determination of gravitational acceleration in the joule balance at NIM

Abstract

The determination of the local acceleration of gravity at the mass position is an important aspect of the joule balance. Since a simultaneous measurement of the absolute gravity at the mass position with the joule balance in place is impossible, seven gravity reference sites are chosen in the joule balance laboratory. Absolute and relative gravity measurements at the reference sites were performed between 2015 and 2019, and varioust transfer methods were used to obtain an absolute gravity value at the mass position. The self-attraction effects of joule balance apparatus are corrected by FEA simulation, and calculation accuracy is also verified by experiment. To obtain real-time local gravitational acceleration values during the joule balance experiment, geophysical effects, such as atmospheric pressure, Earth tides, and polar motion are considered. The absolute gravity results exhibit good stability from 2015 to 2019, and the final gravitational uncertainty contribution in the joule balance is 6.1 µGal.

The upgrade of NIM-2 joule balance since 2017

“…When the vertical position of the magnet is changed, the correction of the self-attraction effect ∆g in the joule balance is also changed and the Finite Element Analysis (FEA) calculation results are given in figure 12 [15]. By adjusting the current I at each position, the difference between ∆f down and ∆f up is less than 10 µg which can be ignored in the calculation.…”

“…It should be noted that the gravitational acceleration g in equation ( 12) is not constant due to the self-attraction effect. When the vertical position of the magnet is changed, the correction of the self-attraction effect ∆g in the joule balance is also changed and the FEA calculation results are given in figure 12 [15]. The final correction of the self-attraction effects in the joule balance g s is obtained based on the equation Therefore, the absolute gravity value used in equation ( 11) is the average of the values along the vertical movement range.…”

“…After the joule balance apparatus was installed, the intermittent measurements of the gravity were performed in 2016, 2018 and 2019, and the long-term stability was also verified. Reference [15] presents the accurate gravimetry activities for the joule balance from 2015 to 2019 in detail. To transfer the absolute gravity value from the gravity reference sites to the mass position, different measurement conditions and transfer paths were used.…”

“…Therefore, the gravity measurement result in September 2019 was used as the final absolute gravity value for the first key comparison of mass. Meanwhile, with the verification of the long-term stability from 2015 to 2019 in [15], a more reasonable uncertainty, which is 6.1 µGal and 6.1 × 10 −9 for relative value, is given for the gravity measurement as shown in table 3.…”