James Prescott Joule (24 December 1818 – 11 October 1889) was an English physicist, born in Salford, near Manchester. In his time he had great contribution to the world of electricity and thermodynamics. He was best known for discovering Joule's law, which described electric heating by saying the amount of heat produced each second in a conductor by a current of Electricity is proportional to the resistance of a conductor and to the square of the current. The unit for this is joule, equal to one watt-second. Later Joule worked with William Thomson to find out that the temperature of gas falls, as gas expands. This principle was then known as the Joule-Thomson effect.