E. de Mirandés

The BIPM Watt Balance: Improvements and Developments

This paper summarizes the various improvements to and new developments for the Bureau International des Poids et Mesures (BIPM) watt balance, which have been implemented over the past two years. The main distinctive feature of the BIPM apparatus is its capability for simultaneous force and velocity measurements. Our first campaign of measurements in air at atmospheric pressure has been carried out with simultaneous force-velocity measurements. The results obtained among 11 series showed a relative reproducibility of the Planck constant h of 5.0 × 10-6. The relative difference between our Planck constant result and the CODATA 2006 recommended value is -4.8 × 10-6, with a relative uncertainty of 4.8 × 10-5.

Progress on the BIPM watt balance

Since the beginning of the development in spring 2005, considerable progress has been made on the BIPM watt balance. We have continued the development of a room temperature experiment to test the feasibility of simultaneous force and velocity measurements. We are now able to simultaneously acquire the velocity and voltage of the moving coil and to deduce their ratio. This article briefly recalls the main ideas of the BIPM watt balance and reports the progress to date and the preliminary results.

Alignment Procedure Used in the BIPM Watt Balance

The Bureau International des Poids et Mesures (BIPM) is developing a watt balance to measure accurately the Planck constant h by linking it to the kilogram. The kilogram is currently the only base unit of the International System of Units still defined by a material artifact. A fixed numerical value of h could be the basis for the expected future redefinition of the kilogram in terms of fundamental constants. One main concern of the watt balance experiment is the alignment of the involved vectors. In particular, the electromagnetic force exerted on the watt balance coil needs to be carefully aligned parallel to the gravitational force exerted on the test mass. To address this requirement, we have developed an original experimental procedure that has been validated with a test coil which is a prototype for the ultimate watt balance coil. The result is accurate enough to enable a determination of the Planck constant with a relative uncertainty of about 1·10-8 , which is the ultimate goal of the BIPM watt balance.

The BIPM watt balance: Improvements and developments

This paper summarizes the various improvements to and new developments for the BIPM watt balance which have been implemented over the past two years. The main distinctive feature of the BIPM apparatus is its capability for simultaneous force and velocity measurements. The fact that all the major components of the apparatus are now available (although not yet necessarily in final form) has enabled crucial tests of this. In addition we expect to soon make a first determination of the Planck constant h, albeit with a relatively large uncertainty.

Superconducting moving coil system to study the behaviour of superconducting coils for a BIPM cryogenic watt balance

The BIPM has built an experimental setup dedicated to the study of the physical behaviour of a superconducting coil during the dynamic phase of a watt balance experiment. We have compared it with the behaviour of a normal conducting coil. First experimental results are presented and preliminary conclusions are drawn.

Proposal for new experimental schemes to determine the Avogadro constant

We propose two experimental schemes for the determination of the Avogadro constant NA with a relative uncertainty of 10 −7 or better combining a watt balance experiment and a cold atom experiment measuring h/m(X) (where h is the Planck constant and m(X) the mass of the atom X). We give some prospects about possible uncertainties, and we discuss the opportunity to test the existence of possible unknown correction factors for the Josephson effect and the quantum Hall effect.

Status of the BIPM watt balance

This paper presents progress on the BIPM watt balance project. After a first measurement of the Planck constant in air at the 10−5 level, an improvement of a factor of five of the type A uncertainty has been obtained by refining the velocity measurement, now based on a three-axis interferometer system. Currently, the experiment is being transferred inside a vacuum enclosure located in a dedicated laboratory with improved thermal and vibrational isolation which we expect will further reduce the measurement uncertainty.

Superconducting coil system to study the behavior of superconducting coils for a cryogenic watt balance

A small scale superconducting coil system has been built at the BIPM. It is a first prototype to study the moving phase of a watt balance where the test coil is superconducting. This preliminary work is a first necessary step towards the development of the expected superconducting BIPM watt balance. The apparatus is presented and first measurements are discussed.

Alignment procedure used in the BIPM watt balance

We present an experimental method used for the BIPM watt balance to align with high accuracy the main vectors involved in the experiment. In particular, the electromagnetic force exerted on the watt balance coil needs to be aligned parallel to the gravitational force exerted on the test mass to within a maximum angle of 100 µrad. Our alignment method has been validated with a test coil which is a prototype for the ultimate watt balance coil. The result is accurate enough to enable a determination of the Planck constant with a relative uncertainty of 1·10−8, which is the ultimate goal of the BIPM watt balance.

Update from the BIPM watt balance

This paper reports the latest progress made on the BIPM watt balance project. The type A uncertainty of the Planck constant measurements was reduced from 1.4 parts in 106 to about 5 parts in 107. The new magnet was successively fabricated and assembled with an expected high uniformity of the flux density of several parts in 104. Several new components are being introduced into the apparatus which we expect to reduce the type B measurement uncertainty significantly.