Brake dynamometers are most widely used in the measurement of the forces generated by motors. In that context, brake dynamometers can be considered varieties of engine dynamometers or motor testers. They take their name from the method by which they produce measurements of force generation in an engine. A brake is a utility that inhibits motion in a machine. When a brake is applied to a motor, it resists the movement that results from the force generated by that motor. The amount of force necessary to inhibit motion can be measured, and the expression of that measurement and its appearance on a display is the fulfillment of a brake dynamometer's purpose.One of the first ever brake dynamometers was the de Prony brake, invented by Gaspard de Prony in 1821. The de Prony brake is a mechanical friction brake. Wrapping a belt or cord around the output shaft of the engine and measuring the force transferred to the belt produces the measurement. Measurement takes place by attaching a pair of spring balances to the brake. While the shaft is rotating, one balance will show an increase in tension and the other will show a decrease in tension. Water brake dynamometers, also called hydraulic dynamometers, are very popular for engine testing because they have a high power capability while remaining inexpensive to operate. The most common type of water brake is called a variable level water brake. In this type of brake dynamometer, water is added until the engine is held at a steady RPM. The water is then kept at that level and replaced by constant draining and refilling that carries away the heat created by absorbing the horsepower. In response to the torque, the housing attempts to rotate, but it is restrained by the torque metering cell.