Joule, James Prescott [key], 1818–89, English physicist. His scientific researches began in his youth when he invented an electromagnetic engine. Joule made valuable contributions to the fields of heat, electricity, and thermodynamics. His work established the mechanical theory of heat, and he was the first to determine the relationship between heat energy and mechanical energy (the mechanical equivalent of heat). Joule discovered the first law of thermodynamics, which is a form of the law of conservation of energy (see conservation laws). He was one of the great experimental scientists of the 19th cent. The mechanical unit of work is named for him.
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