Portait of James Prescott Joule (1818-89), English physicist, the discoverer of the mechanical equivalent of heat. Joule was born at Salford, the second son of a wealthy brewer. Having suffered a spinal injury early in life, he was allowed to study rather than work in his father's company. By and large, Joule was self-taught, and he became devoted to accurate measurements of heat. In 1847 he published his first paper, showing that a given quantity of work always produced a particular quantity of heat. He was also the first to propose the principle of conservation of energy, and that the temperature of a gas drops as the gas expands. The SI unit of work (Joule) is named after him.