Calorimeter from Joules water friction apparatus, 1849.

UNITED KINGDOM - JULY 29: James Joule (1818-1889) proved that energy cannot be created or destroyed, it can only change in form. This idea is now known as the Conservation of Energy. Joule�s experiment of the 1840s consisted of a brass paddle wheel stirring water in a copper vessel. The paddle wheel was turned by falling weights possessing mechanical, or kinetic, energy as they fell. The temperature of the water rose by an amount that depended on how far the weights fell. This showed that the kinetic energy of the weights now existed as heat energy in the water. Joule's experiments also proved that heat is produced by motion, contradicting the caloric theory. The SI unit of energy, the Joule, was named after him. (Photo by SSPL/Getty Images)