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(tăkŏm`ətər), instrument that indicates the speed, usually in revolutions per minute, at which an engine shaft is rotating. Some tachometers, especially those used in automobiles, are similar in construction and operation to automotive speedometersspeedometer,
instrument that indicates speed. A cable from an automotive speedometer is attached to the rear of the transmission of an automobile; the cable turns at a rate proportional to the speed of the car.
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. Other types, often connected directly to the shaft whose speed they indicate, are small electric generators whose output voltage is proportional to speed. This voltage is applied to a voltmeter whose dial is calibrated in speed units. Another type, used only with engines having an ignition system, operates by counting the pulsations of current or voltage in the ignition system, the number of these being proportional to the speed of the shaft.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



an instrument for measuring the speed of rotation of the shafts of machines and mechanisms. Centrifugal, mechanical, eddy-current, and electric tachometers are most common; pneumatic and velocity-head, or hydraulic, tachometers are used less frequently.

In a mechanical centrifugal tachometer (Figure 1), a sliding

Figure 1. Mechanical centrifugal tachometer: (1) weights, (2) arms, (3) sliding coupling, (4) shaft, (5) spring

coupling is mounted on a shaft. The coupling has hinged arms carrying weights that spread apart when the shaft rotates, moving the sliding coupling along the shaft against a counterbalancing spring. The position of the coupling on the shaft depends on the speed of rotation and is transmitted by an arm mechanism to an indicator pointer; the indicator dial is calibrated in revolutions per minute. The tachometer shaft may be driven directly, by the controlled mechanism, or indirectly, by a flexible shaft.

An eddy-current tachometer (Figure 2) uses the interaction of the magnetic fields generated by a permanent magnet and a rotor, whose speed of rotation is proportional to the eddy currents generated. The currents tend to deflect a disk, which is mounted on the shaft and restrained by a spring, through a certain angle. The deflection of the disk, which is rigidly connected to a pointer, is indicated on a dial.

Figure 2. Eddy-current tachometer: (1) permanent magnet, (2) rotor, (3) shaft with pointer, (4) spring

Electric tachometers may be of the generator or impulse type. In tachometer generators the electromotive force of a DC or AC generator is proportional to the angular velocity, from which the shaft speed can be determined; the readings are transmitted to a remote measuring instrument. The operation of impulse tachometers is based on conversion of pulses generated in the primary circuit of an ignition system by the opening of interrupter contacts into a current that is fed to a permanent-magnet indicator. The frequency of pulses in the primary circuit is proportional to the speed of rotation of the engine shaft.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An instrument that measures the revolutions per minute or the angular speed of a rotating shaft.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

tachymeter, tacheometer, tachometer

A surveying instrument designed for use in the rapid determination of distance, direction, and difference of elevation from a single observation, using a short base which may be an integral part of the instrument.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


An instrument that indicates the engine speed either as revolutions per minute (RPM) or as a percentage of the maximum RPM. The instrument gets its information from a tachogenerator.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved


any device for measuring speed, esp the rate of revolution of a shaft. Tachometers (rev counters) are often fitted to cars to indicate the number of revolutions per minute of the engine
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
When we were researching this article, UMA's Sefik Mujakic told us that the company has offered digital-only displays in lieu of its electronic analog tachometers. UMA finds few buyers for the digital version--analog rules.
And the fact that round gauges endure and are preferred for tachometers proves the point.
Tachometers and speedometers process digital and analog inputs.
Two multi-function, battery-operated, portable instruments contain a tachometer, totalizer/counter and timer (stopwatch).
Domed polycarbonate crystal lenses are used on trip odometer speedometers and memory and internal shiftlite tachometers. An antifog solution is applied at the factory to the inside of all glass lenses.
The Regulation on tachometers amends Regulation 3821/85 on recording equipment in road transport, and Directive 88/599 on the application of Regulations 3820/85 and 3821/85.
has introduced a pair of new hand-held tachometers with built-in analog and pulse output.
Frequency/pulse counters for encoders, resolvers and tachometers are available along with electrically isolated inputs for temperature measurement with thermocouples and RTDs, or for measuring DC voltages, DC currents and resistances.
The heavy-duty mechanical line consists of 52 mm temperature/pressure gauges and 86 mm speedometers and tachometers. The temperature/pressure gauges include: air brake; dual air; fuel; manifold; oil and transmission oil temperature gauges, as well as oil; water and turbo boost pressure gauges.
On any older typical outboard installation you'd find a tachometer at the helm and that is about it.
Multi-function, battery-operated, portable instruments containing a tachometer, totalizer/ counter and timer (stopwatch) are available from the company.
Murphy is offering an addition to its line of tachometers/hourmeters, the AT series Tachometer and ATH series techometer/hourmeter.