Watt

James Watt was born in 1736 in Greenock, on the Firth of Clyde, the son of a carpenter and merchant. He apprenticed as an instrument-maker in London and in 1757 was appointed instrument-maker to the University of Glasgow, where he worked near—and eventually with—some of the leading natural philosophers of the Scottish Enlightenment, including Joseph Black, who discovered latent heat. In 1763 or 1764, Watt was given a model Newcomen steam engine to repair. Thomas Newcomen had invented the atmospheric steam engine in 1712; by Watt's day it was fifty years old, well-established in mines for pumping water, and notoriously inefficient.

Watt's analysis of the Newcomen engine identified its central inefficiency: the cylinder was alternately heated and cooled with each stroke, wasting enormous amounts of energy. His solution, developed over years of experiments in 1765–1766, was the separate condenser—a separate chamber where steam was condensed without cooling the main cylinder. This single innovation reduced fuel consumption by 75 percent. Watt patented the separate condenser in 1769. In partnership with Birmingham manufacturer Matthew Boulton from 1775, Watt spent another decade refining his engine, adding rotary motion (converting the up-down pumping stroke into the rotational motion that could drive mills and machinery), a centrifugal governor for speed regulation, and the double-acting engine.

To sell his engines, Watt needed to explain their power to mill owners accustomed to thinking in terms of horses. He conducted trials to measure how much work a horse could do—how much weight a 'mill horse' could lift, per minute, continuously—and defined this as one horsepower (approximately 746 watts by modern measure). He then rated his engines in horsepower, allowing customers to calculate how many horses they were replacing. This unit—horsepower—was itself an eponym of sorts: the unnamed mill horse whose performance Watt measured became the benchmark for all subsequent mechanical power.

The watt—one joule of energy per second—was named in Watt's honor by the Second Congress of the British Association for the Advancement of Science in 1882, one year after the First International Electrical Congress named the ampere and volt. The unit was adopted into the SI system in 1960. Watt's engines drove the textile mills, iron foundries, railways, and steamships of the Industrial Revolution—every system of mechanical power for two centuries. The lightbulb you read by, the phone you charge, the panel on your roof converting sunlight to electricity: all measured in the unit named for the Scottish instrument-maker who noticed that a cylinder should not have to cool down every time it worked.