The establishment of the energy concept in the mid-19th century was possibly the greatest advance in physical science since Newton. Probably the most important contributor to this step forward was James Prescott Joule, son of a Manchester brewing family, who died over 100 years ago, just as the first modern power station was coming on stream. This biography deals with the sources of Joule's ideas and his clarification and proof of the dynamical theory of heat and the conservation of energy. It includes accounts of his discovery of the laws of electrical energy, his contribution to the establishment of the Joule-Thomson effect and to the determination of the universally accepted electrical units. The economic, social and technological implications of Joule's work are fully discussed. The book is based on the author's original papers on Joule and on some 600 letters between Joule and his contemporaries.