About Joule, the American Physical Society says in “December 1840: Joule’s abstract on converting mechanical power into heat” (This Month in History in the APS News, December 2009, Volume 18, Number 11)
Scientists in the early 19th century adhered to caloric theory, first proposed by Antoine Lavoisier in 1783 and further bolstered by the work of Sadi Carnot in 1824. The work of a brewer and amateur scientist on the nature of heat and its relationship to mechanical work would give rise to the first law of thermodynamics.
Born in 1818, James Prescott Joule came from a long line of brewers, so chemistry was in his blood –as was scientific experimentation. Described as “delicate” in contemporary accounts, he and his brother experimented with electricity by giving each other electric shocks, as well as experimenting on the servants. The two boys were tutored at home until 1834, when their father sent them to study under John Dalton, one of the leading chemists of that time, at the Manchester Literary and Philosophical Society. Two years later, Dalton suffered a stroke and was forced to retire from teaching. The Joule brothers’ education was entrusted to John Davies.
Eventually Joule took over as manager of the family brewery, but science remained an active hobby. Fascinated by the emerging field of thermodynamics, Joule jerry-rigged his own equipment at home–using salvaged materials–to conduct scientific experiments–initially to test the feasibility of replacing the brewery’s steam engines with the newfangled electric motor that had just been invented. He found that burning a pound of coal in a steam engine produced five times as much work (then known as “duty”) as a pound of zinc consumed in an early electric battery. His brewery was better off with the steam engines. His standard of “economical duty” was the ability to raise one pound by one foot (the “foot-pound”).
His first experiments focused on electromagnetism and he quickly showed a gift for experimental apparatus; he built his first electromagnetic engine at 19, as well as improved galvanometers for measuring electrical current. Thanks to Dalton’s influence, Joule was a rare subscriber to atomic theory, and sought to explain electricity and magnetism in terms of atoms wrapped by a “calorific ether in a state of vibration.”
This did not match his experimental results, however, and in December 1840, Joule published a short abstract in the Proceedings of the Royal Society suggesting that the heat generated in a wire conveying an electrical current results from the heat generated by the chemical reactions in a voltaic cell. In other words, heat is generated, not merely transferred from some other source in an electromagnetic engine. Based on this work, he formulated “Joule’s Law,” which states that the heat produced in a wire by an electric current is proportional to the product of the resistance of the wire and the square of the current.https://www.aps.org/publications/apsnews/200912/physicshistory.cfm
About the paddlewheel experiment, they say:
In his most famous experiment. Joule attached some weights to strings and pulleys and connected them to a paddle wheel inside an insulated container of water. Then he raised the weights to an appropriate height and slowly dropped them. As they fell, the paddle wheel began to turn, stirring up the water. This friction generated heat, and the temperature of the water began to increase.
It was the very precision of his measurements that caused some scientists to balk at accepting Joule’s findings. He claimed to be able to measure temperatures to within 1/200 of a degree Fahrenheit, which would have been astonishing to a 19th century scientist. Some historians have speculated that Joule’s experience in the art of brewing may have given him skills with experimental apparatus that his colleagues lacked. He also worked with John Benjamin Dancer, England’s finest instrument maker, to build highly accurate thermometers. Among those inclined to accept Joule’s work were Michael Faraday and William Thomson (Lord Kelvin), although they remained skeptical.https://www.aps.org/publications/apsnews/200912/physicshistory.cfm
In “Heat, work and subtle fluids: a commentary on Joule (1850) ‘On the mechanical equivalent of heat‘,” John Young writes:
James Joule played the major role in establishing the conservation of energy, or the first law of thermodynamics, as a universal, all-pervasive principle of physics. He was an experimentalist par excellence and his place in the development of thermodynamics is unarguable. This article discusses Joule’s life and scientific work culminating in the 1850 paper, where he presented his detailed measurements of the mechanical equivalent of heat using his famous paddle-wheel apparatus. Joule’s long series of experiments in the 1840s leading to his realisation that the conservation of energy was probably of universal validity is discussed in context with the work of other pioneers, notably Sadi Carnot, who effectively formulated the principle of the second law of thermodynamics a quarter of a century before the first law was accepted. The story of Joule’s work is a story of an uphill struggle against a critical scientific establishment unwilling to accept the mounting evidence until it was impossible to ignore. His difficulties in attracting funding and publishing in reputable journals despite the quality of his work will resonate with many young scientists and engineers of the present day. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.https://royalsocietypublishing.org/doi/10.1098/rsta.2014.0348