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A graphite cavity chamber for absolute exposure measurements of ⁶⁰Co gamma rays is presented. The chamber is designed to allow the determination of the exposure to rely as far as possible on a theoretical analysis previously given by Allisy. As an extension of this analysis, the problem of the influence of the cavity thickness is treated. The values to be used for the physical constants involved and the correction factors are discussed as well as the related uncertainties.

The influence of the geometry of the sample chamber on the melting behaviour of Ar samples is discussed. Melting curves of Ar samples doped with various small amounts of He, H₂, Ne, N₂, CO, O₂, CH₄ and Kr impurities were determined relative to "pure" samples. It was found that He, H₂ and Ne did not alter the melting curve; N₂, CO, O₂ and CH₄ lowered the liquidus point of Ar by 2.4, 2.4, 1.8 and 3.0 mK for 100 vppm of these impurities, respectively; Kr increased the scatter of points along the melting curve without noticeably altering the liquidus point. – Values for the triple point temperature and pressure and the boiling point were found to be 83.797₆ K and 516.730 ± 0.010 mmHg (68891.6 N/m²) and 87.294₆ K, respectively. – The vapour pressure of solid and liquid Ar can be described by ln P = A_S + (B_S/T) and ln P = A_L + (B_L/T) + C_LT for 81 K ≲ T ≲ 83.8 K and 83.8 K ≲ T ≲ 87.3 K respectively. Numerical values of these constants are compared with some previous determinations. Latent heats of sublimation, vaporization and fusion are calculated to be 7751.6, 6562.0 and 1189.6 joules/mole respectively.

This paper describes in detail the procedures, methods and measurements used to establish a new definition of the U.S. legal volt via the ac Josephson effect. This new definition has been made possible by the use of thin film tunnel junctions (capable of producing 10 mV outputs) and high accuracy voltage comparators. The Josephson junction is used as a precise frequency-to-voltage converter with a conversion factor equal to 2e/h. A series of measurements of 2e/h has been carried out at NBS referenced to the as-maintained unit of emf based on a large group of standard cells. Measurements made at regular intervals over a one year period (1971 to 1972) indicate that the mean emf of this group of standard cells has decreased about 4 parts in 10⁷. Primarily to remove the effects of this drift, on July 1, 1972 a new as-maintained unit was defined by choosing a value of 2e/h consistent with the existing unit of emf. The adopted value of 2e/h is 483593.420 GHz/V_NBS. The precision (one standard deviation) with which the new unit of emf can be maintained with the present techniques and apparatus is about 2 parts in 10⁸. The accuracy of the present system is estimated to be 4 parts in 10⁸. Comparisons of 2e/h systems at different national laboratories have been limited by uncertainties associated with the physical transfer of standard cells. In order to determine the relative agreement of the various 2e/h systems with precision better than 1 or 2 parts in 10⁷, it appears desirable to compare 2e/h systems directly by transporting one of them.