Discusses the term "horse power" and its origin.
James Watt is usually credited with introducing the term horsepower in the late 1700s to market his new steam engines. But just as Watt did not invent the steam engine (his rotative steam engines built on earlier pumping steam engines), he was not the first to compare engine power to a horse’s power. Nearly a century earlier, Thomas Savery, who invented the first steam engine that approached commercial success, also stated the power of his engines in terms of the number of horses they could replace.
Between Savery’s and Watts’ time, the power of a horse was defined inconsistently by different engine makers. Watt estimated that the amount of weight a horse could pull over a given distance in a given period of time was 33,000 foot-pounds per minute, which has been the accepted definition of 1 horsepower ever since. Power is also now measured in watts, and 1 horsepower is equivalent to 746 watts.
Sources differ on how Watt came up with his definition of horsepower. Some say he based it on how quickly a draft horse could turn a mill wheel. According to another account, he based it on ponies lifting coal at a coal mine, but he increased the number by 50 percent to estimate the power of a horse, rather than that of a pony. Another source claims that Watt based it on the power of a horse but deliberately overestimated a horse’s power by 50 percent so that he would not be accused of exaggerating the number of horses his engines could replace.
In pulling contests, draft horses have been observed to have a peak power output of nearly 15 horsepower for a few seconds. The average horse cannot work at a rate of 1 horsepower over long periods, but a fit draft horse can sustain 1 horsepower for hours.
Although the definition of horsepower was standardized more than two centuries ago, the method of measuring an engine’s power has varied. For example, before the 1970s, American automakers measured and advertised their engines’ gross power—the power at the engine’s crankshaft with no belt-driven accessories. Since then automakers have quoted net horsepower—remaining power output after losses caused by standard power-consuming accessories.