The mechanical fuel gauge used in aircraft uses a cork (currently the float is made from Nitrile rubber) that floats on top of the fuel. So when the fuel level goes up or down so does the cork. The cork in turn is attached at the end of a light-weight pipe usually made from tempered aluminium. As the cork (or rubber float) moves up or down so does the pipe. The pipe in turn is attached to a delicate gear system usually made from stainless steel. Through use of the gear mechanism, the up-down movement of the rod is converted into circular movement which is passed to a rod (or cable) that is attached to the fuel gauge via a drive magnet.
http://www.joyfreepress.com/2016/09/01/232499-aircraft-fuel-gauges-how-they-work/
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