One side of impregnated paper capacitors is sprayed with metal.  When such paper is rolled, its gives higher capacitance for a given value. This capacitor has a self healing property, if the voltage across the capacitor is gradually raised to a value at which sparking occurs, the voltage is held constant at this value, and then the sparking is only momentary. This sparking at week points in paper and thin metal film evaporates from these spots without damaging the paper. Aluminium at high temperatures produces aluminium oxide, which is a good insulator. Aluminium is evaporated on lacquered paper tissue in vacuum. Thickness of film is small. Metallised tissue is passed between metal rollers, across which working voltage is applied to remove week points. Two such tissues are superimposed and wound to form a roll. The demutualised margin serves as insulator between the two electrodes. Both the ends are sprayed with copper so that end connections can be made.

SPECIFICATIONS

Range of values                                  1000 Pf to 1 μF

Tolerances                                                                          25%

Voltage rating up to                                                             1.5 KV

Power factor at 1K Hz                               0.05

Stability                                                                              5 to 10%

Temperature coefficient                                 + 150 to 200 ppm/⁰C

APPLICATIONS: It is used for decoupling.