Table of contents

The goal of good performance of a time scale from a small set of atomic clocks has been realized. This time scale is both synchronized to UTC within the internationally agreed goal of a few hundred nanoseconds and syntonized to the SI second to less than 1 × 10⁻¹³. This has been accomplished using an algorithm to combine the readings of the clocks in an adaptive way based upon their actual performance and by a smooth automatic steering operation to maintain synchronism with UTC. This time scale, UTC(INPL), is a virtual time scale, or software clock, and is expected to have the advantage over a single primary clock of being more robust, more reliable and more stable than any of its physical member clocks, and insensitive to environmental perturbations. International measurements and comparisons by the GPS common-view approach have validated the performance. The frequency stability σ_y(τ) is less than 4 × 10⁻¹⁴. The software is PC-based and fully automatic. The concepts used in the generation of a virtual time scale are applicable to other areas of metrology and if incorporated could yield significant improvement in other international standards.

The overall uncertainty between the fixed points underlying the primary calibration of standard platinum resistance thermometers (SPRTs) may be described by terms, each of which is associated with the uncertainty at a given fixed point T_j, within the sub-range covered by the calibration. Each term is calculated, using the appropriate deviation function ΔW(T), by imposing an uncertainty of value zero at the other fixed points. Deviation functions with a non-zero value of the deviation ΔW(T₀) at the triple point of water, T₀, are not defined in the ITS-90, since, nominally, ΔW(T₀) = 0. For this reason the propagation of the uncertainty at T = T₀ is usually accounted for only indirectly through its contribution to the uncertainties δW(T_j), where j ≠ 0. By slightly extending the current mathematical scheme within the formalism provided by the ITS-90 it is shown that the uncertainty δW(T₀) at the triple point of water is also directly propagated.

Measurements of the effective radiance temperature of a high-temperature graphite tube black-body furnace are compared with those calculated from two theoretical models. The first model extends the conventional annular zone model by including effects of vignetting and temperature gradients. Because the black-body surface is not a perfect diffuse reflector, this is found to be insufficiently accurate and a Monte-Carlo integration with a diffuse/Fresnel-specular surface model is presented, which gives good agreement with the measured data.

A new determination of the numerical value of the gravitational constant G was carried out by means of an apparatus consisting of a fibreless torsion balance which was carried by the buoyancy of a floater in a liquid; the angular deflection of the balance was compensated with the aid of a control loop and an electrostatic torque transmitter; the reacting bodies were cylinders. The result obtained for G differs from the CODATA value by about +6 × 10^-3 in relative value. No dependence on the materials or on the distance between the cylinders was found. The apparatus is described, the methods of evaluation and the results are reported, the possible sources of errors are discussed.

The construction, performance and application of a 30 metre high mercury column is discussed. This thermostated, mercury-in-steel column is used to calibrate differential piston gauges up to about 300 MPa. The applied principle is the so-called cumulative method, introduced by Holborn and Schultze. Above 260 MPa the calibrations become problematical, probably due to a strong increase in viscosity of the pressure-transmitting oil. At pressures of about 161 MPa, 199 MPa and 248 MPa comparisons have been carried out with a Harwood controlled-clearance piston gauge. The relative differences in pressure are 12 × 10^-6, 101 × 10^-6 and 28 × 10^-6, respectively.

An analysis of calibration data suggests that W_ref(In) and W_ref(Ga), the resistance ratios at the freezing point of indium and the melting point of gallium, may have been assigned temperatures a fraction of one millikelvin higher and lower, respectively, on the ITS-90, than the values indicated by some high-purity samples. Illustrations are given of how the quality of calibration services can be assessed using issued calibration data.

A detailed description of the He-Ne/¹²⁷I₂ laser wavelength standards developed at the National Metrology Institute of Turkey (UME), using high-frequency modulation and featuring a long-term relative instability of a few parts in 10¹⁴, is presented. The frequency dependence of the lasers on the iodine gas pressure, modulation width and saturating field intensity have been studied with a very low uncertainty level, and the common effect of these parameters on the output frequency has been demonstrated. A new method of accurately measuring modulation width has been realized, making an important contribution to the improvement of reproducibility of this type of wavelength standard. An international comparison between the laser standards of the Physikalisch-Technische Bundesanstalt (PTB) and the UME has been performed. Several iodine cells studied in a laser with an accurate servo-system, reveal a correlation between the spacing of the molecular resonances, the gas contamination in the iodine cell, and the output frequencies of the laser when locked to different iodine components.

Adopting the phrase "quantity of dimension one" instead of "dimensionless quantity" raises a number of questions. After recalling what underlies the concept of dimension, the respective merits and drawbacks of both phrases are reviewed and discussed. In conclusion the author would like to have the current phrase "dimensionless" maintained.

Erratum to Metrologia, 1995, 32, 79-86.

Erratum to Metrologia, 1995, 32, 209-216.