James Prescott Joule

Formulated the principle of conservation of energy and discovered the relationship between heat and mechanical work.

James Prescott Joule (1818–1889) was a British physicist whose experiments established the first law of thermodynamics, demonstrating that energy cannot be created or destroyed. Born into a brewing family in Salford, England, Joule was largely self-taught. His meticulous experiments in the 1840s measured the mechanical equivalent of heat, showing that energy could be converted from one form to another. Collaborating with Lord Kelvin, he also developed the Joule-Thomson effect, critical in refrigeration.

Joule's work debunked the caloric theory and unified the fields of thermodynamics and electromagnetism. His famous paddle-wheel experiment, which used falling weights to heat water, proved that mechanical work could generate heat. The SI unit of energy, the joule (J), is named after him. Joule’s laws of electric heating also advanced electrical engineering.

Despite initial skepticism, Joule’s ideas gained acceptance through the advocacy of William Thomson (Lord Kelvin). His principles underpin modern energy systems, from power plants to renewable technologies. Joule’s legacy as a pioneer of energy conservation remains central to physics and environmental science.