D YNA31031'ETER, a device for measuring the force which does work in in resistance and producing motion. The foot-pound, as a unit of work, has for its factors the force acting and the distance through which it acts. The i larger unit, the horse-power, besides these factors has a third, the time during which the force is exerted. Hence, in getting the data from which the work of a machine is to be calcu lated, we are to observe the force, the distance, and the time required to accomplish a certain result. Strictly speaking, the dynamometer indicates the first of items, but it may be so arranged as to show both the others. Dynamometers are designed to indicate the force of traction, of thrust, or of rotation. A traction dynamometer may be interposed, for example, between a team of horses and a reaper or a plough, to measure the force exerted by the horses in drawing the machine. It is usually some sort of spring balance, fitted with an index and a scale; the figures instrument the scale show the num ber of pounds required to bring the index to the corresponding points, if the nstrument were hung up and weights suspended by it. A dynamometer for thrust is often con
nected with the screw-shaft of a steamship, to measure the force with which the screw is driving the vessel through the water. Rotary dynamometers measure the force of a. mill-shaft, either by showing what force is required to hold the shaft in check, by absorbing the motion, or what force the shaft transmits to other machinery. Nearly all forms of dynamometers are too complex to be described without the help of elaborate drawings and technical descriptions, for which the reader is referred to special works on mechanism. The use of the dynamometer in skillful hands has acquired great value in exchanging the rough and usually overestimated guesses of the efficiency of machines for the exact determination of their performance.