Watt's steam engine

The story

A little model to repair

James Watt was born in 1736 in Greenock, in Scotland, into a family of shipbuilders and merchants. He was a boy in poor health who spent a lot of time at home, and he taught himself geometry and mechanics. As an adult he moved to Glasgow where, since he could not open a shop because he did not belong to the town's guilds, he was taken on by the university as a maker of scientific instruments. Here he met professors such as Joseph Black, the chemist who had studied heat, and who became his friend and adviser.

In the winter of 1763–64 the university asked him to repair a model of a Newcomen engine, used for physics lessons. Watt put it right, but noticed something odd: the little model used up a huge amount of steam, far more than seemed necessary. Why? He studied the problem very carefully, measuring and running tests.

The idea of the condenser

He realised that the fault lay in the way the engine worked. In Newcomen's engines the steam entered the cylinder and then, to create a vacuum, was cooled with a spray of cold water; with every stroke the cylinder had to be heated up again, and this used a great deal of steam and coal. It was as if, to go on a car journey, you had to warm up a cold engine every single time.

One spring day in 1765, while he was walking on Glasgow Green, a large park in the city, the idea came to him: why not condense the steam in a separate container, kept cold all the time, and leave the cylinder hot all the time? The steam would pass from the cylinder into the condenser, where it would turn into water and create the vacuum, without cooling the cylinder. It was a simple and brilliant solution.

From idea to machine

Having the idea was one thing, building an engine that worked was another: the details took years of work and failures. An entrepreneur, John Roebuck, paid for the first attempts, and in January 1769 Watt obtained a patent for his “new method of lessening the consumption of steam and fuel in fire-engines”. But Roebuck went bankrupt, and Watt had to find another partner.

He found one in 1775 in Matthew Boulton, an industrialist from Birmingham who owned a big factory, the Soho Manufactory. Boulton believed in the project and put in money, workshops and workers, and together they founded “Boulton & Watt”. Another decisive contribution came from John Wilkinson, who had invented a machine for boring cannons with precision: he could make perfectly smooth cylinders, which were essential so that the steam did not escape.

Watt's first engine for a customer, in 1776, pumped water out of a coal mine. It used less than a third of the coal of a Newcomen engine, and it was a success. To get paid, Boulton and Watt did not sell the engine, but charged customers a third of what they saved on coal.

From the pump to the factory

Watt did not stop there. The first engines were only for pumping water, with an up-and-down movement. But factory owners also wanted to turn wheels and machinery. With some ingenious devices, such as the gears known as “sun and planet” (1781), he turned the back-and-forth movement into a rotating one, and made the engine “double-acting”, that is, able to push the piston in both directions. He invented a governor with spinning balls, which kept the speed steady automatically (1788).

So steam engines could drive spinning machines, looms, mills, hammers and pumps, even in places far from rivers and waterfalls. By 1800 Boulton & Watt had built about five hundred. To tell customers how much power an engine had, Watt invented a unit of measurement: the “horsepower”, defined by comparing its strength with that of the horses working in the mines.