James Prescott Joule’s paddle-wheel experiment established the equivalence of heat and mechanical work.
In experiments published during the 1840s, Joule allowed falling weights to turn paddles inside an insulated container of water. The paddles stirred the water, and the water’s temperature rose slightly. By comparing the mechanical energy lost by the weights with the heat gained by the water, he estimated the mechanical equivalent of heat.
Joule’s results supported the idea that heat is a form of energy rather than an indestructible fluid. His best-known value was close to 4.2 joules per calorie, although later measurements refined the conversion.
The experiment helped establish conservation of energy and contributed to the first law of thermodynamics. It did not show that heat and work are identical physical processes; rather, it showed that energy transferred as work can appear as thermal energy in a consistent quantitative relationship.