A repair job
In the early 1760s, James Watt was an instrument maker at the University of Glasgow when he was asked to repair a small working model of a Newcomen engine, a type now called an atmospheric engine, used for teaching. The model was small, but it used a surprising amount of steam. Watt began to study why. He measured the properties of steam and water and found that much of the steam in each cycle was spent warming up the cylinder walls, which had been cooled by the cold water spray in the previous stroke.
The problem was built into the Newcomen design. To get a vacuum, the cylinder had to be cooled on every stroke, and to fill it with steam again it had to be reheated. A large engine wasted a large amount of heat in this cycle, and the extra fuel was its main running cost.
The separate condenser
Watt’s idea, which he later said came to him in 1765 while walking on Glasgow Green, was to condense the steam in a separate chamber connected to the cylinder by a valve. The cylinder could then stay hot all the time, and the condenser could stay cold. When the valve was opened, steam rushed into the vacuum in the condenser and condensed there. An air pump removed the water and air from the condenser. Watt received a patent in 1769 for this and related ideas.
The principle is simple: do not cool the part that needs to be hot. Compared with a Newcomen engine of similar size, Watt’s engines used much less coal for the same amount of pumping, with early claims of about a quarter of the fuel. These figures came mainly from the firm itself, so they should be treated as approximate, and performance in daily use varied.
From experiment to business
Building a full-sized engine was harder than building a model. Cylinders needed to be bored accurately and pistons made to fit tightly, otherwise steam leaked. Watt worked first with John Roebuck, a Scottish industrialist, and later with Matthew Boulton, a Birmingham manufacturer. The partnership, formed in 1775 after Parliament extended Watt’s patent to 1800, brought money, workshop skill and customers. The first Boulton and Watt engines were installed in the 1770s, including at an ironworks and at coal and metal mines.
John Wilkinson’s cylinder-boring machine, designed for cannon, gave the accuracy needed for engine cylinders. This shows that the success of the engine depended on advances in metalworking as much as in thermodynamic ideas.
How the engines were sold
Boulton and Watt did not just sell machines. In many cases they supplied designs and key parts, and the customer built the rest locally. They charged an annual premium equal to a third of the value of the coal saved compared with a Newcomen engine of the same output. This arrangement worked well in Cornwall, where coal was expensive and savings were significant. It was less attractive where coal was cheap, which is why some collieries kept using Newcomen-type engines for years.
Beyond pumping
Pumping engines move in a straight line, and many factory machines need rotation. Watt and his colleagues, including William Murdoch, worked in the early 1780s to produce rotary engines. They added a double-acting cylinder so that steam pushed in both directions, and a way of turning the beam’s rocking into rotation. A governor, using spinning weights that moved with speed, helped keep the speed steady. Rotary engines were soon installed in cotton mills, breweries and ironworks, where they drove lines of shafts to power machines. The first mill engines had much in common with the pumping engines, and the cotton mills of the 1780s and 1790s were among the earliest users of the new type.
What the condenser did not change
Watt’s engines still ran at low pressure, using steam at little more than atmospheric pressure. They were big, heavy and stationary, so they were not ideal for ships or road vehicles. The higher-pressure engines of Trevithick and Evans addressed this weakness, and some of their designs got rid of the condenser entirely. The condenser has not disappeared, though; modern steam turbines in power stations use condensers to improve efficiency, which shows how durable Watt’s idea has been.
Credit and context
Watt is sometimes called the inventor of the steam engine, but that is not accurate. Savery and Newcomen had built working machines decades before him, and others worked on engines in the same years. Watt’s most important contribution was the improved design and the engineering and business work needed to make it practical. For many mines and mills of the late 1700s, his engines were the first affordable way to put steam power to work at scale.