# 1911 Encyclopædia Britannica/Horse-power

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​HORSE-POWER. The device, frequently seen in farmyards, by which the power of a horse is utilized to drive threshing or other machinery, is sometimes described as a “horse-power,” but this term usually denotes the unit in which the performance of steam and other engines is expressed, and which is defined as the rate at which work is done when 33,000 ℔ are raised one foot in one minute. This value was adopted by James Watt as the result of experiments with strong dray-horses, but, as he was aware, it is in excess of what can be done by an average horse over a full day’s work. It is equal to 746 watts. On the metric system it is reckoned as 4500 kilogram-metres a minute, and the French cheval-vapeur is thus equal to 32,549 foot-pounds a minute, or 0.9863 of an English horse-power, or 736 watts. The “nominal horse-power” by which engines are sometimes rated is an arbitrary and obsolescent term of indefinite significance. An ordinary formula for obtaining it is 1/15.6D$^2$ ∛ S for high-pressure engines, and 1/47D$^2$ ∛ S for condensing engines, where D is the diameter of the piston in inches and S the length of the stroke in feet, though varying numbers are used for the divisor. The “indicated horse-power” of a reciprocating engine is given by ASPN/33,000, where A is the area of the piston in square inches, S the length of the stroke in feet, P the mean pressure on the piston in ℔ per sq. in., and N the number of effective strokes per minute, namely, one for each revolution of the crank shaft if the engine is single-acting, but twice as many if it is double-acting. The mean pressure P is ascertained from the diagram or “card” given by an indicator (see Steam-Engine). In turbine engines this method is inapplicable. A statement of indicated horse-power supplies a measure of the force acting in the cylinder of an engine, but the power available for doing external work off the crank-shaft is less than this by the amount absorbed in driving the engine itself. The useful residue, known as the “actual,” “effective” or “brake” horse-power, can be directly measured by a dynamometer (q.v.); it amounts to about 80% of the indicated horse-power for good condensing engines and about 85% for non-condensing engines, or perhaps a little more when the engines are of the largest sizes. When turbines, as often happens in land practice, are directly coupled to electrical generators, their horse-power can be deduced from the electrical output. When they are used for the propulsion of ships recourse is had to “torsion meters” which measure the amount of twist undergone by the propeller shafts while transmitting power. Two points are selected on the surface of the shaft at different positions along it, and the relative displacement which occurs between them round the shaft when power is being transmitted is determined either by electrical means, as in the Denny-Johnson torsion-meter, or optically, as in the Hopkinson-Thring and Bevis-Gibson instruments. The twist or surface-shear being proportional to the torque, the horse-power can be calculated if the modulus of rigidity of the steel employed is known or if the amount of twist corresponding to a given power has previously been ascertained by direct experiment on the shaft before it has been put in place.