James Prescott Joule FRS (1818 – 1889) was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature ofheat, and discovered its relationship to mechanical work (see energy). This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The SI derived unit of energy, the joule, is named after him. He worked with Lord Kelvin to develop the absolute scale of temperature, made observations onmagnetostriction, and found the relationship between the current through a resistance and the heat dissipated, now called Joule's law.