James Prescott Joule and the Nature of Heat

James Prescott Joule

James Prescott Joule

On December 24, 1818English physicist and brewer, James Prescott Joule was born. Joule studied the nature of heat, and discovered its relationship to mechanical work. This led to the law of conservation of energy, which led to the development of the first law of thermodynamics. The SI derived unit of energy, the joule, is named after James Joule.

James Prescott Joule was the son of a wealthy brewer and was educated by the famous scientist John Dalton. Further, it is believed that Joule was influenced by chemist William Henry and the engineers Peter Ewart and Eaton Hodgkinson. James Joule began to manage the brewery as an adult, next to his serious hobby, science. Approximately around 1840, Joule began to investigate the feasibility of replacing brewery’s steam engines with the newly invented electric motor and he published his first scientific papers on the topic in William Sturgeon’s Annals of Electricity.

Joule discovered Joule’s first law in 1841, meaning that the heat which is evolved by the proper action of any voltaic current is proportional to the square of the intensity of that current, multiplied by the resistance to conduction which it experiences. He came to realize that burning a pound of coal in a steam engine was more economical than a costly pound of zinc consumed in an electric battery.

In 1843 he published results of experiments showing that the heating effect he had quantified in 1841 was due to generation of heat in the conductor and not its transfer from another part of the equipment. However, these reults were directly challenging the caloric theory which held that heat could neither be created or destroyed. Caloric theory had dominated thinking in the science of heat since introduced by Antoine Lavoisier in 1783. Supporters of the caloric theory readily pointed to the symmetry of the Peltier-Seebeck effect to claim that heat and current were convertible in an, at least approximately, reversible process.

In June 1845, Joule read his paper On the Mechanical Equivalent of Heat to the British Association meeting in Cambridge. The work contained his reports of his best-known experiment, involving the use of a falling weight, in which gravity does the mechanical work, to spin a paddle-wheel in an insulated barrel of water which increased the temperature. He now estimated a mechanical equivalent of 819 ft·lbf/Btu (4.41 J/cal).
Joule’s Heat Apparatus, 1845. Five years later, Joule published a refined measurement of 772.692 ft·lbf/Btu (4.159 J/cal).

After Joule proposed his kinetic theory of heat, he faced much scepticism. However, in Germany, Hermann Helmholtz became aware both of Joule’s work and the similar 1842 work of Julius Robert von Mayer. Though both men had been neglected since their respective publications, Helmholtz’s definitive 1847 declaration of the conservation of energy credited them both. Further, one of Joule’s presentations at the British Association in Oxford was attended by George Gabriel Stokes, Michael Faraday, and William Thomson, later to become Lord Kelvin.

It is believed that Thomson stayed sceptical, however, eventually a fruitful, though largely epistolary, collaboration between the two men evolved. With 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, and the published results did much to bring about general acceptance of Joule’s work and the kinetic theory.

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