prominence by Sir William Thomson, Clerk Maxwell, and others, at various meetings of the British Association; and in 1862, at the Cambridge meeting, the Committee of the Association on Standards of Electrical Resistance, appointed in the previous year, made their first report. In the next report (1863) Joule's name appears, and to him was entrusted the duty of determining the dynamical equivalent of heat from the thermal effects of electric currents. Before this could be done it was necessary to wait for the new standard of resistance, the ‘ohm.’ This was completed by Maxwell and Fleeming Jenkin in 1864, and in 1867 the committee reported that considerable progress in their work had been made, and that Joule's experiments on the heat generated in a voltaic current, the resistance of which was known in absolute measure, when conducted with every possible care, gave 783 as the value of the equivalent. The last experiments by friction had given the value 772, and Joule expressed himself as willing to make a new determination by the frictional method to determine if possible the cause of the discrepancy. An account of the electrical experiments is given in the British Association Report for 1867 (Report of the Committee on Electrical Standards, Appendix vi.)
The results of Joule's final experiments by the direct method of friction appeared in 1878 in a paper ‘On a New Determination of the Mechanical Equivalent of Heat’ (Phil. Trans. 1878, pt. ii.), and lead to the value 772.55, agreeing almost exactly with the value found in 1850. It appeared, therefore, that the cause of the discrepancy lay in the unit of resistance. Any doubt as to this was soon resolved, for Rowland, in the same year as Joule's last paper was published (American Journal of Science and Arts, 1878), showed that the standard of resistance was about 1 per cent. smaller than the committee of 1864 had intended it to be, and that, making this correction, the results of his own experiments by the methods of friction and of electrical heating agreed very closely both among themselves and also with Joule's value, 772.5. This result was confirmed in 1881 and 1882 by Lord Rayleigh, who found that the value of the British Association unit of resistance was .9867 instead of unity, while the value required to bring Joule's two determinations of J. into complete accordance is .9873. Thus the exactness of his work has been amply verified.
The full credit for establishing his great principle belongs to Joule; still others had been working more or less vaguely in the same field. Bacon, in the ‘Novum Organon,’ states his conviction that ‘the very essence of heat is motion and nothing else.’ Boyle, in his book ‘On Cold’ (1665), when discussing the primum frigidum, says: ‘For if a bodie's being cold signify no more than its not having its insensible parts so much agitated as those of our sensories, there will be no cause to bring in the primum frigidum … it suffices that the sun, or some other agent which agitated more vehemently its parts before, does now either cease to agitate them, or agitate them very remissly.’ But these and similar statements, such as that from Locke quoted by Joule in his paper of 1850, which may be found, are merely speculations.
The first experiments of value were those of Rumford about 1798, who produced by friction sufficient heat to raise 26.58 lb. of water from its freezing-point to its boiling-point, and concluded that heat was motion. In 1849 Joule himself called attention to these experiments, and showed that Rumford's numbers led to a value for the equivalent comparable with his own. Towards the end of the last century Sir Humphry Davy showed that ice could be melted by friction, even in a vacuum, when everything in the neighbourhood was at the freezing-point. Seguin in 1837 endeavoured to determine the equivalent from the loss of heat suffered by steam in expanding, and Mayer of Heilbronn in 1842 made a similar attempt by measuring the heat produced in the compression of air; but both of these methods involved the assumption, which was only justified by Joule's experiments of 1845, that all the mechanical energy spent in compressing the air was used in producing change of temperature. Mayer states (Leibig, Annalen, 1842) that he has raised the temperature of water from 12° C. to 13° C. by agitating it, but without indicating the force employed or the precautions requisite to secure an accurate result. Joule devised his own method, and carried out the experiments to a satisfactory conclusion. The great paper of V. Helmholtz, ‘Ueber die Erhaltung der Kraft,’ which did so much to extend the new views, was published in 1847.
In 1852 a Royal medal was awarded by the council of the Royal Society to Joule for his researches. He had been elected a fellow on 6 June 1850, and in 1860 he received the Copley medal from the hands of Sir Edward Sabine for the same experiments. In presenting this Sir Edward said: ‘The award of two medals for the same researches is an exceedingly rare proceeding in our society, and rightly so. The Council have on this occasion desired to mark by it in the most emphatic manner their sense of the special
