Joule determines the mechanical equivalent of heat (1843)

James Prescott Joule experimentally determined the mechanical equivalent of heat in 1843, demonstrating that work converts to heat. His findings, published in the Philosophical Magazine, were presented to the British Association in Cork, where they initially received little attention. This work established the equivalence of mechanical work and heat, contributing to the conservation of energy.

Episodes

Background and detail

Background. Joule, an English physicist, studied the relationship between heat and mechanical work. His experiments on energy transformations were first published in 1843. The concept that heat and work are equivalent was proposed independently by Joule in 1843 in the leading British physics journal, following earlier work by Mayer in 1842. Joule's work was crucial for the development of thermodynamics.

Experimental Method. Joule conducted experiments showing that heating effects were due to heat generation in conductors, not transfer. He estimated the mechanical equivalent of heat as 4.1868 joules per calorie. His results from electrical and purely mechanical means agreed to at least two significant digits, providing compelling evidence for the convertibility of work into heat.

Publication and Reception. Joule announced his results at the British Association for the Advancement of Science in Cork in August 1843, where he was met by silence. His paper was read to the Royal Society in June 1844 but rejected for publication. He subsequently published in the Philosophical Magazine in 1845, which helped establish the scientific acceptance of his findings.

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