Variable capacitors

If you have followed my previous post there I have given the basic idea of a capacitor but now I will give you a clear cut idea of a variable capacitor.In the previous post you have seen how  a capacitor works, now you are going to see how a variable capacitor works.A variable capacitor is the capacitor in which area of the plates can be manipulated ,in simple words you can adjust the plate areas.In this modern day world ,many types of variable capacitors are present but here I will mention a few which in my view are important.

• Variable air capacitor(image ^)-This type of capacitors are mainly found in the tuning circuits of radio's. Here in this figure you will be able to observe that there are fixed and movable parallel plates ,movable plates can be distinctly observed as they are mounted on a shaft.By rotating the shaft the second set of plates(movable one) can be made to interleave the first set by a lesser or a greater extent .But there is one drawback which is that this type of capacitors have less capacitance.
• Paper capacitor(image at the right side)-a paper capacitor is one which cotains paper as the on and only dielectric and you can figure this out from the figure itself .Usually what is done
  
  is that a large sheet of paper (impregnated or you can say pinned with the help of wax) is placed between fold of tin and rolled so as to create a shape of a cylinder.One of the positive feature of this kind of capacitor is that it takes a relatively small amount of  volume to fit in any circuit.
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Electrolytic capacitor(image^)-This type of capacitors are made by passing direct current between the layers of aluminium containing a electrolyte between them.As you can see from the figure itself that a small layer of aluminium oxide is formed on the anode plate(+ve side).The film which is formed acts as a insulator and forms the dielectric between the two plates  whereas the electrolyte is an good conductor.Here you can see that the oxide layer that is formed is relatively small  as a result of which capacitance becomes very large.If the oxide layer breaks down then the whole thing is of no use .
Any suggestions are welcome!!!