Silver mica capacitor
Encyclopedia
Silver mica capacitors are high precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...

, stability and reliability capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s. They are available in small values, and are mostly used at high frequencies.

Clamped Mica capacitors

Now obsolete, these were in use in the early 20th century. They consisted of sheets of mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 and copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 foil sandwiched together and clamp
Clamp
Clamp may refer to:*Clamp , a device used to hold an object in a fixed position*Wheel clamp, a device used with road vehicles to prevent theft or enforce parking restrictions*Riser clamp, a device used to support vertical piping...

ed. 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

Generally known as silver mica capacitors, these rendered clamped mica capacitors obsolete. Instead of being clamped with foil
Metal leaf
Metal leaf, also called composition leaf or schlagmetal, is a thin foil used for decoration. Metal leaf can come in many different shades. Some metal leaf may look like gold leaf but not contain any real gold...

s these are assembled from sheets of mica coated on both sides with deposited metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

. The assembly is dipped in epoxy
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

. The advantages are:
  • greater stability, since there are no capacitive airgaps that can change dimension
    Dimension
    In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

  • airtight enclosure removes the risk of oxidation or corrosion
    Corrosion
    Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

     of plates or connections
  • greater capacitance per volume
    Volume
    Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

    , since
    • there are no airgaps between plates and mica
      Mica
      The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

      ,
    • the conducting surfaces can be thinner.
    • no clamp
      Clamp
      Clamp may refer to:*Clamp , a device used to hold an object in a fixed position*Wheel clamp, a device used with road vehicles to prevent theft or enforce parking restrictions*Riser clamp, a device used to support vertical piping...

      ing mechanism is needed


Silver mica capacitors are one of the less popularly used types because of their cost. Example prices:
  • 100pF £0:99
  • 10nF 1% silver mica £4:40


These 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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