Table of contents

In the present investigation the values of the gold, silver and aluminium freezing points have been determined on the thermodynamic scale. The method used is based on Planck's radiation law, with the antimony freezing point as a reference temperature on the thermodynamic scale.

Taking for c₂ the value 1.4388 × 10^-2 m. °K and for the freezing point of antimony Moser's value of 630.67 °C, it is found that the value of the gold point on the International Practical Temperature Scale needs a correction of +1.4 ± 0.4 °C, that of the silver point of +1.4 ± 0.3 °C and that of the aluminium point of +0.3 ± 0.07 °C.

Present knowledge of the refractive index of air is reviewed. Regarding the absolute values there are as yet no definite indications that the standard adopted in 1953 on the basis of Barrell and Sears' measurements should be changed, but new experiments aiming at reducing the present uncertainty of about ± 5 × 10^-8 would be desirable.

Several recent investigations have contributed important new information on the dispersion of air, which has made it possible to derive an improved dispersion formula for standard air, (n − 1)_s × 10⁸ = 8342.13 + 2406030 (130 − σ²)^-1 + 15997 (38.9 − σ²)⁻¹, where σ is the vacuum wave-number in μm^-1. The deviations from the 1953 formula are small and practically negligible in most spectroscopic work.

An equation for the dependence of refractivity on temperature and pressure based on theoretical considerations has been derived. For the range of atmospheric conditions normally found in a laboratory the equation can be approximated by the formula (n − 1)_tp = (n − 1)_s × 0.00138823 p/(1 + 0.003671 t), with p in torr, t in °C, and (n − 1)_s given by the dispersion formula for standard air.

The effect of carbon dioxide and water vapour is discussed. From Erickson's dispersion data for water vapour, combined with Barrell and Sears' absolute measurements, one obtains the equation n_tpf – n_tp = −f (5.722 − 0.0457 σ²) × 10^-8 for the difference in refractive index of moist air, containing f torr of water vapour, and dry air at equal temperature and total pressure. The equation is valid for visible radiations and normal atmospheric conditions.

The performance of commercially produced, standard platinum resistance thermometers has been investigated in the region 630 - 900 °C. Particular attention was paid to resistance stability, quenching effects, and electrical insulation leakage. The limit of 900 °C was dictated by the use of mica insulation in these instruments.

The most serious problem encountered was that of insulation leakage at both high and low temperatures. The low temperature leakage was due to water that had been released from mica insulation when the thermometers were used at high temperatures, and this problem is studied in some detail here. A relationship between the magnitude of the galvanometer `wet kick' and the insulation resistance has been established. The useful lifetime of the dry air filling in a thermometer has been estimated for various conditions of use.

Based on these studies, procedures have been recommended for stabilizing platinum resistance thermometers, annealing-out quenching effects, and reducing insulation leakage over this temperature range.