A tachometer an instrument that indicates the revolutions per minute (rpm) of a rotating shaft, such as an automobile or airplane engine crankshaft, or of other machinery. Most tachometers have a pointer that moves over a dial to provide a direct reading of rpm.

Hand tachometers, either mechanical or electrical in operation, are brought into contact with the rotating end of a shaft so that the tachometer rotates at the shaft speed. In mechanical tachometers, weights attached to a rotating spindle swing out by centrifugal force against the action of a spring and move the indicating pointer over a dial. In electric types, a rotating armature generates a current that is indicated as rpm on the dial. Such a tachometer is really a milla-meter or a voltmeter, indicating the output of a small generator turning at shaft speed.
A vibrating-reed tachometer is used when it is impossible for instrumentation to reach a rotating shaft. This tachometer has a number of metallic strips (reeds), each attuned to vibrate at a frequency in the normal operating range of a machine. The rotational speed of the machine is thus shown by the visible vibration of a reed in resonance with the speed of rotation. In other instances in which instrumentation cannot reach the rotating shaft, belt drive or gearing is used to drive a tachometer, usually of the electric type.

The strobotac and similar instruments are also used as tachometers. In a strobotac, an electronically controlled light, the frequency of which can be changed, flashes on a rotating part of the machine. When the flashes reach synchronization with the machine, the machine appears to be standing still; a calibrated dial then indicates the speed of rotation or frequency of oscillation.

In another device a magnetic or mechanical pickup coupled to the shaft transmits a series of impulses which are integrated by a timing device, such as an electric clock, to provide a direct reading of rpm. Still another arrangement incorporates a so-called Arago's disk- a metal plate that, rotating in a magnetic field, creates a torque. This torque, when resisted by spring action, can be calibrated to indicate the speed of the shaft that drives the disk.

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