Table of contents

A new method, using a novel double-ended interferometer, is described for the highly efficient calibration of gauge blocks. Unlike the conventional Kösters interferometric method, the new method does not require wringing of the gauge block onto an auxiliary platen. Although the gauge block length defined by International Standard 3650 (Gauge blocks) can be obtained using the Kösters method, the wringing operation is a laborious process, which requires skill, sometimes causes contact errors and decreases measuring efficiency, and prevents a fully automated measuring operation. The new interferometer can also be used as a conventional Kösters interferometer by changing some of the optical systems. Thus, after preliminary investigations measuring some samples using the Kösters method, we can determine an accurate correction value, dependent on the phase shift of light at the reflecting surface, to obtain the gauge block length defined by ISO 3650. The results, obtained from experimental work and theoretical analysis, show that the combined standard uncertainty of a gauge block length up to 100 mm is less than 16 nm, and this value is nearly the same as that obtained by the conventional Kösters method.

A 543 nm He-Ne laser stabilized to iodine using modulation transfer spectroscopy is described. A beatfrequency comparison between it and a reference iodine-stabilized laser showed that it had a stability better than 2 × 10^-12 after 140 seconds. The average frequency difference between the two lasers was only 12 kHz despite the lasers using different stabilization schemes. Two mechanisms for frequency shifts in modulation transfer spectroscopy are discussed, both arising from the presence of amplitude modulation on the phase-modulated saturating beam. The first is produced by the phase modulator itself and the second when the saturating beam propagates through the absorbing medium. For the laser described here, the frequency shifts are smaller than the uncertainty of the frequency measurement (~10 kHz).

This paper describes a recent determination of the von Klitzing constant and the fine-structure constant by comparisons of values of the ohm as defined in the International System of Units (SI), derived from the National Institute of Standards and Technology (NIST) calculable cross-capacitor, and values of the international practical unit of resistance derived from the integral quantum Hall effect. In this determination, the comparisons were made in a series of measurements lasting three years. A small difference is observed between this determination and an earlier comparison carried out in this laboratory and reported in 1988. The most recent value of the fine-structure constant based on the experimental value and theoretical expression for the magnetic moment anomaly of the electron, which has the smallest uncertainty of any value currently available, is consistent with both of these results. The new value exceeds the 1990 conventional value of the von Klitzing constant R_K-90 by slightly more than twice the relative standard uncertainty of the present measurement, which is 2.4 × 10^-8.

A world-wide comparison of gauge block measurement by interferometry, decided on by the Comité Consultatif pour la Définition du Mètre (CCDM), has been carried out. Four national metrology institutes, each representing one of the regional metrology organizations EUROMET, NORAMET, COOMET and APMP, measured a set of seven rectangular gauge blocks of steel with nominal lengths between 1 mm and 100 mm. This paper reports on the results of the comparison and draws conclusions with respect to the execution of future CCDM (now CCL) key comparisons.

The vacuum wavelengths of hyperfine components of three different rovibronic transitions of the B 0_u⁺ - X ¹E_g⁺ spectrum of molecular ¹²⁷I₂ in the near-infrared were determined with a relative standard uncertainty of δλ/λ = 1.3 × 10^-10. The measurements were performed by interferometric wavelength comparison using as standards an iodine-stabilized He-Ne laser at 633 nm and a dye laser stabilized to the intercombination line of Ca at 657 nm. The vacuum wavelengths of hyperfine component a₁₀ of the R(96) lines of both the (0-14) and (0-15) bands and the component a of the P(166) line of the (0-14) band were determined to be 781.488 441 03(10) nm, 793.527 910 44(10) nm and 790.109 812 76(10) nm, respectively.

This paper presents a general formulation of least-squares estimation using Lagrange multipliers. It is general in the sense that it takes into account the covariance matrix of all measured quantities and allows any known constraint to be imposed on the quantities being estimated. The technique may be used to estimate parameters of a calibration curve for a transducer taking into account the uncertainty and correlation coefficients of the measured stimuli and responses. It may also be used to adjust measured fundamental constants to a coherent set obeying the appropriate physical laws.

A new high-pressure instrument has been developed with the pressure uncertainty characteristic of free-piston gauges in the 3000 MPa range. It consists of an end-loaded piston-cylinder apparatus modified to use argon as a pressure medium. Pressure is determined from the piston force and the piston seal area with minor corrections for vessel bore dilation and seal friction. Bore dilation is approximately 0.36% and double-valued friction for the piston seal ranges from about 1% to 2%. A force-balance analysis of the piston seal indicates that the friction is highly symmetric. The equilibrium pressure of the Bi I-II transition has been determined in this apparatus to serve as a reference point on the practical pressure scale. Transition pressure measurements were made on 99.9999% pure polycrystalline bismuth. Small piston force excursions were made with both phases present to determine the equilibrium pressure which is insensitive to reaction rate. The Bi I-II equilibrium pressure is (2520 ± 5) MPa at 25 C. This value is 30 MPa (1.2%) lower than Heydemann's [1] value of (2550 ± 6) MPa. It is 8 MPa (0.3%) higher than the value of (2512 ± 3) MPa from Sekoyan et al. [2].

An air refractometer has been developed for accurate measurements of the refractive index of gases under well-defined conditions. Measurements have been performed for dry and moist air at four wavelengths distributed over the visible part of the spectrum. The measured values have been compared with values determined by modified Edlén's formulae and measured parameters. A fitting procedure is described to adjust the humidity dependence and the constant term of the dispersion formula so that optimum agreement is obtained. A standard deviation of 3 × 10^-9 for the residuals describes the reproducibility of the measurements and the success of fitting. For 50 % relative humidity, a standard uncertainty of 10^-8 for the fitted formulae is claimed, arising mainly from the uncertainty contributions of the air parameters.