Liquid Air—Production and Applications 1

Abstract

IN the former of these papers the author details experiments showing the trustworthiness of a German silver platinum couple to measure temperatures in the neighbourhood of those of liquid air and liquid and solid hydrogen. The electric resistance of metals is an unsafe guide at very low temperatures, and the manipulation of gas thermometers involves much time and care. A thermo-electric junction would be much more convenient if trustworthy. That it is trustworthy the experiments go to show, but only within limits. If the constants of the formula for interpreting the observations be determined at temperatures between 90½° and 123½° abs., the formula will then give the temperature of solid hydrogen at low pressure as 15°.27 abs., whereas if the constants be deduced from experiments at a lower temperature, 20½° to 77½°, the interpretation formula then makes the temperature of solid hydrogen at low pressure 1¾° lower, i.e. 13°.5 abs., which the author considers more correct. Bearing in mind that at this very low temperature a difference of 1¾° is equivalent to a difference of 37° at the ordinary temperature, we see that the method has no confirmatory value, and can itself be trusted only over the range for which it has been verified by the careful use of gas thermometers. If, therefore, helium be procured in sufficient quantity for liquefaction or solidification, its lower temperatures, possibly within 5° of the absolute zero, will have to be ascertained by the low-pressure helium thermometer. For ranges of temperature over which its indications can be verified, the thermo-electric junction thermometer will have a useful sphere of work in saving the inconvenience of employing gas thermometers. Among important cautions given by the author is a warning that junctions made with soft solder are affected by the low temperature. The junctions should be made with hard silver solder, and the indications at the temperature of liquid oxygen compared before and after exposure to the temperature of liquid hydrogen, to see whether there has been any change produced. A German silver platinum junction was employed, but as the result of his experience the author recommends German silver gold.