Y. Nezu

Absolute determination of the magnetic flux quantum using superconducting magnetic levitation

The current status of our superconducting magnetic levitation experiment for determining the magnetic flux quantum is described. The flux-up system has been improved significantly by using a Josephson voltage standard. Studies are also in progress to improve the mechanical measurement relevant to the floating body with the goal of reducing the uncertainty to less than 1 ppm. >

International intercomparison of silicon density standards

In order to obtain a more accurate value for the Avogadro constant N/sub A/ the reliability of density values obtained by different measuring techniques for silicon single crystals was checked by density determinations with four silicon spheres at PTB, IMGC, and NRLM. The diameters of the spheres (two of PTB and two of NRLM) ranged from 90 mm to 94 mm. All the mass measurements (at PTB, IMGC, and NRLM) were carried out in air. At IMGC and NRLM the volume was determined by interferometric measurements, whereas at PTB the volume was obtained from hydrostatic weighings using two Zerodur volume standards. In addition, at PTB the densities of the four spheres were cross-checked by flotation measurements. The maximum relative density difference was found to be 1/spl times/10/sup -6/, whereas the typical values were 2/spl times/10/sup -7/.

Accurate determination of the density of a crystal silicon sphere

For an independent determination of the Avogadro constant, the density of a 1-kg crystal silicon sphere was determined by direct measurements of its mass and volume. A scanning-type interferometer was used to measure the diameters, and the volume was calculated from the mean of uniformly distributed diameters. The sphere was weighted using a balance for the prototype kilogram at the National Research Laboratory of Metrology (NRLM), and the mass was determined accurately by direct measurements of the buoyancy force acting on the sphere. The total uncertainty of the density is estimated to be 0.34 ppm. Effects of a thin oxide layer and impurities on the density of a pure crystal are evaluated. >

Improvement of the superconducting magnetic levitation system for the determination of the magnetic flux quantum

An improvement of the preliminary superconducting magnetic levitation system for the absolute determination of the magnetic flux quantum is described. This improvement includes the development of the flux-up method to determine the flux in terms of the Josephson voltage. The improvement is essential for the determination of the magnetic flux quantum as well as of the coil current, in terms of the Josephson voltage and quantized Hall resistance. >

Evaluation of equilibrium trajectory of superconducting magnetic levitation system for the future kg unit of mass

The superconducting magnetic levitation system for absolute determination of magnetic flux quantum /spl Phi//sub 0/, which is aimed at the future replacement of the kilogram unit of mass, has been developed at the National Research Laboratory of Metrology (NRLM) with a small floating body of approximately 25 g mass. The consistency of the relation between potential energy of the floating body and the electromagnetic energy of the superconducting magnetic levitation system has been evaluated with 1 ppm order resolution. In this paper, the results of the evaluation as well as the improvements made in the apparatus in recent years at NRLM are described.

A buoyancy-free mass comparator with precision better than 10/sup -6/ for use in a superconducting magnetic levitation system

A system for measuring mass without the necessity of correcting for the buoyancy of air with a relative standard uncertainty better than 10/sup -6/ has been developed. The system is intended for use in determining the mass of the floating body in the superconducting magnetic levitation system.

A determination of the magnetic flux quantum using superconducting magnetic levitation system

A refinement of a preliminary superconducting magnetic levitation system is in progress for the determination of the magnetic flux quantum Phi /sub 0/. A novel superconducting coil with a conic section has been fabricated, so that the Meissner effect of the system seems to remain intact and the basic characteristic of the levitation system is much improved.<<ETX>>

Present status of the NRLM superconducting magnetic levitation system for absolute determination of the magnetic flux quantum

The NRLM superconducting magnetic levitation is based on the concept of quasi-static energy injection process, where the energy injected from external source to the coil system is equal to the sum of the increase in magnetic energy and gravitational potential energy. The levitation system uses a niobium coil and floating body with mass of 25 g. The measurement consists of two processes: (1) to increase the magnetic flux through the coil in terms of the Josephson voltage which we call flux-up, (2) to measure the coil current and the vertical displacement of the center of gravity of a floating body in equilibrium. By the repetition of these processes, the relation which we call equilibrium trajectory is obtained. The absolute value of the magnetic flux quantum is calculated in terms of the energy equation using the trajectory data. Hence, a quasi-static process is not vital. The levitation system has been developed at NRLM and the reproducibility of magnetic flux and vertical displacement in equilibrium at 0.1 ppm level was confirmed including Meissner effect of the superconductive material. In this paper, we report a recent improvements concerning mechanical energy determination as well as the changes to the electrical system.

Development of a new levitated-mass mechanism toward replacing the kilogram

A new superconducting levitated-mass mechanism is now being developed with the final target of replacing the kilogram in terms of the fundamental constants. Improving the stability of trajectory by means of introducing a superconducting linear bearing is the issue in question for this development. The strategy of the development, experimental results and further prospects are discussed.

Improvements of NRLM superconducting magnetic levitation system for absolute determination of /spl Phi//sub 0/

The NRLM superconducting magnetic levitation system is now refining for the determination of Magnetic Flux Quantum /spl Phi//sub 0/ with small 25 g mass floating body. Recent study to investigate error sources and improvements are described in this paper.