Table of contents

The thermodynamic temperature scale was realized over the range from 0 °C to -183 °C by using a helium constant volume gas thermometer, and platinum resistance thermometers were calibrated to derive an accurate relation between the resistance of pure platinum and the thermodynamic temperature. It is shown that the International Practical Scale of Temperature, as defined by the platinum resistance thermometer using the Callendar-Van Dusen equation, is in error by -0.036 °C at -80 °C and +0.023 °C at -150 °C, the estimated limit of error on these differences being ±0.006 °C.

A refractometer for the measurement of the refractive index of the surrounding air is described, in which interference fringes are produced between two beams, one propagated in air, the other in vacuum. The accuracy aimed at is one part in 10⁸, corresponding to the accuracy of the best vacuum wavelength standards used in the length measurement of mechanical standards. The refractometer is placed with the comparator inside an airtight isothermal enclosure and is operated from outside.

The place of absolute measurements of gravity in the system of fundamental constants of physics and celestial mechanics is discussed and the history of absolute determinations is summarised. Two classes of method are mainly used at present, the reversible pendulum and the free-motion experiment. The principles of these methods are discussed in detail, particular attention being given to the definition of experimental parameters and to perturbing effects. The results so far obtained are described and experiments still in progress are mentioned. Three of the completed determinations are in Europe - at Leningrad (reversible pendulum), Teddington (reversible pendulum) and at Sèvres (free-fall of line standard) and three are in N. America - at Washington (reversible pendulum), at Ottawa (free fall of graduated scale) and at Princeton (free fall of interferometer component). Comparisons between the results can only be made through measurements of the differences of gravity between the six sites and the problems of obtaining reliable differential results are indicated. It appears that the N. American results are more scattered than the European ones and that they differ from them by more than 2 mgal, an apparently significant amount. It is most likely that the differential measurements have systematic errors but there is a great need for further absolute measurements, especially in North America, to confirm or invalidate this conclusion.

In recognition of the October 1964 declaration of the International Committee of Weights and Measures that the physical measurement of time be based on a particular transition between two hyperfine levels in the ground state of cesium 133, a review of the characteristics of cesium beam atomic frequency standards is presented. This article discusses the general requirements for frequency and time standards, advantages offered by the atomic standard as compared to astronomical standards, various other atomic standards in brief, the operating principles of cesium standards, measures of performance, error sources in cesium standards, characteristics of several standards in current operation, comparison of cesium standards, and atomic time standards derived from atomic frequency standards.