Joule's heat-equivalence experiment. 19th-century illustration of British physicist James Joule (1818-1889) measuring the heating in wires resulting from the passage of an electric current. Joule was interested in the physical properties of heat and, in 1840, determined that the heat generated in a wire by an electrical current was proportional to the resistance and the square of the current (Joule's Law). In 1843 he determined the amount of mechanical work required to produce a given amount of heat. This work established the equivalence of heat and other forms of energy, and the principle of conservation of energy: energy is neither created nor destroyed. This illustration is from 'Physique Populaire' (Emile Desbeaux, 1891).