Diagram of the Watt Steam Engine in its most basic form showing the improvement of the separate condenser, which was not found on the Newcomen steam engine.

The Watt steam engine was the next great step in the development of the steam engine after the Newcomen engine. A Scottish instrument maker, James Watt arranged a separate vessel for condensing the steam. This condenser, C, was connected with the cylinder through a valve V'. When the piston had reached the top of the cylinder, the valve V was closed and V' was opened. Then the steam rushed from the cylinder into the condenser, which was kept cold and under less than atmospheric pressure. At first these valves V and V' had to be operated by hand, but later, it is said, a boy named Potter, whose job it was to turn these valves, connected the valve handles by cords to the beam ED in such a way that the machine became automatic.

In its more advanced form, the improvements over the Newcomen engine were:

• the separate condenser, which saved fuel by making better use of energy (75% fuel saving)
• the double acting engine, which alternately acted on each end of the piston
• the sun and planet gear, which translanted the reciprocating motion into a rotary motion.

The oldest working Watt engine in the world is the Smethwick Engine, at Millennium Point in Birmingham (formerly at Birmingham Museum of Science & Industry).

See also

• Boulton and Watt