Sheet of Glass
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cut into sheets by an electrical device and taken away to be cut into
PLANIMETRY. Sheet glass has a fire-polished surface; its
thickness is determined by the skill of the operator or the accuracy of
the machine, and as such the surface is infinitely superior to any
artificially ground and polished surface, but it is definitely inferior
to the artificial surface as regards planimetry. Perfect flatness would
turn it into perfect plate glass and this is not yet within our scientific
reach. The world demands an article with a higher degree of flatness than
sheet glass for mirrors, motor-cars, shop windows, etc., and this is
provided by ground and polished plate glass. For the purpose of measuring
planimetry in comparative terms we use an optical method, depending on the
distortion which can be observed by reflection from the surface under
examination, of an illuminated screen ruled with squares.
Figure 4a is reproduced from a
photograph of the ruled screen reflected from the surface of a sheet of plate
glass ¾" thick. Figure 4b is a similar
photograph of the screen after reflection from a piece of quite good sheet
glass, and by way of comparison Figure 4c is an ordinary
¼" plate glass. The amount of distortion might suggest that the
departure from flat glass is considerable. The concavity shown here has in
fact a radius of 300 feet.
P L A T E G L A S S
The making of Plate Glass involves two processes:
(1) The manufacture of the rough glass blanks.
(2) The grinding and polishing of these rough blanks.