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The analog speedometer used on cars indicates the speed of the car
and records the distance the car has traveled. A speedometer is
driven by a flexible cable connected to the speedometer pinion in the
transmission. Speedometers are calibrated in miles per hour and/or in
kilometers. The instrument also records the distance traveled,
recorded in miles or kilometers. That portion of the instrument is
known as the odometer. Most odometers record the total distance
traveled. Some also record the distance of individual trips. These
can be reset to zero.
The speedometer and odometer are driven by a cable in a casing. The
cable is connected to a gear at the transmission. This gear is
designed for a specific model, tire size, and rear axle ratio. The
speed indication of an analog speedometer works on the magnetic
principle. It includes a revolving permanent magnet driven by the
cable connected to the transmission. The magnet sets up a rotating
magnetic field which exerts a pull on the speed cup, making it
revolve in the same direction. The movement of the speed cup is
slowed and held steady by a hairspring attached to the spindle of the
speed cup. The speed cup comes to rest where the magnetic drag is
just balanced by the retarding force created by the hairspring. The
hairspring also pulls the pointer of the instrument back to zero when
the magnet stops rotating. As the speed of the magnet increases, due
to movement of the car, the magnet drag on the speed cup increases
and pulls the speed cup further around. In that way, a faster speed
is indicated by the pointer on the face of the dial. The magnetic
field is constant, and the amount of movement of the speed cup is
always proportional to the speed at which the magnet is being
rotated.
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