Inside a mechanical speedometer- paul-ryan.info Drivecable Flexibletube Hairspring Needle Milometer Dial Metaldrum Magnet
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A flexible drive cable inside a flexible tube links a small magnet inside the speedometer to the gearbox output shaft.

The magnet rotates with the shaft and its magnetic field attracts a metal drum to turn the speedometer needle against the force of a hair spring. The needle moves round the dial until the restraining force of the hair spring brings it to rest to give a reading of the road speed.

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