Joule, James Prescott (1818–1889), British physicist. Joule is famous for his experiments on heat. In a series of investigations he demonstrated that heat is a form of energy (regardless of the substance that is heated) and later determined quantitatively the amount of mechanical and electrical energy expended in the generation of heat energy. In 1843 he published his value for the amount of work required to produce a unit of heat, now known as the mechanical equivalent of heat. He established the principle that various forms of energy—mechanical, electrical, and heat—are basically the same and are interchangeable. This principle forms the basis of conservation of energy. The joule unit of work or energy was named in his honor in 1882.