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 1 of Joule's scientific papers was published in 1884. It is organised chronologically and reveals the range of Joule's interests and the development of his thought. The topics of the papers include the measurement of heat, voltaic batteries, electromagnets, specific heat, meteorology and thermodynamics. Joule's careful experiments in these areas were fundamental to the development of significant areas of twentieth-century physics, although he was slow to gain recognition from his contemporaries.