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speedometer,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. In a very common type of speedometer the other end of the cable is attached to a simple magnetic device inside the speedometer. In response to the rotating cable, this device moves a needle along a calibrated dial to indicate the speed of the automobile. Another type of automobile speedometer uses centrifugal force and operates in a manner similar to a flyball governorgovernor,
automatic device used to regulate and control such variables as speed or pressure in the functioning of an engine or other machine. A governor may be an electric, hydraulic, or mechanical device, or it may employ some combination of electric, hydraulic, and mechanical
..... Click the link for more information. . The airplane speedometer is called an air-speed indicator. A dial that registers the speed in kilometers or miles per hour is actuated by the wind pressure in a tube located where it is not affected by the air stream from the engine. The speed of a watercraft is frequently determined by means of a patent or taffrail log. In this device a small propeller, which is towed astern, revolves as it moves through the water, activating a calibrated dial aboard the vessel. In another type of marine speedometer a tiny fin projects from the hull below the waterline. The resistance of the water to the passage of the fin is converted on a dial into terms of speed in knots.
an instrument used in motor vehicles to measure the speed of travel and the distance traveled. The speed indicator of a speedometer is magnetically operated, and the distance counter (odometer) operates mechanically by means of rollers. In mechanically driven speedometers, both the speed indicator and the odometer are connected by means of a flexible cable to a set of reduction gears, one gear of which is driven from the driven shaft of the vehicle’s transmission. In electrically driven speedometers, the transmission’s driven shaft is connected to a contact breaker that functions as a pickup and converts direct current into three-phase alternating current. The frequency of the alternating current is proportional to the speed of rotation of the driven shaft. The current feeds an electric motor, whose rotor rotates at the same speed as that of the pickup.
REFERENCESGalkin, Iu. M. Elektrooborudovanie avtomobilei i traktorov, 2nd ed. Moscow, 1967.
Historical note: One computer, the GE 600 (later Honeywell 6000) actually had an *analog* speedometer on the front panel, calibrated in instructions executed per second.