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Various absolute steady-state methods of measuring thermal conductivity can be made to yield, verifiably, experimental results accurate to within ± 2% from 100 to 1200 K, and for conductivity values from 0.1 to 5 W/cmK. This statement is substantiated by a detailed analysis of the results obtained by eight different systems in three laboratories on ten metals: Cu, Ag, Au, Pd, Pt, W, Cr, Armco Iron, and the alloys Nb-10 W and Inconel 702. The experimental systems by which these results were obtained are also briefly described.

The achievement of such accuracy, however, requires that careful attention be paid to the design and operation of the experimental apparatus. In general, the thermal conductivity is computed from three parameters: the power developed in a specific heater; the temperature difference due to that heater; and a geometrical factor relating the two through the geometry of the heat flow pattern. The determination of each parameter presents problems of its own; these we outline, and present solutions to them which are often a matter of judicious compromises. However, because the individual problems are not equally severe for the different methods here discussed, the close agreement of the results that we show provides a gratifying reassurance that the experimental approaches and theoretical analysis, including estimates of error, are basically sound.

The Stefan-Boltzmann constant σ has been evaluated by using an absolute radiometer of the electrical substitution type to measure the radiance of a cavity radiator at the freezing-point of gold, T_Au. The value obtained for σ, based on the IPTS-68 value of 1337.58 ± 0.20 K for T_Au, is (5.6644 ± 0.0075) × 10^-8 W m^-2 K^-4. Unlike most earlier experimental values, this result is not inconsistent with the theoretical value of (5.6696 ± 0.0025) × 10^-8 W m^-2 K^-4 calculated from the present best values of c, h, and k. Alternatively, the experiment may be regarded as a radiometric determination of the gold-point based on the theoretical value for σ, the result then being T_Au = 1337.27 ± 0.40 K. This value is 0.31 K below the IPTS-68 value, derived by gas thermometry.

The problem of measuring a number of length etalons in terms of one well-known reference etalon may be solved by various intercomparison schemes. The optimal scheme will provide equal prescribed precision for all derived values with a minimal number of measurements. Criteria are established and comparison schemes developed which provide the optimal results.

A calculation of the thermal neutron self-absorption for cylindrical or spherical neutron sources has been made. The calculations are confirmed by the experimentally-measured difference in manganous sulfate bath activity for bare and cadmium-covered Pu-Be and Am-Be neutron sources. The calculation is done in single interaction approximation and assumes that the incident thermal neutron flux is isotropic. The source material may be fissionable and be covered by up to three cladding materials. A computer program has been written for the numerical calculations.

This is a report on the International Conference on Precision Measurement and Fundamental Constants held at the National Bureau of Standards, Gaithersburg, Maryland, August 3–7, 1970. It includes a brief discussion of the purpose and significance of the Conference as well as some of the organizational details as they pertain to the Conference sponsors, topics covered, publication of the Conference Proceedings, etc. The bulk of the report is devoted to a summary by topic of the Conference highlights and conclusions.