A brewer with a paddle wheel and a thermometer proved energy is never lost
James Prescott Joule spent years lowering a weight to spin a paddle wheel inside an insulated tank of water, then measuring the tiny rise in temperature it produced. His painstaking experiments showed that mechanical work converts into heat at a fixed, measurable rate — evidence that became a cornerstone of the law of conservation of energy, and eventually earned him a unit of energy named after him.