Joule, James (Prescott)

(born Dec. 24, 1818, Salford, Lancashire, Eng.—died Oct. 11, 1889, Sale, Cheshire) English physicist.

After studying under John Dalton, in 1840 he described “Joule's law,” which stated that the heat produced in a wire by an electric current is proportional to the product of the resistance of the wire and the square of the current. In 1843 he published his value for the amount of work required to produce a unit of heat, called the mechanical equivalent of heat, and established that heat is a form of energy. He established that the various forms of energy are basically the same and can be changed from one into another, a discovery that formed the basis of the law of conservation of energy, the first law of thermodynamics. In his honour, the value of the mechanical equivalent of heat is usually represented by the letter &math.J;, and a standard unit of work is called the joule.