Dielectric Materials. Mica.  Of all dielectrics mica is the most suitable for condensers, since it has very high insulation resistance and also high specific inductive capacity, and furthermore may be obtained in very thin sheets.  High-grade condensers, such as are used for measurements and standardization purposes, usually have mica for the dielectric.

[Illustration:  Fig. 121.  Rolled Condenser]

Dry Paper. The demands of telephonic practice are, however, such as to require condensers of very cheap construction with large capacity in a small space.  For this purpose thin bond paper, saturated with paraffin, has been found to be the best dielectric.  The conductors in condensers are almost always of tinfoil, this being an ideal material on account of its cheapness and its thinness.  Before telephony made such urgent demands for a cheap compact condenser, the customary way of making them was to lay up alternate sheets of dielectric material, either of oiled paper or mica and tinfoil, the sheets of tinfoil being cut somewhat smaller than the sheets of dielectric material in order that the proper insulation might be secured at the edges.  After a sufficient number of such plates were built up the alternate sheets of tinfoil were connected together to form one composite plate of the condenser, while the other sheets were similarly connected together to form the other plate.  Obviously, in this way a very large area of plates could be secured with a minimum degree of separation.

[Illustration:  Fig. 122.  Rolled Condenser]

There has been developed for use in telephony, however, and its use has since extended into other arts requiring condensers, what is called the rolled condenser.  This is formed by rolling together in a flat roll four sheets of thin bond paper, 1, 2, 3, and 4, and two somewhat narrower strips of tinfoil, 5 and 6, Fig. 121.  The strips of tinfoil and paper are fed on to the roll in continuous lengths and in such manner that two sheets of paper will lie between the two strips of tinfoil in all cases.  Thin sheet metal terminals 7 and 8 are rolled into the condenser as it is being wound, and as these project beyond the edges of the paper they form convenient terminals for the condenser after it is finished.  After it is rolled, the roll is boiled in hot paraffin so as to thoroughly impregnate it and expel all moisture.  It is then squeezed in a press and allowed to cool while under pressure.  In this way the surplus paraffin is expelled and the plates are brought very close together.  It then appears as in Fig. 122.  The condenser is now sealed in a metallic case, usually rectangular in form, and presents the appearance shown in Fig. 123.

[Illustration:  Fig. 123.  Rolled Condenser]

A later method of condenser making which has not yet been thoroughly proven in practice, but which bids fair to produce good results, varies from the method just described in that a paper is used which in itself is coated with a very thin conducting material.  This conducting material is of metallic nature and in reality forms a part of the paper.  To form a condenser of this the sheets are merely rolled together and then boiled in paraffin and compressed as before.