James Watt

James Watt trained as an instrument maker in Glasgow, where he was asked to repair a model Newcomen engine and saw how much heat it wasted. His answer, a separate condenser patented in 1769, cut fuel use so sharply that steam power became practical far beyond pumping water out of mines. In partnership with the manufacturer Matthew Boulton he built and sold engines across Britain, and his rotative engine let steam turn the machinery of mills and factories directly. He was a member of the Lunar Society, a Birmingham circle of industrialists and natural philosophers that included Josiah Wedgwood and Erasmus Darwin. The unit of power, the watt, is named after him.