With this apparatus, the Salford-based physicist James Joule discovered the 'mechanical equivalent of heat', quantifying how heat can be converted into mechanical work and vice versa. The apparatus used a system of paddles to stir water vigorously in the vessel. The resulting rise in water temperature was related to the mechanical work expended in moving the paddles, which was provided by falling weights. This led ultimately to the formulation of the first law of thermodynamics, concerning the conservation of energy. The standard unit of energy is now called the Joule.

Inv. 1876-492