James Watt

James Watt was working as an instrument maker at the University of Glasgow when he grew interested in steam engines, then clumsy devices that wasted energy by heating and cooling the same cylinder over and over. His insight - the separate condenser - changed everything: suddenly steam engines were powerful, efficient, and cheap enough to move the world. He struggled to commercialize the design until partnering with Matthew Boulton in 1775; their firm made both men wealthy. Later Watt adapted the engine to produce rotary motion, expanding its use far beyond pumping water. In retirement he kept inventing, though nothing matched the steam work. He also developed the concept of horsepower, and today the SI unit of power carries his name.