Temperature measurement with the einthoven galvanometer

Accurate measurement of instantaneous temperatures during a cycle of variation is of great importance in the application of the theory of thermodynamics. In the past, temperatures of the working charge in the internal-combustion engine have been deduced from a conditional equation for the gas, values obtained by measurement and estimation being assigned to the other variables in the equation. Serious errors of unknown magnitude may be involved in such estimation, and the deduced temperatures are liable to corresponding inaccuracies. Thermo-couples and platinum-resistance thermometers have been used by a number of experimenters for obtaining records of the cyclically varying temperature, but these records do not directly give the temperature of the charge, even if they represent accurately the temperature of the thermometer. The object of the work undertaken is the investigation of the relationship between the temperature of the gas in which the thermdmeter is situated and the thermometric record obtained. Attention has been devoted primarily to the platinum-resistance thermometer. As it is required to obtain a continuous record of temperature throughout a single cycle, the method adopted is to measure the out-of-balance current in the Wheatstone bridge by means of an Einthoven galvanometer. The forces on the fibre of the Einthoven galvanometer are considered, and the suitability of various types of fibre are compared. The mathematical theory of the unbalanced Wheatstone bridge is investigated. The general case is considered, and alternative arrangements are compared with respect to sensitivity and error in compensation. An estimate of the accuracy with which the galvanometer deflection corresponds to the temperature attained by the measuring wire is given. The apparatus designed to investigate the problem is described, and experimental results are given and discussed.