Boulton and Watt Rotative Engine (the Lap Engine)
Boulton and Watt, Soho, Birmingham · 1788
A tall wooden beam pivots on an iron column above a cylinder and a great spoked flywheel, all painted and polished, near the entrance of the Energy Hall. This is the Lap Engine, built by Boulton and Watt in 1788, and it is the oldest essentially unaltered rotative steam engine in the world. It worked for seventy years at Matthew Boulton's Soho Manufactory in Birmingham, where it drove forty three metal polishing machines, the laps that gave it its name, turning out buttons, buckles and coins. When it was finally retired in 1858 it came straight to the museum.
Almost every idea that made James Watt famous is built into it. The separate condenser, patented in 1769, let the cylinder stay hot and cut fuel use by two thirds. The parallel motion, a hinged linkage at the end of the beam that Watt was prouder of than anything else, keeps the piston rod straight. The sun and planet gear beside the flywheel turns the up and down motion into rotation, a device Watt used because a rival had patented the simple crank. Look also at the centrifugal governor above, two spinning brass balls that automatically throttle the steam when the engine runs too fast, the first widely used feedback control device.
Engines like this took steam power out of the mines and into the factories, and this particular machine was one of the models that Watt used to fix the standard of horsepower. It sits in the museum beside the earlier Old Bess engine and a series of later machines, so you can follow the whole story in one hall. A memorable detail is that the engine's original governor was among the first ever fitted, installed within months of Watt adopting the idea.
Did You Know? This engine helped Watt define the horsepower, the unit still printed on car brochures today.
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