Horsepower now no longer existed. But I am sure all of you are curious: what is this weird unit?Where did it originate from? What does it measure?What relation to the SI units does it have? What is its disadvantage over the other units?
I will be answering all of these questions today.
What is this weird unit?Where did it originate from?
Good question.
The development of the steam engine provided a reason to compare the output of horses with that of the engines that could replace them. In 1702, Thomas Savery wrote in The Miner's Friend: "So that an engine which will raise as much water as two horses, working together at one time in such a work, can do, and for which there must be constantly kept ten or twelve horses for doing the same. Then I say, such an engine may be made large enough to do the work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained and kept for doing such a work…" The idea was later used by James Watt to help market his improved steam engine. He had previously agreed to take royalties of one third of the savings in coal from the older New comensteam engines.This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt determined that a horse could turn a mill wheel 144 times in an hour (or 2.4 times a minute). The wheel was 12 feet in radius, therefore the horse travelled 2.4 × 2π × 12 feet in one minute. Watt judged that the horse could pull with a force of 180 pounds. So:
This was rounded to an even 33,000 ft·lbf/min.
Adapted from:
http://en.wikipedia.org/wiki/Horsepower#History_of_the_unit
What does it measure? What is its relation to the SI units?
As the name suggests, it is used to measure power, by using horses. Now, the SI unit to measure power is......
sttaW:rewsnA (read the other way round )
What is its disadvantage compared to the other units?
This unit is not good as the horse power varies between the respective horses. Like how the Egyptians used to measure one cubit ( the length between the elbow and the hand ), it changes and is not fixed. Therefore, it is very ambiguous.
I feel that it is important to study the old and outdated units. The lessons we learnt from failure of the old units will help us create a more stable and reliable unit.
