Born Dec. 24, 1818, in Salford, Lancashire; died Oct. 11, 1889, in Sale, Cheshire. English physicist; member of the London Royal Society (beginning in 1850). Owner of a brewery near Manchester.

Joule made important contributions to the study of electromagnetism and thermal phenomena, to the establishment of low-temperature physics, and to the justification of the law of conservation of energy. He established (in 1841; published in 1843) that the amount of heat dissipated in a metal conductor carrying an electric current is proportional to the electrical resistance of the conductor and to the square of the current.

In 1843-50 Joule proved by experiments that heat can be produced by mechanical work and defined the mechanical equivalent of heat; this work furnished an experimental foundation for the law of conservation of energy. In 1851, regarding heat as the motion of particles, he determined theoretically the heat capacity of some gases. Working with W. Thomson (Lord Kelvin), Joule found by experiment that the temperature of a gas changes upon slow, stationary adiabatic flow through a porous barrier (the Joule-Thomson effect). He also discovered the phenomenon of magnetic saturation, which occurs during the magnetization of ferromagnetic materials.