The English physicist James Prescott Joule (1818-1889) proved that mechanical and thermal energies are interconvertible on a fixed basis, and thus he established the great principle of conservation of energy.

On Dec. 24, 1818, James Joule was born at Salford near Manchester, the second of the five children of a wealthy brewery owner. A rather frail boy, he received his early education at home. In 1839, in the laboratory in his home, he began his studies of electrical motor efficiency, which ultimately led to his development of the mechanical theory of heat. In connection with this work he became one of the first to realize the necessity for standard units in electricity and to advocate establishing them.

In the course of his efficiency experiments Joule made his first discovery--now known as Joule's law: the heating of a conductor depends upon its resistance and the square of the current passing through it...