Silver mica capacitor
Silver mica capacitors are high precision, stable and reliable capacitors. They are available in small values, and are mostly used at high frequencies and in cases where low losses ( high Q) and low capacitor change over the time is desired.
There are 2 distinct types of mica capacitor.
Clamped Mica capacitors
Now obsolete, these were in use in the early 20th century. They consisted of sheets of mica and copper foil sandwiched together and clamped. These had even worse tolerance and stability than other clamped capacitors since the mica surface is not perfectly flat & smooth. References to mica capacitors from the 1920s always refers to this type.
Silvered mica capacitors
Commonly known as silver mica capacitors, these rendered clamped mica capacitors obsolete. Instead of being clamped with foils these are assembled from sheets of mica coated on both sides with deposited metal. The assembly is dipped in epoxy. The advantages are:
- Greater stability, since there are no capacitive airgaps that can change dimension
- Airtight enclosure removes the risk of oxidation or corrosion of plates or connections
- Greater capacitance per volume, since there are no airgaps between plates and mica, the conducting surfaces can be thinner
- No clamping mechanism is needed
Silvered mica capacitors are one of the less popularly used types due to their high cost.
They are sometimes informally referred to as mica capacitors. Any modern reference to mica capacitors can be assumed to mean these, unless pre-war equipment is being discussed.
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