Joule, James Prescott (24.12.1818–11.10.1889) was an English physicist, one of the discoverers of the law of conservation of energy, and a Fellow of the Royal Society (1850). Born in Salford. He was educated at home. His first lessons in physics were given to him by J. Dalton, under whose influence Joule began his experimental research. His work concerned electromagnetism, heat and the kinetic theory of gases. In 1841 he established how the quantity of heat released in a conductor carrying an electric current depends on the magnitude of the current and on the resistance of the conductor (the Joule–Lenz law). In 1843 he showed experimentally that heat can be obtained from mechanical work and calculated the mechanical equivalent of heat, thereby giving experimental proof of the law of conservation of energy. He studied thermal phenomena in the compression and expansion of a gas and in particular, by an experiment on the expansion of a rarefied gas, showed that the internal energy of an ideal gas does not depend on its volume (1845). Together with W. Thomson he discovered in 1853–54 the cooling of a gas as it flows slowly and steadily under adiabatic conditions through a porous plug (the Joule–Thomson effect). He constructed a thermodynamic temperature scale and determined theoretically the heat capacity of certain gases. He regarded heat as the motion of particles. He calculated the speed of the molecules of a gas and established its dependence on temperature, and held the pressure of a gas to be the result of the impacts of its particles on the walls of the vessel (1848). He discovered magnetic saturation in ferromagnets (1840) and magnetostriction (1842).