Mostly, the capacitors are classified based on the size and shape of the plates used e.g. parallel plate, cylindrical, concentric spherical etc. they may be classified based on the nature of the dielectric used as follows :
i) Air capacitors : This type of capacitor consists of one set of fixed plates and another set of movable plates. This type is mainly used for radio work where the capacitance is required to be varied.
ii) Paper capacitors : This consists of metal foils interleaved with paper impregnated with wax or oil and it is rolled into a compact from. These are used power supplies.
iii) Mica capacitors : It consists of alternate layers of mica and metal foil clamped together tightly. Use of mica makes its cost high. It is mainly used in high frequency circuits which requires greater accuracy, high voltages and less dielectric loss.
iv) Poly carbonate capacitors : This is a recent development where a film of polycarbonate, metallised with aluminium is wound to from the capacitor elements. It has a relative permittivity of 2.8 and has a high resistivity with very low dielectric loss.
v) Ceramic capacitors : it has a metallic coatings on the opposite faces of a thin disc of ceramic material like barium titanate, hydrous silicate of magnesia, etc. It is used in high frequency radio and electronic circuits.vi) Electrolytic capacitors : These are most commonly used and consists of two aluminium foils, one with an oxide film and one without. The foils are interleaved with a material such as a paper saturated with a suitable electrolyte. The aluminium oxide film acts as a dielectric. These are used where very large capacitance values are required so used in electronic and filter circuits. The main limitations of this type are the low insulation resistance and suitably only for those circuits where the voltage applied to capacitor never reverses its direction.