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taken from:
http://floridafieros.org/osg/speedo.html
The speedo needle is driven by a so called air-core meter.
Regular meters work by a needle which is pulled back to zero
by a spring and a coil forces a magnet away from the zero
position if current flows through the coil. An air core meter
uses two coils which are at an angle of 90 degrees to each other.
These coils are called the sine coil and cosine coil.
Each coil produces a magnetic field proportional to the current
flowing through it. Since the two coils are at a 90 degree angle,
the two fields add up to a "magnetic vector".
If a magnet is placed in the middle of this field, it will
point in the direction of this magnetic vector.
By varying the current through the two coils,
the needle can utilize all 360 degrees of the gauge.
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Must admit, this type of gauge is one i haven't met yet.
This mentions varying the current,
i think that includes reversing the current, if needed.
I suppose if the needle assembly is very light,
and it were positioned on a friction basis,
then it would be highly vibration resistant.
Regards, John