Watt, James

Scottish engineer who developed the steam engine in the 1760s, making Thomas Newcomen's engine vastly more efficient by cooling steam in a condenser separate from the main cylinder. He eventually made a double-acting machine that supplied power with both directions of the piston and developed rotary motion. He also invented devices associated with the steam engine, artistic instruments and a copying process, and devised the horsepower as a description of an engine's rate of working. The modern unit of power, the watt, is named after him.

At Glasgow University, Watt was asked to repair a small working model of Newcomen's steam engine, which was unpredictable and difficult to operate without air entering the cylinder and destroying the vacuum. It was also extremely costly to run in terms of the coal required to keep a large enough head of steam in a practical engine. Newcomen's engine was impractical for widespread use in the new mills that were beginning to appear in the early years of the Industrial Revolution; although the concept was correct, it needed further development for commercial use.

In Newcomen's engine, the steam in the cylinder was condensed by a jet of water, creating a vacuum. The vacuum, in turn, was filled during the power stroke by the atmosphere pressing the piston to the bottom of the cylinder. On each stroke the cylinder was heated by the steam and cooled by the injected water, thus absorbing a tremendous amount of heat. Watt investigated the properties of steam and made measurements of boilers and pistons. He had the idea of a separate condenser (separate from the piston) that would allow the cylinder to be kept hot, and the condenser fairly cold by lagging (covering with an insulating material), thus improving the thermal efficiency.

Working with manufacturer Matthew Boulton in 1782, Watt improved his machine by making it double-acting. Using a mechanical linkage known as ‘parallel motion’ and an extra set of valves, the engine was made to drive on both the forward and backward strokes of the piston, and a ‘sun-and-planet’ gear (also devised by Watt in 1781) allowed rotary motion to be produced. Watt's engine could now drive a machine with a forward and backwards, up and down, and circular motion. This new and highly adaptable engine was quickly adopted by cotton and woollen mills.

During the period 1775–90, Watt developed the automatic centrifugal governor (controller), which cut off the steam when the engine began to work too quickly and turned it on again when it had slowed sufficiently to a safe speed. He also created a steam engine indicator that showed steam pressure and the degree of vacuum within the cylinder. Because of the secretarial duties connected with his business, Watt developed a way of copying letters and drawings with a chemical process that was replaced only with the invention of the typewriter and photocopier.

Watt devised a mathematically rational method to rate the capability of his engines by considering the rate at which horses worked. After many experiments, he concluded that a ‘horsepower’ was 33,000 lb/15,000 kg raised through 1 ft/0.3 m each minute.