Tale of the Horsepower Unit

Horsepower. A unit of power. Devansh Malik tells the story behind it and it is very interesting.

 

When the steam engine was invented, it served one purpose only. For 50 years!

“(A steam engine) did only one job: pumping water out of coal mines, which flooded quite often. They could only move up and down, which is perfect for a pump but useless for anything else.”

 

James Watt (“the same dude from your physics textbooks”) was a builder of things – barometers, compasses, toys, musical instruments etc. Once, someone asked him to fix a broken steam engine. Those initial steam engines were very inefficient. So Watt decided to both fix it and improve its design.

“His version did the same amount of work while burning about 75% less coal. He also worked out a gear system that turned the engine’s up-and-down motion into a spinning one. Which meant a steam engine could now turn a wheel.”

 

This hugely expanded the possible uses and applications of the steam engine.

“Until then, a steam engine was a machine for coal mines. But now it could run a flour mill. A brewery. A distillery. A cotton factory. Basically anywhere there was a wheel that needed turning. Which, at the time, meant anywhere there were horses.”

 

But… building something and making money of it are two different things.

“Watt had no idea what to charge for the engine. The problem was that the engine’s value wasn’t the machine but the coal it saved. And that was different for every single buyer. Put a fixed price on it and you’d either undercharge or overcharge.”

So he came up with a new way of deciding the price. He wouldn’t charge for the engine itself! Instead:

“He worked out how much coal people were burning with the old engine, and how much they burned with his. Then he charged them a third of the difference, every year.”

Buyers liked the idea – they paid in proportion to their savings, so they were sure it was profitable for them. And Watt got paid every year, like royalty.

 

But there was a snag. His pricing method (difference in coal used) only worked for folks who already had a steam engine, i.e., only coal mines. But, as we saw, Watt’s engine had far more applications. And those other potential buyers (flour mills, breweries, distillers etc) used horses to do the job. How could one compare the horse v/s steam engine? So Watt asked himself a new question:

“How many horses is my engine worth?”

 

Seems like apples and oranges, right? Aha, Watt was a maths wizard, a quantifier long before measurements became a management and individual fad (steps walked, calories burnt). He observed horses at work. What was the radius of the beam it was tied to? 12 feet. Therefore, the distance it walked in one round (circle) was 2πr = 75 feet. Then he timed the horse – how many laps per minute? Around 2.4 laps per minute. So, in one minute, it covered 180 feet (75 x 2.4).

 

Work done = Force x Distance. Watt had the distance worked out. But with how much force was the horse pulling? He plucked a number out of thin air – 180 pounds of force. With that, the work done = 32,580 foot pounds. Per minute. Watt rounded that off to 33,000 foot pounds per minute. Watt called that one horsepower.

 

Was it scientific? Well, kind of. Was it useful? Very much so.

“Watt could now walk into a factory with horses and say: this engine will do the work of twenty of them. And then charge for it accordingly. He basically invented a unit to solve a sales problem.”

 

Not surprisingly, future measurements of a horse’s power yielded very different values. Different horses. How long they worked. Decades later, with industrialization, a more scientific, objective unit of power was created. It was named watt (in honour of James Watt). And based on the accepted value of horsepower at that time, one horsepower was defined as 746 watt.

“Which is NOT the power of one horse.”