The basic construction of air ganged capacitor is as shown in fig. It consists of two sets of aluminium plates separated from each other by air. These metal pates may be rotary or concentric type. The rotary type configuration has a rotor and a stator. The concentric type configuration has two cups of aluminium. The concentric type variable capacitor consists of two cups of aluminium, one moving in other. The movement is actuated by a threshold screw. One set of the plates in fixed, while the other set is connected to a shaft and can be rotated. The fixed set of plates is insulated from the body of a capacitor on which it is mounted. The set of moving aluminium plates with the help of a suitable knob connected to a rotating shaft. As the plates are moved in and out of the fixed plates, the capacitance value varies. The capacitance is minimum, when the moving plates are completely out and it is maximum, when the moving plates are completely in. the fixed plates are called stators, which are normally made of brass, copper or aluminium. The cadmium plated steel is used for the frames in low cost capacitors. The outer set of plates is called rotors. They get interleaved with stators, when the shaft is rotated. Sometimes, two or more such capacitors are operated by a single shaft.