ham. The subject of his inaugural dissertation (3 Nov. 1846) was ’De Motu Caloris per Terrae Corpus.' He held this professorship till 1899. Admittedly a bad expositor, he proved himself to be a most inspiring teacher and a leader in research. With the slenderest material resources and most inadequate room, he created a laboratory of physics, the first of its kind in Great Britain, where he worked incessantly, gathering around him a band of enthusiastic students to collaborate in pioneering researches in electric measurement and in the investigation of the electro-dynamic and thermoelectric properties of matter. In the lecture theatre his enthusiasm won for him the love and respect of all students, even those who were unable to follow his frequent flights into the more absruse realms of mathematical physics. Over the earnest students of natural philosophy he exercised an influence little short of inspiration, which extended gradually far beyond the bounds of his own university.
From his first days as professor Thomson worked strenuously with fruitful results. By the end of four years (1850), when he was twenty-six, he had pul3lished no fewer than fifty original papers, most of them highly mathematical in character, and several of them in French. Amongst these researches there is a remarkable group which originated in his attendance in 1847 at the meeting at Oxford of the British Association, where he read a paper on electric images. But a more important event of that meeting was the commencement of his friendship with James Prescott Joule [q. v.] of Manchester, who had for several years been pursuing his researches on the relations between heat, electricity, and mechanical work. Joule's epoch-making paper, which he presented on this occasion, on the mechanical equivalent of heat, would not have been discussed at all but for Thomson's observations. Thomson had at first some difficulty in grasping the significance of the matter, but soon threw himself heart and soul into the new doctrine that heat and work were mutually convertible. For the next six or eight years, partly in co-operation with Joule, partly independently, he set himself to unravel those mutual relations.
Thomson was never satisfied with any phenomenon until it should have been brought into the stage where numerical accuracy could be determined. He must measure, he must weigh, in order that he might go on to calculate. ’The first step,' he wrote, 'toward numerical reckoning of properties of matter ... is the discovery of a continuously varying action of some kind, and the means of observing it definitely, and measuring it in terms of some arbitrary unit or scale division. But more is necessary to complete the science of measurement in any department, and that is the fixing on something absolutely definite as the unit of reckoning.' It was in this spirit that Thomson approached the subject of the transformation of heat.
Sadi Camot in 1824 had anticipated Joule in his study of the problem in his 'Reflexions sur la Puissance Motrice du Feu,' where was discussed the proportion in which heat i% convertible into work, and William John Macquom Rankine [q. v.] had carried the inquiry a stage farther in 1849; while Helmholtz in 'Die Erhaltung der Kraft' (1847)— 'On the Conservation of Force' (meaning what we now term Energy) — denied the possibility of perpetual motion, and sought to establish that in all the transformations of energy the sum total of the energies in the universe remains constant. Thomson in June 1848 communicated to the Cambridge Philosophical Society a paper 'On an Absolute Thermometric Scale founded on Camot's Theory of the Motive Power of Heat, and calculated from Regnault's Observations.' There he set himself to answer the question : Is there any principle on which an absolute thermometric scale can be founded ? He arrived at the answer that such a scale is obtained in terms of Camot's theory, each degree being determined by the performance of equal quantities of work in causing one imit of heat to be transformed while being let down through that difference of temperature. This indicates as the absolute zero of temperature the point which would be marked as — 273° on the air thermometer scale. In 1849 he elaborated this matter in a further paper on 'Carnot's Theory,' and tabulated the values of 'Carnot's function' from 1° C. to 231° C. Joule, writing to Thomson in December 1848, suggested that probably the values of 'Carnot's function' would turn out to be the reciprocal of the absolute temperatures as measured on a perfect gas thermometer, a conclusion independently enunciated by Clausius in February 1850.
Thomson zealously continued his investigation. He experimented on the heat developed by compression of air. He verified the prediction of his brother, Professor James Thomson, of the lowering by pressure of the melting-point of ice. He gave a thermodynamic explanation of
