A brewer's cost question grew into the law of conservation of energy
Around 1840, James Joule was running his family's brewery in Salford and wondering whether new electric motors could replace its steam engines. He did the sums: a pound of coal burned under a boiler beat a pound of expensive zinc eaten up by a battery. Then the deeper puzzle took over.
To compare the two machines fairly, Joule measured their output in foot-pounds, the effort of lifting one pound through one foot. That habit of turning everything into a common currency led him to ask how much work any source could yield. In 1841 he found that the heat from an electric current rises with the square of the current times the resistance. By 1843 he had shown this heat was generated inside the wire rather than shuffled from elsewhere, a direct blow to caloric theory, the view since Lavoisier's day that heat was a fluid that could never be made or destroyed.
When he announced a mechanical equivalent of heat at the British Association meeting in Cork that August, the room went quiet. So he tried other routes. Pushing water through a perforated cylinder gave 770 foot-pounds per British thermal unit; compressing gas gave 798. The Royal Society turned down his 1844 paper. His most famous rig, shown in 1845, used a falling weight to spin a paddle wheel inside an insulated barrel of water, and by 1850 he had refined the figure to 772.692. Critics doubted he could read temperatures to 1/200 of a degree Fahrenheit, overlooking the precision brewing had taught him.
Support crept in. Helmholtz's 1847 statement of energy conservation credited both Joule and Julius Robert von Mayer. At an Oxford meeting that year, a young William Thomson, later Lord Kelvin, listened with interest and doubt. They met again by chance in Chamonix, where Joule was on honeymoon after marrying Amelia Grimes, and the pair plotted to check whether water at the foot of a waterfall ran warmer than at its lip, an attempt that proved impractical.
Thomson slowly came round. From 1852 to 1856 they worked mostly by letter, Joule running experiments and Thomson analysing them, and found the Joule–Thomson effect. As a boy Joule had been taught by John Dalton; today the SI unit of energy bears his name.
Source: James Prescott Joule
