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The Smoke Stack

The Origin of “Horsepower”

Posted on October 20, 2017


Although James Watt wasn’t the true inventor of the steam engine, this Scottish inventor developed the first way to measure its power… in the mid-1700s!

Types of experimental steam-powered engines have been in the works for hundreds of years beginning in ancient Egypt. These were devices generally used by inventors to demonstrate the power and properties of steam. The 1600s brought more useful tools such as a water pump for draining flooded mines and a piston that raised weights.

The first steam engine that generated power transmitted directly to a machine was called the “atmospheric engine”, invented in 1712 by Thomas Newcomen. This was a successful item that was used commercially, in some cases providing a reusable water supply to turn waterwheels at factories.

In the 1760s, James Watt improved on the atmospheric engine by using a condenser versus applying pressure to expand steam; this innovation resulted in the use of half as much coal. In order to effectively market his new product, Watt developed a measurement system called horsepower, which we still use today!

This measurement system is based on the number of times a horse could turn a mill wheel, which is 144 times per hour, and this calculation was compared to the size of the wheel, the distance the horse walked and the force used by the horse to do all of this. Using physics, Watt formulated that one horsepower equaled 33,000 foot-pounds per minute, and in turn, he calculated how many horses just one of his engines could replace (PLAN/33000 where P=Mean Effective Pressure in Cylinder, L=Length of Stroke in feet, A=Area of Piston in inches and N=Revolutions per minute).

This system helped to promote Watt’s steam engine which soon became the industry standard… eventually leading to the invention of the first steam locomotive in 1804.

Oh, and as it stands, one “mechanical horsepower” equals 745.7 Watts, a rate of energy transfer James Watt invented and named after himself, one Watt representing 1 joule per second.

Portrait of James Watt (1736–1819)
by Carl Frederik von Breda

Plan a visit to Essex Station to see one of our many steam locomotives!

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