Which scientist demonstrated the mechanical equivalent of heat?

James Prescott Joule demonstrated the mechanical equivalent of heat. His experiments showed that mechanical work and heat are interchangeable forms of energy.

In his best-known experiment, published in the 1840s, falling weights turned paddle wheels inside an insulated container of water. The mechanical work done by the weights warmed the water slightly. By comparing the work performed with the temperature increase, Joule estimated how much mechanical energy corresponded to a unit of heat.

Joule’s results challenged the older caloric theory, which treated heat as an indestructible fluid. They helped establish conservation of energy and contributed to the first law of thermodynamics. The SI unit of energy, the joule, is named in his honour.

Julius Robert von Mayer independently proposed a similar relationship in 1842, so Joule was not the only scientist associated with the idea. However, Joule’s careful experimental demonstrations made him the canonical answer to this question.