James Prescott Joule (December 24, 1818 - October 11, 1889) was an English physicist, born in Salford, near Manchester, England.

Joule studied the nature of heat, and discovered its relationship to mechanical work (see energy). This led to the theory of conservation of energy (the First Law of Thermodynamics). The SI unit of work, the joule, is named after him. He worked with Lord Kelvin to develop the absolute scale of temperature, made observations on magnetostriction, and found the relationship between the flow of current through a resistance and the heat dissipated, now called Joule's law.

The son of a wealthy brewer, Joule was tutored as a young man by the famous scientist John Dalton. Fascinated by electricity, he and his brother experimented by giving electric shocks to each other and to the family's servants. Joule ran the brewery as an adult, and science was merely a serious hobby. His work on energy can be traced to his attempt to build an electric motor that would replace steam engines.

Joule's ideas about energy were not accepted at first, partly because they depended on extremely precise measurements, which had not previously been common in physics. His best-known experiment involved the use of a falling weight to spin a paddlewheel in an insulated barrel of water, whose increased temperature he measured. He claimed to be able to measure temperatures to an accuracy of 1/200 of a degree Fahrenheit, which his contemporaries did not believe possible. Joule's experiments complemented the theoretical work of Clausius, who is considered by some to be the coinventor of the energy concept.

Further resistance came because Joule's work contradicted the widespread belief that heat was a fluid, the "caloric." That theory stated that caloric could be neither destroyed nor created, whereas Joule claimed that heat was only one of many forms of energy, and only the sum of all the forms was conserved. Although it may be hard today to understand the allure of the caloric theory, at the time it seemed to have some clear advantages. Joule was proposing a kinetic theory of heat (he believed it to be a form of rotational, rather then translational, kinetic energy), and this required a conceptual leap: if heat was a form of molecular motion, why didn't the motion of the molecules gradually die out? Joule's ideas required one to believe that the collisions of molecules were perfectly elastic. We should also remember that the very existence of atoms and molecules was not widely accepted for another hundred years. Carnot's successful theory of heat engines had also been based on the caloric assumption, and only later was it proved by Lord Kelvin that Carnot\'s mathematics were equally valid without assuming a caloric fluid.

Although the discovery of energy conservation was one of the keystones of the new science of thermodynamics, Joule and his contemporaries did not initially understand that thermodynamic processes could be irreversible. Instead, they interpreted energy conservation as implying a kind of static universe, in which the same processes could be repeated indefinitely by recycling the same energy. This view was only later shown to be invalid with the discovery of the second law of thermodynamics and the concept of entropy.

Joule, like Dalton with whom he worked, is buried in Westminster Abbey.