The above figure shows the variable capacitor which can be varied gradually. This is achieved by varying effective area between the plates. These plates are usually made up of brass or aluminium and semi circular shape. This capacitor consists of two sets of plates. One set of plates is fixed in position and is called the stator. The other set of plates can be rotated over the stator by rotating the piston. This set called rotor. During the rotation of the rotor, the area common to the plates of stator and rotor is varied.