A flow method for comparing the specific heats of gases part I.―The experimental method

Abstract

Recent theoretical developments of kinetic theory, especially in the theory of pseudo-unimolecular reactions, have led to the demand for further measurements of the specific heats of vapours and gases, especially at high temperatures. Again many technical problems are demanding a better knowledge of the specific heats of gases over a wide range of conditions. The method to be described enables the ratio of the specific heats of two gases to be measured accurately. Since the monatomic gases are available as reliable standards, the fact that the method gives relative values only is of little account. There appear to be possibilities of wide application for the method on account both of its simplicity and its adaptability to a wide range of temperature and pressure. The method consists essentially in passing a slow stream of gas through a narrow tube, along which a temperature gradient has been established. The change of the temperature distribution along the tube depends on the properties of the gas and the rate of flow. It is found possible to choose the arrangement so that the measured temperature difference between two parts of the tube is a direct measure of the specific heat of the gas flowing in the tube. It will be seen that, though the method belongs to the "constant flow" class, it differs considerably in principle from that originally devised by Callendar and used for liquids by Callendar and Barnes, and Lang; and for gases by Swann, Nernst, and Scheel and Heuse. In the first place, all these experimenters measured the rate of supply of heat electrically, whilst the heat lost by conduction and radiation appeared as an experimentally determined correction; in the method to be described, the heat losses, though playing an essential part in the theory of the apparatus, do not enter at all into the measurement of the specific heat. Secondly, in previous constant flow methods the temperature of the gas itself has been measured by resistance thermometers, a proceeding which involves very careful design of the thermometer, the thorough mixing of the gas, and which has constituted the essential difficulty of specific heat measurements. In the present method the difficulty is avoided by never requiring to measure the exact temperature of the gas. Instead the temperature of the tube containing the gas is measured without in any way disturbing the flow.