II. Continuous electrical calorimetry

Abstract

The method of Continuous Electrical Calorimetry, described in the following paper, was originally devised as part of a Fellowship Dissertation on applications of the platinum thermometer, at Trinity College, Cambridge, in the year 1886, but, on account of unforeseen difficulties, the experiments did not at that time get beyond the preliminary stage. In the first rough apparatus, a steady flow of water, passing through a tube about 30 centims. long and 3 millims. in diameter, was heated by an electric current in a fine spiral of platinum wire of about 5 ohms resistance, nearly fitting the tube. The steady difference of temperature between the inflow and the outflow was measured by a pair of delicate mercury thermometers, which it was of course intended to replace in the final apparatus by a differential pair of platinum thermometers. The electrical energy supplied was measured by the potentiometer method in terms of a set of 5 Clark cells and a large German-silver resistance of 5 ohms in series with the platinum spiral. The potentiometer was specially made for the work, and consisted of a metre slide-wire, and ten resistances, each equal to the slide-wire, for extending the scale so as to secure sufficient accuracy of reading. This potentiometer was still in existence at the Cavendish Laboratory in 1893. The set of 5 Clark cells were tested by Glazebrook and Skinner (‘Phil. Trans’. A, 1892), and were still in good condition at a later date. The external heatloss in these experiments was found to be much larger than had been anticipated, and so variable that the results were of little or no value. In order to remedy this defect, I designed the vacuum-jacket, which was suggested by some experiments of Sir William Crookes ('Roy. Soc. Proc.’, vol. 31, 1881, p. 239), which appeared to indicate that the rate of cooling of a mercury thermometer in a very good vacuum was ten to twenty times less than in air. I therefore regarded the vacuum-jacket as a most essential part of the experiment, and expected a great improvement to result from its use. Unfortunately I failed to make the jacket for want of sufficient skill in glass-work, and abandoned the experiment for the time, until my appointment as Professor of Physics at McGill College, Montreal, gave me greater facilities for carrying out the work. Eventually it proved that the effect of the vacuum-jacket in diminishing the external loss of heat was not nearly so great as I had been led to imagine, but it possessed several advantages as a heat insulator over such materials as cotton wool or flannel. The thermal capacity of a vacuum being negligible, the time required for attaining a steady state was much shortened. Moreover there was no risk of error from damp, which is the worst drawback of ordinary lagging. I had not originally intended to employ the electrical method for determining the variation of the specific heat of water, but only for comparing the electrical and thermal units at ordinary temperatures. In the meantime the work of Griffiths, with which I was intimately acquainted, had shown that the electrical units were probably in error, and appeared to indicate a smaller rate of variation of the specific heat than that given by Rowland. In reconsidering the problem, in 1893, I therefore determined to attempt the absolute measurement of the ohm and the Clark cell, in addition to the variation of the specific heat of water over as wide a range as possible. The method of steady-flow calorimetry appeared to be particularly adapted to the latter object, as it afforded much greater facility than that of Griffiths or Rowland in varying the conditions of experiment over a wide range. For the absolute measurement of the ohm, I immediately obtained estimates for a Lorenz apparatus of Professor V. J ones’ pattern, which was eventually ordered in October, 1894, and is briefly described in Section 6 of this paper. For the absolute measurement of the Clark cell in terms of the ohm, after spending some time in designing various forms of electrodynamometer, I decided to employ the British Association pattern, with certain modifications, which are explained below, Sections 10 to 16. At the same time I commenced a series of investigations into the defects of the form of Clark cell described in the Board of Trade Memorandum, in which I was assisted by Mr. H. T. Barnes. This work included an accurate determination of the variation of the E. M. F. with temperature and with strength of solution, in addition to measurements of the solubility of zinc sulphate and of the density of its solutions. It extended further than I had at first anticipated, and was not completed till the summer of 1896. The results were published in the ‘Proceedings of the Royal Society,’ vol. 62, pp. 117-152.