Joule, James Prescott (1818-1889), a British physicist. He proved that heat is a form of energy, and he made the first accurate measurement of the amount of mechanical energy needed to produce a given amount of heat. The measurement of this value, called the mechanical equivalent of heat, was fundamental to the formulation of the principle of conservation of energy. Joule was unaware of similar investigations by Julius von Mayer.

Joule was born near Manchester, England. He studied the physical sciences under private tutors, including John Dalton. Joule's earliest research was in electromag-netism. He discovered the law (now known as Joule's Law) that describes the rate of heating in electrical circuits. After 1847 many of Joule's investigations were made in collaboration with Sir William Thomson (Lord Kelvin). Their studies of the properties of gases led to improvements in the design of steam engines.