Mechanical equivalent of heat (Joule)

1843-1850 Energy and motion observation

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A measured relation between work and heat: a given amount of mechanical work always yields the same amount of heat.

The series of measurements was carried out by the English brewer and amateur physicist James Prescott Joule: the magneto-electric experiments of 1843 and the paddle wheel of 1845-1849. The final paper 'On the Mechanical Equivalent of Heat' appeared in Philosophical Transactions 140 (1850) and gave the final value of the equivalent (the figure itself has not yet been checked against an open source and is not given here).

This directly contradicted the premise of the conservation of caloric and became the foundation of the first law of thermodynamics.

Why this date

exact: first value 1843, final paper in Phil. Trans. 1850; the year of first publication is used for the axis

What grew out of it

• Kinetic theory of gases and statistical mechanics

  influenced · Confidence: high

  The mechanical equivalent of heat and the conservation of energy made the idea of heat as molecular motion admissible for serious consideration, and so opened the way for the kinetic theory in the 1850s.

  Link sources

  • Stephen G. Brush, History of the Kinetic Theory of Gases (Storia della Scienza, vol. 7, ch. 44) (2004) Verified 20 Sep 2026

    The 'kinetic theory of heat' -- the old idea that heat is directly related to the kinetic energy of atomic motion -- had to be given serious consideration as soon as energy conservation and thermodynamics had been introduced in the middle of the 19th century.

  • Stephen G. Brush, History of the Kinetic Theory of Gases (Storia della Scienza, vol. 7, ch. 44) (2004) Verified 20 Sep 2026

    The kinetic theory was not widely accepted in the 18th century; most scientists preferred the Newtonian repulsion theory, which was compatible with the idea that heat is a fluid, 'caloric,' rather than the energy of atomic motion.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Other links

• Caloric (calorique) as a weightless conserved fluid

  refuted by · Confidence: high

  A constant mechanical equivalent of heat means that heat is created from work, which denies the conservation of caloric.

  Dispute: A gap between experiment and acceptance: Joule obtained his first value of the equivalent in 1843 and the measurements continued until 1850; the scientific community accepted the mechanical theory of heat in 1850-1851 through the work of Clausius and Thomson, and Thomson himself, according to Mendoza, long found it hard to accept Joule's results because he held to caloric. The historian's judgement that it was Joule's work that overturned caloric is Daub's: Clausius held the first law to have been established by Joule's experiments.

  Link sources

  • Daub E. E. «Clausius, Rudolf», Dictionary of Scientific Biography (via Encyclopedia.com) (1971) Verified 20 Sep 2026

    The denial was based, of course, on what has become the first law of thermodynamics—that whenever work is produced by heat, a quantity of heat equivalent to that amount of work is consumed—a premise that Clausius believed had been firmly established by Joule's experiments.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Links under review

• Rumford: heat from friction in cannon boring

  influenced · Confidence: medium

  The only source is the Wikipedia article 'Caloric theory' (citing Fox 1971); a verbatim quotation from Fox 1971 is needed.

  Rumford's experiment served as a model and motive for Joule's measurements.

  Dispute: No direct quotation from Joule about Rumford has been extracted from the primary source (Phil. Trans. 1850); the claim rests on a secondary source.

  Link sources

  • Wikipedia «Caloric theory» (2024) Verified 20 Sep 2026

    Rumford's experiment inspired the work of James Prescott Joule and others towards the middle of the 19th century.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

• The two laws of thermodynamics (Clausius 1850, Thomson 1851)

  prerequisite for · Confidence: high

  The claim rests on a summary in the Wikipedia article. A verbatim quotation is needed from the paper itself: Clausius R., «Über die bewegende Kraft der Wärme», Annalen der Physik 79, 1850, or from Fox R., 1971.

  Without Joule's equivalent, Clausius and Thomson could not have replaced the conservation of heat with the conservation of energy.

  Link sources

  • Chemistry World, «Carnot's heat engine and the birth of thermodynamics» (2024)

    By 1850, he and Rudolf Clausius separately reconciled Carnot's ideas about motive power with James Joule's measurements of the equivalence of heat and work, using the concepts of absolute temperature and entropy.

  • Wikipedia «Caloric theory» (2024) Verified 20 Sep 2026

    In 1850, Rudolf Clausius published a paper showing that the two theories were indeed compatible, as long as the calorists' principle of the conservation of heat was replaced by a principle of conservation of energy.

  "Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.

Node sources

• Joule J. P. «On the Mechanical Equivalent of Heat», Phil. Trans. R. Soc. 140, 61-82 (1850)

  III. On the mechanical equivalent of heat. James Prescott Joule. Phil. Trans. R. Soc. (1850) (140): 61–82.

• Thomson W. «An Account of Carnot's Theory of the Motive Power of Heat», Trans. R. Soc. Edinburgh 16 (1849) Verified 20 Sep 2026, quotation assembled from fragments

  The extremely important discoveries recently made by Mr JOULE of Manchester, that heat is evolved in every part of a closed electric conductor, moving in the neighbourhood of a magnet, and that heat is generated by the friction of fluids in motion, seem to overturn the opinion commonly held that heat cannot be generated, but only produced from a source, where it has previously existed either in a sensible or in a latent condition. ... (See a paper “On the Calorific Effects of Magneto-Electricity, and on the Mechanical Value of Heat; by J. P. JOULE, Esq.” Read before the British Association at Cork in 1843...)

"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.

Further reading

Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.

• Mendoza E. «A sketch for a history of early thermodynamics», Physics Today 14(2) (1961)

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