POWER, in mechanics and physics, the rate at which work is performed; or (what is the same thing), thequantity of work that is per formed per unit of time. In ordinary engineer ing practice in the United States and England, the unit of work is the •foot-pound;" a foot pound being defined as the quantity of work that is performed when a force eou-I to the weight of one pound of matter is exerted through a distance of one foot. (See MECHANICS). Nothing is here it will be observed, as to the time in which this work is done. A small motor, for example, might require 10 seconds to accomplish it, while a more powerful one might perform it in a very small fraction of one second. The work that is performed by a given motor or engine in one second, or in one minute, is therefore the true measure of the "'power* that is being developed. James Watt adopted the ((horse power') as the unit of power for steam engines and other large motors, and this has been universally adopted, since his time, in all English-speaking coun tries. He estimated that a strong horse is capa ble of performing 33,000 foot-pounds of work per minute; and although it is known that the horse cannot work continuously at this rate, his definition has never been modified, and in steam and hydraulic engineering a horse is still understood to signify the performance of 33,000 foot-pounds of work per minute, or (what is the same thing) 550 foot-pounds per second. The horse power of an engine or a
water-wheel is therefore determined by observ ing how foot-pounds of work it is per forming per minute, and dividing this number by 33,000. In the centimeter-gram-second sys tem of units, which is in extensive and increas ing use in modern physics, the unit of work is the eerg,D and the unit of power is the awate; a watt being defined as the power that is de veloped when 10,000,000 (that is 10') ergs of work are performed per second. The watt is employed in practical electrical engineering, as well as in experimental physics; it being then defined as the rate at which work is being done in a circuit in which the current is one ampere, and the electromotive force is one volt. This change in definition is merely a matter of con venience, since it may be shown that the two definitions amount to precisely the same thing. A horse power is approximately equal to 746 watts. Consult Everett, and Physical Constants.' See also the article 'Chars OF MEASUREMENT.