1-155-821 - Oxford Science Archive/Heritage Images

Joule's apparatus for determining the mechanical equivalent of heat, 1872. A vessel of water, oil or mercury encloses vanes attached to a spindle. Cord is wound round the cylinder and drum. The weight descending against the scale rotates the spindle and vanes. Raising and lowering the weight raises the temperature of the fluid. From the rise in temperature and the distance travelled, the energy used can be calculated. British physicist James Prescott Joule (1818-1889) determined the mechanical equivalent of heat. The unit of energy called the joule is named after him.