If the air is replaced by a dielectric of relative permittivity ∈_r in the capacitor it is found that the value f the capacitance is greatly increased; and the ratio of the capacitance of a capacitor having a certain dielectric to the capacitance of that capacitor with vacuum (or air) as medium is called the relative permittivity of that dielectric and is represented by the symbol ∈_r . Values of the relative permittivity of some of the most important insulating materials (dielectrics) are given below:

Dielectric                           Relative Permittivity
Air__________________________  1
Bakelite_______________________4.5
Ebonite_______________________ 2 8
Fibre__________________________2.5
Mica _________________________ 5-9
Oil___________________________ 2-5
Paper_________________________2-4
Porcelain______________________(-7.5
Rubber________________________2-3.5
Slate_________________________ 6- 7.5

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