Sir James Prescott Joule (1818-1889) became one of the most significant physicists of the nineteenth century, although his original interest in science was as a hobby and for practical business purposes. The son of a brewer, he began studying heat while investigating how to increase the efficiency of electric motors. His discovery of the relationship between heat and energy contributed to the discovery of the conservation of energy and the first law of thermodynamics. Volume 2 of his collected papers, published in 1887, contains those which he co-authored with other noted physicists, such as Scoresby, Playfair and William Thomson, later Lord Kelvin. Because he was based in Manchester, and was not an academic, Joule's work was at first ignored by the scientific establishment, but Thomson's approval helped him gain acceptance. His joint work with Thomson on thermodynamics was fundamental to the development of significant areas of twentieth-century physics.