One Horsepower — What Watt Actually Measured
On the left, a horse gin: horses harnessed to arms, walking a circle to wind a kibble out of a mine shaft. On the right, one of Watt's steam engines doing the identical job at the identical rate. One slider rates the engine in horsepower, and the gin is given exactly that many horses. It is the definition of the unit, turned into a machine you can set.
How to use this simulation
- Rating of the engine — the single slider, 1 to 8 hp. The gin gains or loses a horse, an arm and a trace with every step.
- Drag anywhere on the scene to walk round the pit head.
What to look for
- The same power written four ways: watts, kilowatts, horsepower, and Watt's own foot-pounds-force per minute.
- Add a horsepower and another 2446 N goes up the shaft — at the same steady foot per second, never faster.
- Both kibbles stay level with each other all the way up, which is the comparison Watt wanted his customers to be able to make.
- The check line P = Fv returns the rating exactly, because the definition of the unit is a force times a speed.
The physics behind it
When Watt came to sell his engines, his customers already knew what a team of horses cost them and what it could shift, so he expressed his engines in the same currency. He defined one horsepower as a rate of doing work of 33 000 foot-pounds-force per minute — 550 ft·lbf every second — which is 550 × 4.448 N × 0.3048 m/s = 745.7 W, about 746 W. Both machines here raise 2446 N per horsepower at 0.3048 m/s, so P = Fv reproduces the rating by construction. The point worth keeping is that horsepower is a convention for comparing, not a measurement of any particular animal: real horses fall above and below it.