By 1847, Thomson had already gained a reputation as a precocious and maverick scientist when he attended the
British Association for the Advancement of Science annual meeting in
Oxford. At that meeting, he heard
James Prescott Joule making yet another of his, so far, ineffective attempts to discredit the
caloric theory of
heat and the theory of the
heat engine built upon it by
Sadi Carnot and
Émile Clapeyron. Joule argued for the mutual convertibility of heat and
mechanical work and for their mechanical equivalence.
Thomson was intrigued but skeptical. Though he felt that Joule's results demanded theoretical explanation, he retreated into an even deeper commitment to the Carnot-Clapeyron school. 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.
In his publication, Thomson wrote:
- but a footnote signalled his first doubts about the caloric theory, referring to Joule's
very remarkable discoveries. Surprisingly, Thomson did not send Joule a copy of his paper but when Joule eventually read it he wrote to Thomson on
6 October, claiming that his studies had demonstrated conversion of heat into work but that he was planning further experiments. Thomson replied on
27 October, revealing that he was planning his own experiments and hoping for a reconciliation of their two views.
Thomson returned to
critique 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". Moreover, his
theological beliefs led to speculation about the
heat death of the universe.
Compensation would require
a creative act or an act possessing similar power.
In final publication, Thomson retreated from a radical departure and declared "the whole theory of the motive power of heat is founded on ... two ... propositions, due respectively to Joule, and to Carnot and Clausius." Thomson went on to state a form of the second law:
In the paper, Thomson supported the theory that heat was a form of motion but admitted that he had been influenced only by the thought of Sir
Humphry Davy and the experiments of Joule and
Julius Robert von Mayer, maintaining that experimental demonstration of the conversion of heat into work was still outstanding.
As soon as Joule read the paper he wrote to Thomson with his comments and questions. Thus began a fruitful, though largely epistolary, collaboration between the two men, Joule conducting experiments, Thomson analysing the results and suggesting further experiments. The collaboration lasted from 1852 to 1856, its discoveries including the
Joule-Thomson effect, sometimes called the Kelvin-Joule effect, and the published results did much to bring about general acceptance of Joule's work and the
kinetic theory.
Thomson published more than 600 scientific papers and filed over 70 patents.