The work done in a given time, divided by the time, gives the average rate of doing work, or power.
The C. G. S. unit of power is the erg per second. In practical work the joule per second is used; this is called the watt in honor of James Watt, the inventor of the steam engine.
1 watt = 1 joule per second = 107 ergs per second.
In the F. P. S. system, the unit of power is the foot poundal per second. This is a small unit and is seldom used, the practical unit being the horse power, which means a rate of 33,000 foot pounds per minute, or 550 foot pounds per second; 1 hence the expression for horse power is
This unit was introduced by James Watt and its value was assigned by him. It is the work that would be done in one minute by a horse walking at the rate of three miles per hour and raising a weight of 125 pounds at the same rate by rope passing over a pulley. One horse power is rated at 746 watts or 0.746 kilowatts. Hence the horse power is practically three fourths of a kilowatt and one kilowatt equals one and one third horse power.
The kilowatt is used to measure the power output of electric generators, while the steam power input, used in engines or turbines, is measured in horse power or myriawatts (1 myriawatt = 10 kilowatts).
