James Joule. Portrait of James Prescott Joule (1818-89), the British physicist who discovered the mechanical equivalent of heat. In 1840 Joule described the Joule effect, stating that the heat generated in a wire by an electrical current was proportional to the resistance and the square of the current. He made a series of accurate measure- ments to show that a given quantity of work, from whatever source, always produced a particular quantity of heat. From this he deduced that energy was never destroyed, but was always conserved in a different form. He also showed that the tempera- ture of a gas falls as it expands, even when it does no external work. Portrait of Joule in 1863.