Editing Lord Kelvin (section)

=== Thermodynamics ===
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 [[Nicolas Léonard Sadi Carnot|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 sceptical. 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 ''[[perpetual motion|perpetuum mobile]]''. Experimental confirmation in his laboratory did much to bolster his beliefs.

In 1848, he extended the Carnot–Clapeyron theory further through his dissatisfaction that the [[gas thermometer]] provided only an [[operational definition]] of temperature. He proposed an ''[[absolute temperature]] scale''<ref>{{cite book | author=Chang, H. | title=Inventing Temperature: Measurement and Scientific Progress | publisher=Oxford University Press | year=2004 | isbn=978-0-19-517127-3|chapter =4 }}</ref> 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."<ref>{{cite book | title=Mathematical and Physical Papers | chapter=On an Absolute Thermometric Scale founded on Carnot's Theory of the Motive Power of Heat, and calculated from Regnault's observations | publisher=Cambridge University Press | date=1848 | doi=10.1017/cbo9780511996009.040 | pages=100–106 | author = Thomson, W.| isbn=978-1-108-02898-1 }}</ref> 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. "Reflections on the Motive Power of Heat", published by Carnot in French in 1824, the year of Lord Kelvin's birth, used −267 as an estimate of the absolute zero temperature. Thomson used data published by Regnault to [[calibration|calibrate]] his scale against established measurements.

In his publication, Thomson wrote:

{{Blockquote|... The conversion of heat (or ''caloric'') into mechanical effect is probably impossible, certainly undiscovered}}—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 sides.

Thomson returned to critique Carnot's original publication and read his analysis to the [[Royal Society of Edinburgh]] in January 1849,<ref>{{cite book | title=Mathematical and Physical Papers | chapter=An Account of Carnot's Theory of the Motive Power of Heat; with Numerical Results deduced from Regnault's Experiments on Steam | publisher=Cambridge University Press | date=1849 | doi=10.1017/cbo9780511996009.042 | pages=113–164 | author = Thomson, W.| isbn=978-1-108-02898-1 | url=https://zenodo.org/record/1634618 }}</ref> 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. 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 [[theology|theological]] beliefs led Thomson to [[Extrapolation|extrapolate]] the second law to the cosmos, originating the idea of [[heat death of the universe|universal heat death]].

{{Blockquote|I believe the tendency in the material world is for motion to become diffused, and that as a whole the reverse of concentration is gradually going on – I believe that no physical action can ever restore the heat emitted from the Sun, and that this source is not inexhaustible; also that the motions of the Earth and other planets are losing ''[[vis viva]]'' which is converted into heat; and that although some ''vis viva'' may be restored for instance to the earth by heat received from the sun, or by other means, that the loss cannot be ''precisely'' compensated and I think it probable that it is under-compensated.<ref name="Sharlin 1979">[[#Sharlin|Sharlin]], p. 112.</ref>}}

Compensation would require ''a creative act or an act possessing similar power'',<ref name="Sharlin 1979" /> resulting in a ''rejuvenating universe'' (as Thomson had previously compared universal heat death to a clock running slower and slower, although he was unsure whether it would eventually reach [[thermodynamic equilibrium]] and ''stop for ever'').<ref>{{cite magazine |last1=Otis |first1=Laura |year=2002 |title=Literature and Science in the Nineteenth Century: An Anthology |url= https://oxfordworldsclassics.com/view/10.1093/owc/9780199554652.001.0001/isbn-9780199554652 |magazine=OUP Oxford |volume=1 |pages=60–67}}</ref> Thomson also formulated the [[heat death paradox]] (Kelvin's paradox) in 1862, which uses the second law of thermodynamics to disprove the possibility of an infinitely old universe; this paradox was later extended by [[William Rankine]].<ref>{{cite magazine |last1=Thomson |first1=William |year=1862 |title=On the Age of the Sun's Heat |url=https://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html |magazine=Macmillan's Magazine |volume=5 |pages=388–393}}</ref>

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."<ref>{{cite book | title=Mathematical and Physical Papers | chapter=On the dynamical theory of heat; with numerical results deduced from Mr. Joule's equivalent of a thermal unit and M. Regnault's observations on steam | publisher=Cambridge University Press | date=1852 | author = Thomson, W. | doi=10.1017/cbo9780511996009.049 | pages=174–332| isbn=978-1-108-02898-1 }}</ref> Thomson went on to state a form of the second law:

{{Blockquote|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.<ref>{{cite journal|last=Thomson|first=W.|title=On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam|journal=Transactions of the Royal Society of Edinburgh|date=March 1851|volume=XX|issue=part II|pages=261–268; 289–298}} Also published in {{cite journal|last=Thomson|first=W.|title=On the Dynamical Theory of Heat, with numerical results deduced from Mr Joule's equivalent of a Thermal Unit, and M. Regnault's Observations on Steam|journal=Phil. Mag. |date=December 1852 |volume=IV |series=4 |issue=22 |pages=8–21 |url=https://archive.org/details/londonedinburghp04maga }}</ref>}}

In the paper, Thomson supports the theory that heat was a form of motion but admits 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.<ref>Thomson, W. (1851) ''p.''183</ref> 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<ref>{{cite book | title=Mathematical and Physical Papers | chapter=On the thermal effects of fluids in motion | publisher=Cambridge University Press | date=30 June 2011 | doi=10.1017/cbo9780511996009.050 | pages=333–455| author1 = Joule, J. P. | author2 = Thomson, W.}}</ref> did much to bring about general acceptance of Joule's work and the [[kinetic theory of gases|kinetic theory]].

Thomson published more than 650 scientific papers<ref name="britannica.com" /> and applied for 70 patents (not all were issued). Regarding science, Thomson wrote the following:

{{Blockquote|In physical science a first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind: it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of ''science'', whatever the matter may be.<ref>{{cite book|last=Thomson|first=W.|title=Popular Lectures and Addresses, Vol. I|year=1891|publisher=MacMillan|location=London|page=80|isbn=978-0-598-77599-3|url=https://books.google.com/books?id=JcMKAAAAIAAJ|access-date=25 June 2012}}</ref> }}