La conception d'une expérience de balance du watt fait appel à des compétences pluridisciplinaires et nécessite de nombreuses études sur chaque élément qui la constitue.Un exemple d'une telle étude est présenté dans cet article.Afin de concentrer, en un point unique, les points d'application de deux forces à comparer (un poids et une force électromagnétique équilibrant ce poids), un système à liaisons élastiques monobloc à lames flexibles a été réalisé en alliage de cuivre-béryllium.Il permet l'articulation des suspensions de la masse et de la bobine engendrant la force électromagnétique selon quatre axes coplanaires qui se coupent en un seul point (centre virtuel de rotation).Les caractéristiques statiques d'élasticité des lames en termes de raideur et de limite d'élasticité ont été étudiées.De plus, l'étude du comportement dynamique sous vide de ce dispositif a été effectuée en termes de fréquence de résonance, d'amortissement dû aux frictions internes et de couplage entre les deux systèmes d'articulation.

In the French watt balance experiment, in order to have a single point of application of a weight and an electromagnetic force balancing this weight, a system of double gimbals with. flexure pivots was made out of copper-beryllium alloy. This allows the articulation of the suspension devices of the mass (generating the weight) and coil (on which the electromagnetic force acts) around four coplanar axes which intersect at one point (equivalent to a virtual centre of rotation). The study of the free dynamic behaviour under vacuum of this device dealt with resonance, damping due to internal friction of the material and coupling between the two systems of gimbals. The results obtained show that this novel device presents suitable characteristics for concentrating two different forces at a single point and aligning them around a common vertical axis. It can be used not only for watt balance experiments but also for any other mechanical applications where alignment and concentration of two forces are necessary.

Since 1980, the different national metrology institutes use the same procedure and formula for air density determination. The formula employed assumes some hypotheses on the composition of dry air and is expressed in terms of its molar mass and the four environmental parameters: air pressure, air temperature and concentrations of carbon dioxide and water vapour. In this formula (known as CIPM-1981/91) recommended by the Comité International des Poids et Mesures, the mole fraction of argon is fixed at 9.17×10−3. This value is now questioned and new measurements involving different techniques are of great interest.

A new Watt balance project is now in progress at the Bureau National de Metrologie (BNM), Paris, France. In this paper, the general configuration and the main parts of the experimental setup currently in development are presented. The aim is to contribute to the international effort in monitoring the kilogram toward a new definition of the mass unit with a relative accuracy of 10/sup -8/ or better...

We report a key comparison of 1 kg stainless steel mass standards carried out between February 1995 and October 1997. Organized within the Consultative Committee for Mass and Related Quantities (CCM), the participants included 14 national metrology institutes (NMIs) and the BIPM, with the BIPM serving as pilot laboratory. The 14 NMIs were divided into two groups, each of which determined the mass of two cylindrical travelling standards. In preliminary measurements, the pilot laboratory had determined the important physical properties of the four travelling standards and had monitored their mass stability over time. Measurements within the two groups then proceeded in parallel, with the four travelling standards returning several times to the pilot laboratory in order to monitor their stability. One of the present authors (MG) shows in appendix 3 how to treat comparisons among participants in different 'loops'. As shown in appendix 4, the mass reported by each NMI was derived indirectly from the mass of its national prototype, introducing a significant correction for air buoyancy during this step.

Adsorption and desorption of cleaning products are among the most important contributions to the instability of mass standards. An optical method, the mirage effect, is proposed to investigate the adsorption/desorption of cleaning solvents on the surface of mass standards at conditions close to normal temperature and pressure (NTP). A theoretical model of the mirage effect in the presence of an adsorbable vapour is recalled, relating the amplitude and phase of the mirage deflection to the adsorption isotherm. Representing this isotherm with the BET (Brunauer, Emmett and Teller) equation and introducing an inverse procedure, the parameters characterizing the adsorption process, such as the amount of adsorbed matter in the first monolayer and the differential heat of adsorption, are obtained. Experimental results for the adsorption of acetone, ethanol and isopropanol on XSH alacrite are presented that agree qualitatively with results from more conventional methods.

Among all the basic SI units, the mass unit is the only one to be defined in terms of a material standard: a prototype called K. All the industrial countries possess their own standards which were compared to the K during the last international comparison showing that unknown evolution occurs, but the adsorption-desorption of cleaning products plays a relatively important role. A few years ago, several laboratories in the U.S.A., Germany, and France reported interesting results of photothermal measurements to detect desorption at normal temperature and pressure (NTP). This paper presents a mirage set-up built to detect the film of condensable gasses on metallic surfaces at NTP conditions. In order to quantify these measurements, an inverse method has been developed to determine the adsorption isotherm involved in the physical process of adsorption-desorption and the linked parameters such as absorbability, type of adsorption, and differential heat of adsorption. The results will be discussed to imagine possible tracks to reduce the instabilities of the standards in the future and for possible new definitions of standards built with silicon.

The Institut National de Métrologie has set up an equipment for primary pressure calibration of standard condenser microphones by the reciprocity technique in a closed coupler.

This article only presents the force comparator designed and developed by the CNAM-INM in collaboration with the LNE to be used for the French watt balance experiment. The principle of the force comparison in this experiment is explained. The design of the force balance using flexure strips as pivots is presented and the two first prototypes of flexure strip beam balance are described and some results are discussed.

A « watt balance » type project, based on a comparison between electrical and mechanical powers is developed. Due to its interdisciplinarity, it requires the participation of several BNM and external laboratories. The purpose of the project is to characterise the time dependence of the kilogram by linking it to an invariant quantity -the Planck's constant- that could be used as a basis for the elaboration of a new definition of the mass unit in the future. The principle of the experiment, the selected options for the measurement device and its main components under development are described.