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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 It might be outdated or ideologically biased.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 ?
If our mechanical tach failed and we had to replace it tomorrow, we'd be tempted to go with the UMA electronic tachometer after marking it for our application.
After a three-and-a-half hour search in atrocious weather conditions and pitch dark light, trudging through murky fields, Tach and his super sensitive nose picked-up the missing man's scent and started barking.
L'un des objectifs de cette recherche d'authenticite est de reduire les activites absurdes que l'on propose parfois aux apprenants pour augmenter leur motivation a realiser les taches proposees.
Les ecrits scientifiques en EPS revelent peu d'information sur la nature simple ou complexe des taches d'evaluation generalement proposees aux eleves.
Deleguer des taches, lorsqu'elles sont bien realisees, est un avantage pour tout le monde.
Statistiquement les entretiens ne permettent pas de savoir si la facon de repartir les taches menageres au sein du couple represente un heritage familial ou plutot une revendication personnelle.
Les deux articles suivants portent sur l'emploi des taches avec les jeunes apprenants.
La decision a prevu les taches et specialisations visant a faciliter et fournir les services necessaires pour les pelerins.
Nous executions nos taches ne cherchant que la satisfaction d'Allah[beaucoup plus grand que], a-t-il dit, soulage.
However, the most enduring character from Bleasdale's seminal show was that of Yosser Hughes, the bushy 'tached, sallow faced apparition who'd turn up unannounced at building sites with his raggle-taggle kids in tow pleading with flustered foremen to "gizza job".
The Teesside Taches went to extreme lengths to grow their upper lip hair ahead of the tournament, in which they took on clean shaven opponents.