Before you start visualizing a set of broad pectorals and sleek swimmer's body wrapped in a skimpy swimsuit, we'll correct you -- we're talking about the speedometer today.
It's pretty tricky to pin down exactly where and when the speedometer came into everyday use. Far before anything went fast enough to need a speedometer, odometers were invented as a way of measuring distance so you could be billed relative to the distance traveled. For example, ol' Ben Franklin dabbled with an odometer in the 18th century and we can pretty much blame the taxi-meter on him.
Almost all the vehicles produced in the last century have used one of two types of speedometer: mechanical or digital. You might have a digital speedometer that still utilizes a needle to display speed -- as opposed to digits like you see on a digital watch. In general, though, there is a trigger device -- often a hall-effect transistor -- that acts as a signal generator. Hall-effect transistors turn on when they are near a magnet, so designers mount magnets on particular moving parts and place the transistor nearby. The faster the magnets pass in front of the transistor, the shorter the square-wave pulse.
Huh? It means the electronics in the speedometer instrument part translates the pulses into a needle position and converts the pulses into a signal to advance the odometer. You can plug a square-wave generator's output into certain leads of BMW speedometers, for example, to calibrate and test them.
For mechanical speedometers the process is a bit different. The speedometer cable, which taps into the transmission, turns at given rate per turn of the wheel and tire. The speedometer is set up with gears to drive the odometer directly from the speedometer cable. The needle "top" end of the speedometer that indicates speed is coupled to the so-called "bottom" of the speedometer via a magnetic coupling. Two disks are aligned just within magnetic reach of each other. A spring provides counter-torque so the needle does not fly around wildly.
You can imagine the various symptoms you might see caused by a weak spring and so forth. In theory, you should be able to get your speedometer needle to react to the field of a strong magnet brought within magnetic "reach" of the needle.
Thanks to Rick Borch of Overseas Speedometer (speedometer.com) for the shop tour and for tweaking the speedometer on our Mini Cooper.
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