James Prescott Joule (1818-1889), British physicist. In 1840 he 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 measurements 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 temperature of a gas falls as it expands, even when it does no external work. The International System (SI) unit of energy is named in his honour.