History of Film CapacitorsFilm capacitors appeared soon after glass and air capacitors. The first experimenters needed high-capacitance capacitors of quite small size, and finally this problem was solved. One of the first dielectrics used in capacitors was paper treated with wax or oil. The body of such a capacitor may also be filled with oil. They were simple to make, and were produced with capacitance varying from hundreds picofarads to tens of microfarads. Their modern versions are still used for specific purposes, say, for acoustic issues. Their disadvantages are the following: the insulator depends on the frequency which makes it not applicable for high frequencies. Besides, it is highly inductive. Moreover, their size is not very suitable for miniature transistor equipment (compare the much more compact ceramic capacitors on the left in the following picture, with the paper ones on the right). A little bit later mica capacitors appeared. They had higher specific capacitance and were more suitable for high frequencies. However, these capacitors can not be considered the film ones as mica is not enough flexible to be folded into a roll. The next type of dielectric used for film capacitors were polymeric films: from polystyrene to modern lavsan and polypropylene. In most load-bearing and digital units these capacitors work very well. One just needs to remember that special capacitors should be used for high frequencies and high powers.
Tags: capacitor, dielectric, history of technics
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