A dynamometer can measure torque, rotational speed and power. Dynamometers often measure these things in a driven machine such as a pump or an engine, but they are also used in emissions tests to simulate loading the engine and in the medical community to measure limb strength in rehabilitating patients. One type of dynamometer is hooked up to a rotating engine output shaft. An arm attached to the dynamometer's housing prevents it from rotating. This arm is hooked up to a weighing scale to measure how much force is produced by the dynamometer trying to rotate.
Invented in 1821 by Gaspard de Prony, a de Prony brake is a relatively simple device for measuring the torque produced by an engine. The de Prony brake's ability to measure torque and power gives us the term "brake horsepower." The output shaft of the engine is attached to a rotating drum around half of which a friction band resists the rotation. Attached to the other end of the friction band are spring scales that measure the change in force as the engine shaft starts rotating.
When applying a specified amount of torque to a nut or bolt is crucial, a torque wrench is the right tool for the task. The wrench was initially designed to prevent overtightening of bolts in New York City's water system. Torque wrenches are imprecise in that they cannot measure the friction between the bolt and the fastener, but they are often the only practical way to measure bolt tension. The simplest torque wrenches have a main arm that bends as the bolt is tightened and the indicator arm remains straight; the space created between the bent arm and the straight arm causes the needle on a calibrated scale to move.
A torque sensor is used in dynamic systems where an ongoing measurement of torque is needed while the machine is in use. Some torque sensors rely on magnetism, with areas of the rotating members conditioned to be magnetic. When these members rotate, the magnetic areas produce small electrical currents, and the measurements of these currents can be manipulated to figure out the torque produced. Strain gauges or angular position sensors measure the differences once rotation begins.