At the end of the eighteenth century, horses were the most used means of moving loads and were also used to operate machinery, for example to raise water wells. With the invention of the steam engine, it was necessary to establish a conversion between the power of this engine and that which can be obtained from a horse. The goal was, for example, to answer this kind of question: what power must have a steam engine to replace a horse extracting water from a well?
James Watt and the horsepower
The great British scientist James Watt, like others before him, attacked this problem and he found a solution. He took as reference the horse power needed to run a grain mill and used the units of his country: pounds (lb), feet (feet, or ft) and seconds. Thus was born the “horsepower”.
Value of a horsepower
The power unit is, later, become … the watt (W). This is defined in the SI (international system) as a newton.meter per second.
A horsepower, or hp, is worth 745,699,872 watts.
The trailer (used to carry heavy loads) is the first motor vehicle with steam engine. It was made in 1771 by the military engineer Nicolas-Joseph Cugnot (here, the second model, with its final size, preserved at the Museum of Arts and Crafts, in Paris). During the next century, the generalization of steam engines, gradually replacing horses, led to adopt a more powerful unit of power than the watt of the SI (International System of Units): the “horse”. The habit will survive until the 21st century … © Roby, GNU
Value of a horsepower
The rest of the world, using the metric system, defined the “horsepower” as the power developed by a horse to go up 1 m to a mass of 75 kg in 1 s. A mass of 1 kg weighs 9.81 newtons, because the value of Newton is given by the acceleration of gravity (due to Earth’s gravity) and is, by convention (it is an average), 9.81 m / s / s.
A horsepower of the metric system, or horse, is therefore 1 s x 75 kg x 9.81 m / s / s, or 735,498.75 W.
Tax horses, horsepower …
When steam engines began to replace horses, their power was expressed in horsepower or horsepower. The habit continues, and we keep the hp and the horsepower, to which were added the electric watt, the fiscal horses (CV, an administrative value) and even, for the cars, the horses SAE and the horses DIN .. While it would be enough for everyone to count in watts!