CALCULATION OF HORSEPOWER.

Auckland Star, Volume LIV, Issue 183, 2 August 1923, Page 7

CALCULATION OF HORSEPOWER.

(To the Editor.) Sir, —Could I encroach on your valuable space in regard to the above? Your correspondent John Martin has not very adequately defined horse-power; that is, what ia known as horse-power in 1 mechanics and physics. There is only one definition which is accepted in English-speaking countries to-day, and that is 33,000 foot pounds in one minute. The whole question must be I taken to the beginning to see where that value ie derived from. | Now, power is the rate of doing work, | and work is defined as force, multiplied by distance; so that in power we have three elements—force, distance and time. j When Watt invented the first steam' i engine he had in view the raising of ] I coal from low levels in a mine to the surface, and naturally compared his j then new invention with what it was : to replace. He recognised, co various | treatises tell us, that a good, well-grown horse would be able to perform 22,000 foot pounds of work in one minute, \ and he placed his engine horse-power j at half as much again, or 33,000 foot pounds to be fair to old Dobbin. j As before stated, there is only one definition, but many are the methods I of calculating horse-power, one accurate i and others only theoretical. Brake- j horse-power is the only really accurate ' value, as it is determined by test, and can he obtained for all machines with rotary movements, and some few ■ others. Indicated horse-power is derived from a diagram made on a special | paper by an instrument fitted to each cylinder of the engine under test. The area of each diagram is taken, and with certain other necessary data the horsepower is worked out according to formula. This is at best only approximate, as certain inaccuraeifes are bound to creep in. Electrical horsepower is determined 'by the multiplication of the 1 current (amperes) by its pressure (volts), flowing through a motor or other circuit, and dividing the value in watts so obtained by 746, the number of watts in one horse-power. The constant 746 has been found to be correct after many exhaustive tests and experiments. The formula used to calculate the horse-power of petrol engines in England is based on the theory of a motor speed of 1000 revolutions pef minute, and for that speed has been found to be approximately correct. However, when the engine runs at a lower speed, of course it is manifestly incorrect. Now, my point is that very few, in fact we can say none, of the motors used to-day for car or commercial vehicle purposes, when running with open throttle, fall under that speed except on steep praties, so that instead of bring as Mr. Martin says only one-third of the standard value per given horse-power, they develop power considerably in excess of it, in direct proportion to their pnjrine speed. Still the value of the horse-power does ; not change at all. it is as before. 33.000 I foot pounds in one minute. —I am. etc., FOOT POUNDS.