Joule was the son of a very successful brewer in Manchester, England. He was educated privately and with ambitions to spend his life in science. His first papers dealt with “electromagnetic engines,’’ and in the course of his experiments he focused on what we now call Joulean heating in wires carrying electric currents. This drew attention to the production of heat in both physical and chemical processes. From his experiments and a developing theory, he concluded that an electric generator enables one to convert mechanical power into heat through the electric currents induced into it. Joule also implied that an electromagnetic ‘‘engine’’ producing mechanical power would also convert heat into mechanical power. By 1843, he presented his first paper to the British Association for the Advancement of Science (BAAS) on the “mechanical value of heat.’’ Joule believed that only the Creator could destroy or create and that the great agents of nature, such as electricity, magnetism, and heat, were indestructible yet transformable into each other. As his experiments progressed, he elaborated his mechanical theory of nature. Important to observers, and to Thomson, was Joule’s idea that mechanical work and electricity could be converted into heat and that the ratio between mechanical work and heat could be measured. In a series of papers on the mechanical equivalent of heat presented annually to BAAS and politely received, Joule labored alone until the stark reactions of Thomson in 1847. Some scientists could not believe Joule’s contentions because they were based on such small temperature changes. Presumably, their thermometers were not that accurate. However, Thomson understood that Joule was challenging caloric theory head on. Until 1850, Thomson continued publishing papers within a caloric framework and an account of Carnot’s theory. He had finally located a copy of the treatise. Clearly, he began to doubt its validity, noting that more work on the experimental foundations of the theory were ‘‘urgent,’’ especially claims that heat was a substance. It took Joule 3 years to convince Thomson of the correctness of his insight. Only in 1850 did Thomson concede that heat was convertible into work after he read Rudoph Julius Emmanuel Clausius’s papers and those of William McQuorne Rankine on heat. He then reinterpreted some parts of his caloric papers that depended on heat as a substance in terms of the interconversion of heat and work. Also, he rederived and reinterpreted his expression for the maximum amount of work that could be extracted from the cycle of a perfect heat engine and other results and then gave his version of the second law of thermodynamics.