James Prescott Joule was born on Christmas Eve 1818, in Salford, the son of a wealthy brewer. Joule was educated at home, and was taught for a time by John Dalton. He did not attend university, in part because of his family's dissenting background.
Joule worked at the family brewery until it was sold in 1854, and his earliest researches were carried out in his spare time. In 1838, he published his first work in William Sturgeon's Annals of electricity. Sturgeon (1783-1850), an evangelist for electricity, was an important early influence on Joule. Joule followed Sturgeon in believing that electromagnetism might be the source of virtually limitless, cheap motive power. Joule's practical experiments with electromagnets however convinced him that this hope was unfounded.
Joule continued to experiment with electricity, examining the properties of heat generated in electrical circuits and in electro-chemical reactions. One of his earliest discoveries was that heat produced in an electric circuit was proportional to the square of the current, and the resistance that a given weight of fuel burnt in a battery generates was the same amount of heat as it would produce if burned in an atmosphere of oxygen. In January 1843, Joule delivered an important paper to the Manchester Literary and Philosophical Society (to which he had recently been elected), 'On the heat evolved during the electrolysis of water’, where he noted direct equivalences between heat generated and energy expended [Joule did not use the term 'energy', but typically referred to 'work done' or vis viva], and concluded, "Electricity may be regarded as a grand agent for carrying, arranging, and converting chemical heat" (Joule, Scientific papers, vol. 1 p.120).
Joule followed up with experiments to determine a precise value for the relationship between work done and heat produced ('the mechanical equivalent of heat'). At the Cork meeting of the British Association in 1843, he delivered a paper, 'On the calorific effects of magneto-electricity and the mechanical value of heat' [later published in Philosophical Magazine 3.23, 1843]. Although this made little impact at the time, it was highly significant in its description of the interconvertibility of heat and work, and in containing a value for the mechanical equivalent of heat. Joule argued that heat was generated by mechanical or chemical action, not merely redistributed as the caloric theory of heat held, and he believed this was caused by the mechanical energy of constituent atoms as they vibrated during the reaction. Joule described experiments with electromagnets to measure electricity generated (using an accurate galvanometer) and the calorific effect of the coil (measured by the change of temperature in the water surrounding it). He contended that "heat is generated by the magneto-electrical machine, and that by means of the inductive power of magnetism we can diminish or increase at pleasure the heat due to chemical changes" (Joule, Scientific papers, vol.1, p.149). Joule held further that a constant ratio might exist between the heat and the mechanical power gained or lost, termed the 'mechanical value of heat’, and he offered a provisional finding for this: "The quantity of heat capable of increasing the temperature of a pound of water by one degree of Fahrenheit's scale is equal to, and may be converted into, a mechanical force capable of raising 838 lb. to the perpendicular height of one foot."
Between 1843-7 Joule attempted to demonstrate that a constant ratio existed between energy and heat, in a range of experiments involving the compression of air, the overcoming of friction between iron discs and the fluid friction between different liquids. Joule also refined his calculation for the mechanical value of heat. In 1845 at the British Association meeting in Cambridge, Joule first described an experiment using a paddle wheel and a cylindrical vessel fitted with baffles in which the wheel rotated. This vessel was filled with water and the wheel driven by falling weights. The energy expended by the wheel in overcoming the viscous drag of the water was converted to heat. The experiment was repeated with whale oil and mercury, yielding results of a similar value. Although this experiment again made little impact, when he expounded a revised version to the 1847 Oxford meeting of the British Association, it had a better reception. A young William Thomson [later Lord Kelvin] took up Joule's arguments, which were later developed into the concept of the conservation of energy, i.e. that energy can be transformed, but cannot be created nor destroyed. Thomson, rather than Joule, was to become a leading exponent of the new theories of thermodynamics, although Joule had played a major role in undermining the established science of heat, predicated on the idea of the conservation of heat.
In April 1847 Joule delivered a lecture 'On matter, living force and heat' at St Ann's Church reading room, Manchester, which outlined this concept of the conservation of energy. Professional recognition of Joule's work can came when "On the mechanical equivalent of heat", which reported a definitive value for the 'exchange rate' between heat and work, was communicated to the Royal Society in 1849 by Michael Faraday, and published in the Society's Philosophical Transactions.
After the 1850s Joule played a lesser role in the emerging subject of thermodynamics, but he continued to experiment. He established, with William Thomson, the Joule-Thomson effect, by which the temperature of a gas or liquid changes when it is forced through a valve while kept insulated so that no heat is exchanged with the environment. This was to be used in the liquefaction of gases. In one of his last papers 'New determination of the mechanical equivalent of heat' (1878) he repeated his 1849 experiments for the mechanical equivalent of heat, with his findings agreeing with his earlier experiments [the figures were later revised by others].
Joule was elected F.R.S. in 1850. He became a member of the Society's Council in 1857 and was recipient of its Gold Medal in 1852, and the Copley Medal in 1870. He was awarded of £200 civil list pension in 1878. Joule was an active member of the Manchester Literary & Philosophical Society; having first been elected in 1842, he served as the Society's librarian, honorary secretary, and vice-president, and was twice president (1860 and 1868).
Joule married Amelia Grimes in 1847, the daughter of the Liverpool Comptroller of Customs; they had a son and daughter. His wife died following the birth of a second son (who also died) in 1854. Joule died on 11 October 1889 at his home in Wardle Road, Sale, Cheshire.
In 1948 the ninth general conference on weights and measures introduced a new set of scientific units known as the Joule.