An electric vehicle speedometer is a device used to measure the traveling speed of a vehicle, usually for the purpose of maintaining a sensible pace. Its development and eventual status as a standard feature in automobiles led to the enforcement of legal speed limits, a notion that had been in practice since the inception of horseless carriages but had gone largely ignored by the general public. Today, no automobile is equipped without a speedometer intact; it is fixed to a vehicle's cockpit and usually shares a housing with an odometer, which is a mechanism used to record total distance traveled. Two basic types of automobile speedometer, mechanical and electronic, are currently produced.
In a mechanical speedometer, a rotating cable is attached to a set of gears in the automobile's transmission. This cable is directly attached to a permanent magnet in the speedometer assembly, which spins at a rate proportional to the speed of the vehicle. As the magnet rotates, it manipulates an aluminum ring, pulling it in the same direction as the revolving magnetic field; the ring's movement, however, is counteracted by a spiral spring. Attached to the aluminum ring is the pointer, which indicates the speed of the vehicle by marking the balance between these two forces. As the vehicle slows, the magnetic force on the aluminum ring lessens, and the spring pulls the speedometer's pointer back to zero.
Electronic speedometers are almost universally present in late-model cars. In this type of gauge, a pulse generator (or tach generator) installed in the transmission measures the vehicle's speed. It communicates this via electric or magnetic pulse signals, which are either translated into an electronic read-out or used to manipulate a traditional magnetic gauge assembly.
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