Shell No. 2 is made from a ceramic material. When
it was fired, the more fusible materials melted
and formed glasses. The glassy bond between the
unmelted grains is all that holds the body together
and it serves as a matrix from which crystals form
and interlock. This glassy bond gives the material
its dielectric strength. The quasi-crystalline
structure tends to be non-porous but is not wholly
so. Usually it is covered in by a glaze --
chemically and physically a glass not more than a
few thousands of an inch thick! In a comparison
with Shell No. 1 this composite shell has higher
coefficient of expansion, larger dielectric constant
and power factor. The inherent dielectric strength
is very much lower so that thicker sections are
needed to withstand the same voltage. The surface
resistivity is simply that of the glassy glaze.
MANUFACTURE
There can be no doubt that shell No. 1 is
manufactured by processes which are fewer in number,
simpler, and easier to control. Because it is pressed
in an accurately machined mold under high pressure,
and because it shrinks very little in the process,
the external and internal surfaces, including the
pin-hole, can be accurately and uniformly
maintained. Because they are transparent these
shells can be visually inspected, clear to the
bottom of the pin-hole, at every step in their
manufacture.
. . . On the other hand, the glass design, once
it is established, cannot be readily changed, for
the mold equipment is too expensive.
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Nor can the glass be readily furnished in colors,
to correspond to different glazes. Because of
pressing limitations some shapes -- guy strain
insulators, for example -- are difficult to make
at all. Glass costs vary so greatly with quantity
that the lowest costs can be obtained only with
relatively large volume.
During manufacture, glass is likely to acquire
permanent internal stresses as a result of cooling
from a hot, softened state to a cooler, hardened
state. The same thing is true of the competitive
material. Then they are called "firing stresses."
Sometimes these stresses are detrimental -- if so,
in glass, they are removed by annealing.
On the other hand, it if often advantageous to
deliberately introduce and maintain stresses of
controlled type and magnitude. Tempered
or "toughened" glass is the result.
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