A determination of the Planck constant h using the LNE Kibble balance in air was carried out in the spring of 2017. Substantial improvements since 2014, chiefly related to the mass standard, mechanical alignments, voltage measurements and type A evaluation uncertainties, leads to a h value of 6.626 070 41(38) × 10 -34 J • s, with a relative standard uncertainty of 5.7 × 10 -8 .

After separate developments of the different elements with continuous characterizations and improvements, the LNE watt balance has been assembled. This paper describes the system in detail and gives its first measurements of the Planck's constant h. The value determined in air is h = 6.626 068 8(20) × 10 −34 Js which differs in relative terms by −0.05 × 10 −7 from the h 90 value and by −1.1 × 10 −7 from that of the 2010 CODATA adjustment of h. The relative standard uncertainty associated is 3.1 × 10 −7 .

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This paper describes the mechanical and electrical modifications carried out on the LNE watt balance to reduce the noise level associated with the static phase. The mechanical improvements concern the home-made balance beam using flexure hinges as pivots of the force comparator. The electrical improvements involve the source used to servo-control the equilibrium position of the beam during the static phase. All these modifications have led to a significant improvement of the repeatability and reproducibility of the results of static phase measurements.

The value of the Planck constant h was determined in 2014 by means of the LNE watt balance experiment. The relative standard uncertainty was 31 parts in 10 8 . This first determination was performed in air with a 500 g mass standard made from XSH Alacrite. The main uncertainty components in air associated with the mass involve the calibration, the mass stability, the buoyancy correction and the magnetic interaction correction. The combined relative uncertainty due to the mass is 7.2 parts in 10 8 . The use in 2016 of a mass standard made from platinum iridium alloy significantly reduces the component of uncertainty arising from the mass standard for a Planck constant measurement either in air or under vacuum. The relative uncertainty due to this contribution is estimated to be about 3 parts in 10 8 in air and one part in 10 8 under vacuum. The future system for the dissemination of the mass unit using the LNE watt balance will be based on a primary realization with three 500 g mass standards made from platinum-iridium alloy, pure iridium and Udimet 720 respectively, coupled with a pool of kilograms made from different materials. Pure iridium and Udimet 720 are new materials to make reference mass standards proposed by CNAM and LNE respectively and have never been used by any NMI for manufacturing mass standards until now. Some new results concerning their surface behavior are given.

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Abstract. The Planck constant h was determined in 2014 by means of the French watt balance experiment at the LNE (Trappes). The relative standard uncertainty was 3.1×10 -7 . This first French determination was performed in air with a 500 g mass standard made from a Co-based superalloy. The two main corrections are the buoyancy correction of the order of 66 mg, and the magnetic interaction correction of about 37 µg. This interaction force appears between the mass artifact and the residual magnetic field coming from the permanent magnet used in the experiment. The main uncertainty components associated with the mass are the calibration uncertainty, the stability uncertainty, the buoyancy correction uncertainty and the magnetic interaction correction uncertainty. The combined uncertainty component due to the mass is about 36 µg, i.e. 7.2×10 -8 in relative value.

Abstract. Since 2005, the French watt balance experiment, installed at the LNE, has been using a balance beam with removable flexure hinges designed at the CNAM. With this balance, a first determination of the Planck constant in air was performed in 2014. This balance beam presents some drawbacks. First, it is difficult to mount and adjust the hinges. Secondly, the use of clamped flexure strips used as pivots limits the beam sensitivity and measurement repeatability, moreover the central flexure strip twists under the effect of unwanted horizontal forces at the ends of the beam, which leads to a potential measurement error. For these reasons, a new, single-piece balance beam has been designed. It is not only easier to mount and adjust than its predecessor, but also free of unwanted stresses such as shear force on the hinges. This ensures that, once the beam is loaded, the rotation axes are by construction parallel and coplanar. In addition, the use of a double central hinge strongly reduces the torsion effect.