A tachometer is a measuring device with the purpose of measuring the speed of a circular plate or disk, or sometimes of a shaft. The word comes from the Greek words meaning speed (tachos) and measure (metron). A tachometer is typically used in measurement of engine speed in an automotive engine, but was originally used to describe centrifugal force. The German engineer Dietrich Uhlhorn is believed to be the inventor of the tachometer. His interest was in measuring the speed of machines. The use of the tachometer in measuring the speed of locomotives has been common since about 1940, and it is also used in measurements of light rail axle speeds. The early tachometers were entirely mechanical, but it is interesting to note that the modern day version relies entirely on electronic measurements to register as part of a computerized system. Find more info at http://www.tachometerworld.info For instance, the tachometer, although described in revolutions per minute (RPM’s) is actually derived in older vehicles from impulses recorded from the low tension side of the ignition coil. Engines which have no ignition system, such as diesel engines, measure the alternator output voltage. In neither case is the engine actually “revolving”, although the pistons perform a somewhat circular dance, when in operation. Originally, tachometers were used to register and display the range where the engine could be safely operated. To “red line” an engine meant to run the engine at a dangerously high speed. The theory was that over racing the engine would lead to excessive wear and damage to the equipment. Even knowing the possibility of engine damage was never enough to prevent the sound of “revving up” the engine as an invitation to drag race, or perhaps just to let others know that your engine was “hot” In modern engines, a rev limiter prevents damage due to engine speed over safe levels.