Table of contents

The technique of Nuclear Quadrupole Resonance thermometry is described. It is shown that the N.Q.R. thermometer exhibits unique properties of sensitivity, range, and reproducibility that are all desirable qualities for a temperature standard. The response to temperature is a unique characteristic of the working substance, which in this case is KClO₃. Experimentally, the center frequency of the absorption line of Cl³⁵ in pure samples of KClO₃ at 77 °K is reproducible to approximately 2 Hz. This corresponds to an absolute accuracy of better than ± 0.001 °K. The purity of the samples is achieved by recrystallization of commercial reagents. The range of the thermometer extends from 10 °K to approximately 450 °K. A description of a simple system which locks an oscillator to the N.Q.R. absorption line is given. A new method of determining accurately the frequency of the center of the line using magnetic field broadening is also described. With this technique a sensitivity of ± 0.0002 °K is expected. In line with these results, evaluation of a thermometer based on the temperature dependence of the nuclear magnetic resonance frequency in antiferromagnetic salts is made.

Comparative measurements of monochromatic black-body radiation in the infra-red (2 to 4 μm) over temperature ranges between 175 °C and 1063 °C have been made. Temperatures were measured on the International Practical Scale, and the differences between the thermodynamic and practical scales determined. Assuming a value of 419.58 °C for the freezing point of zinc, in conformity with recent gas thermometry, and a value of 0.014 388 m.deg for the constant c₂ in Planck's equation, agreement with gas thermometry is good up to 780 °C, but thereafter temperatures measured on the radiation scale are lower. Recent gas thermometer values for the gold point range from 1063.7 °C to 1064.5 °C, with a mean of 1064.2 °C, while the radiation measurements give 1063.7 °C, with limits of ± 0.6 deg calculated from the random errors. It is not thought that systematic errors add seriously to the uncertainty.

The various aspects of the concept of temperature as a basic quantity in physics are discussed. After a brief historical introduction, the thermodynamic basis of the definition of temperature is given. In § 3 the statistical definition of temperature in terms of the canonical ensemble of statistical mechanics is presented and the equivalence with the thermodynamic definition is stressed. Attention is given also to the physical meaning of negative temperatures. In § 4 the realization of the thermodynamic temperature scale based on the laws of ideal gases, ideal paramagnetic and nuclear magnetic substances and black body radiation is discussed. The international practical temperature scale is very briefly discussed in § 5. Finally in § 6 some remarks are made about the "basic" character of temperature concept and it is stressed that proposals to change the definition of this quantity have so far not offered an improvement.

A definition of time-scales and criteria of uniformity of a time-scale are introduced in the first part of the paper. These considerations are applied to time-scales currently used in Astronomy: Sidereal and Universal Times. Each of them is defined, their equivalence is proved, and the precision with which each can be determined is discussed. The irregularities of the rotation of the Earth are the causes of the non-uniformity of these time-scales. A uniform gravitational time, Ephemeris Time, is then defined and the precision of its determination is discussed. All the three astronomical time-scales have a necessity of their own and cannot be interchanged. In order to reach a better understanding of the concept of time, it is necessary that all time-scales — including atomic time — should coexist and be compared with the highest possible precision.