William Thomson

W illiam Thomson, 1st Baron Kelvin (26 June 1824 – 17 December 1907) was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second Laws of Thermodynamics, and did much to unify the emerging discipline of physics in its modern form. For his work on the transatlantic telegraph project he was knighted by Queen Victoria, becoming Sir William Thomson. On his ennoblement in honour of his achievements in thermodynamics, and of his opposition to Irish Home Rule, he adopted the title Baron Kelvin of Largs and is therefore often described as Lord Kelvin. The title refers to the River Kelvin, which flows close by his laboratory at the university of Glasgow, Scotland.

Lord Kelvin is widely known for realising that there was a lower limit to temperature, absolute zero; absolute temperatures are stated in units of kelvin in his honour. He predicted that the melting point of ice must fall with pressure, otherwise its expansion on freezing could be exploited in a perpetuum mobile. Experimental confirmation in his laboratory did much to bolster his beliefs. In 1848, he extended the Carnot–Clapeyron theory still further through his dissatisfaction that the gas thermometer provided only an operational definition of temperature. He proposed an absolute temperature scale in which a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T−1)°, would give out the same mechanical effect [work], whatever be the number T. Such a scale would be quite independent of the physical properties of any specific substance. By employing such a ‘waterfall’, Thomson postulated that a point would be reached at which no further heat (caloric) could be transferred, the point of absolute zero about which Guillaume Amontons had speculated in 1702. Thomson used data published by Regnault to calibrate his scale against established measurements.

Thomson critiqued Carnot’s original publication and read his analysis to the Royal Society of Edinburgh in January 1849, still convinced that the theory was fundamentally sound. However, though Thomson conducted no new experiments, over the next two years he became increasingly dissatisfied with Carnot’s theory and convinced of Joule’s. In February 1851 he sat down to articulate his new thinking. However, he was uncertain of how to frame his theory and the paper went through several drafts before he settled on an attempt to reconcile Carnot and Joule. During his rewriting, he seems to have considered ideas that would subsequently give rise to the second law of thermodynamics. In Carnot’s theory, lost heat was absolutely lost but Thomson contended that it was ‘lost to man irrecoverably; but not lost in the material world’. Thomson went on to state a form of the second law:It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects.