Joule, James Prescott (1818–1889)

English physicist. His work on the relations between electrical, mechanical, and chemical effects led to the discovery of the first law of thermodynamics.

He determined the mechanical equivalent of heat (Joule's equivalent) in 1843, and the SI unit of energy, the joule, is named after him. He also discovered Joule's law, which defines the relation between heat and electricity; and, with Irish physicist Lord Kelvin in 1852, the Joule–Kelvin (or Joule–Thomson) effect.

Joule was born in Salford, Lancashire, and educated by private tutors, including scientist John Dalton. Having a private income, he dedicated his life to precise scientific research. Until neighbours protested, he kept a steam engine in his house in Manchester.

Joule showed experimentally that the ratio of equivalence of the different forms of energy did not depend on how one form was converted into another or on the materials involved.