Joule, jool or jowl, James Prescott (1818-1889), a British physicist, helped prove the law of conservation of energy. This law states that energy is neither created nor destroyed when it changes from one form to another. Joule's experiments demonstrated that energy seemingly lost is actually converted into heat. His results supported the theories of German physicists Hermann von Helmholtz and Julius Robert von Mayer, and British physicist Lord Kelvin.