A floatstick is a device to measure fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 levels in modern large aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. It is made up of two tubes, one enclosed within the other, around which is a ring-shaped float that contains a magnet

Magnet

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...

. There is also a magnet attached to the top of the floatstick. The floatstick is withdrawn from the bottom of the wing

Wing

A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...

 until the magnets stick, indicating the level of the fuel.

Older aircraft use a dripstick

Dripstick

A dripstick is a thin hollow tube installed vertically in the bottoms of fuel tanks of many large aircraft, used to check fuel levels. To read a dripstick, it is withdrawn from the lower surface of the wing. When the top of the dripstick is withdrawn below the level of the fuel, fuel enters the...

.