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This data is interesting as showing the wide divergence in specific
inductive capacities of various materials, and also showing the wide
divergence in different observations of the same material.
Undoubtedly, this latter is due mainly to the fact that various
materials differ largely in themselves, as in the case of paraffin,
for instance, which exhibits widely different specific inductive
capacities according to the difference in rapidity with which it is
cooled in changing from a liquid to a solid state.

We see then that the capacity of a condenser varies as the area of its
plates, as the specific inductive capacity of the dielectric employed,
and also inversely as the distance between the plates.

Obviously, therefore, in making a condenser of large capacity, it is
important to have as large an area of the plate as possible; to have
them as close together as possible; to have the dielectric a good
insulating medium so that there will be practically no leakage between
the plates; and to have the dielectric of as high a specific inductive
capacity as economy and suitability of material in other respects will
permit.

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 c