Watt was born in Greenock on 19 January 1736, the son of a chandler and joiner. Throughout his life he suffered from serious attacks of migraine, and at school both his peers and his teachers took a poor view of this ‘weakness’. His great delight was to work in his father's workshop, where a corner had been set aside for him with his own forge and workbench. Soon he developed great skill, and he wished to become an instrument-maker. In his attempt to find an apprenticeship in this trade, he went first to Glasgow, where he worked with an optician and odd-job man for a year. Then, on advice from a friend, he went to London. Eventually, he secured a position with very unfavourable conditions. He did, however, learn the skills of instrument-making before illness forced him to return home to Greenock. After recovering, he set up in business as an instrument-maker in Glasgow and in 1757 obtained work from Glasgow University that allowed him to work in a room within its precincts, and he proudly described himself as ‘Instrument Maker to Glasgow University’.

During this period he was asked to repair a small working model of Newcomen's steam engine. The machine proved to be temperamental and difficult to operate without air entering the cylinder and destroying the vacuum. He set about investigating the properties of steam and making measurements of boilers and pistons in the hope of improving Newcomen's machine, which was, at best, slow, temperamental, inefficient, and extremely costly to run in terms of the coal required to keep a sufficient head of steam in a practical engine. During a short period of inspiration, in the course of a Sunday afternoon walk, he had the idea of a separate condenser (separate from the piston). In Newcomen's engine, the steam in the cylinder was condensed by a jet of water, thus creating a vacuum that, 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. With his separate cylinder, Watt could keep the cylinder hot, and the condenser fairly cold by lagging, thus improving the thermal efficiency of the machine and the economics of its operation. Watt's original engine of 1765 is now in the Science Museum, London. It was only a working model, and reveals the haste in which it was built.

As far as practical engines were concerned, Watt had a great deal of trouble in efficiently lagging the cylinder so that heat was retained and at the same time allowing the piston to move freely. He was helped with facilities and labour by John Roebuck of Kinneil, who eventually employed Watt's engine to pump water from his mines. In 1767 Watt again travelled to England, this time to patent his engine (patent granted in 1769). On his way back to Scotland he visited some friends of Roebuck in Birmingham and met Matthew Boulton. Boulton was a major manufacturer in Birmingham and had the finance to exploit Watt's engine. Because of the patent arrangements between Watt and Roebuck it was not until the latter got into severe financial difficulties that Boulton could buy him out and begin manufacturing the engine. In fact, between 1767 and 1774, Watt made his living as a canal surveyor. Although he was successful at this, his health was not up to an outside job in harsh weather and he suffered accordingly.

From 1775, financial difficulties being solved, Boulton and Watt went into partnership and manufactured Watt's engines at the famous Soho Foundry, near Birmingham. In 1782 Watt improved his machine by making it double-acting. By means of 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) allowed rotatory motion to be produced. This new and highly adaptable engine was quickly adopted by cotton and woollen mills. A universally practical means of producing power for the evolving British industry was therefore at hand, with the consequent rapid rise in the adoption of larger machines.

During this same period, 1775–90, Watt developed the automatic centrifugal governor, which cut off the steam when the engine began to work too quickly and turned it on again when it had slowed sufficiently. He also devised a steam engine indicator, which showed steam pressure and the degree of vacuum within the cylinder. Because of the secretarial duties connected with the business, Watt invented a way of copying letters and drawings; this was a chemical process and was displaced only with the advent of the typewriter and photocopier. Although his steam engines were usually built for specific purposes and individually priced, it was important to have a rational method upon which charges could be made. For this he considered the rate at which horses worked and, after many experiments, concluded that a ‘horsepower’ was 15,000 kg/33,000 lb raised through 0.3 m/1 ft each minute. He rated his engines in horsepower and in the English-speaking world this method of describing the capability of an engine continued until recent years.

In 1785 Watt was elected a fellow of the Royal Society. During the last decade of the 18th century the active management of the Soho Works was taken over increasingly by Boulton and Watt's sons and in 1800, when the patent rights to the engine expired, Watt retired. He then kept an attic workshop and busied himself designing and constructing copying machines.

Watt died on 25 August 1819, aged 83, leaving the legacy of highly useful machine power for the development and proliferation of industry. His name has become immortalized as the unit of power; a watt is one joule per second, and one horsepower is equivalent to about 746 watts.

Categories: biography; technology and manufacturing

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