Dynamometer

Dynamometer (Gr. force, and a measure), an instrument originally designed to ascertain the strength of men and animals, of the limbs of the body, the fingers, etc. Its application was afterward extended to the determination of the power exerted by machines, or of any portions of them, and the instrument has hence come into use as a meter of the power of engines. The principle of the earlier contrivances was to weigh the force exerted by the amount of compression or of deflection produced upon an elliptical steel spring; this in the former case being drawn together by the application of the power and of the resistance at the two opposite ends, and in the latter separated by the force and resistance being applied upon the opposite sides of the spring, on the line of the minor axis of the ellipse; an index upon a graduated arc attached to the spring showed the amount of deflection. Another contrivance was a spiral spring enclosed in a tube, the force being applied in the direction of compression. By such means the greatest power exerted by one impulse was indicated; but as in most instances the power is not constant for any determinate time, the index must fluctuate in such a manner that the mean effort it should represent cannot be ascertained.

If known, its amount multiplied by the time of continuance of the operation would give as a result the value of the whole power exerted. Instruments have been devised by MM. Poncelet, Morin, and others, which register upon papers, made to pass by a clockwork movement under the index, curved lines from which the whole power is directly calculated from the areas enclosed; the ordi-nates of the curves representing the power exerted, and the abscissas the length of time, or in some instances the space run over. The apparatus might be fixed to a carriage, the length of the index paper in this instance bearing a certain proportion to the length of the road gone over. A great number of different forms of this instrument have been devised by eminent engineers of France, England, and the United States. One by Watt, improved by Macknaught, gives the force exerted by the piston of a steam engine against a spiral spring, a style attached to the piston inscribing a line representing its position during the unrolling of the paper which moves at an even rate against it. In the Dictionnaire des arts et manufactures the subject is fully treated in the article Dynamometre, by M. Laboulaye. The descriptions of the various forms of the apparatus are made intelligible by many illustrations.

In Appleton's "Dictionary of Mechanics," also, many forms of the apparatus are figured and described; and the following simple contrivance, applicable in some instances, is proposed: A cylinder of some material heavier than water is suspended in this fluid by a rope passing over a pulley. As power is applied to this rope to draw the cylinder out of the water, the increasing weight of this, as more is raised into the air, will at last cause the resistance to equal the force applied, the cylinder being sufficiently large and long. By means of a scale properly arranged, the amount of the power applied may be accurately measured.