Up: Glassmaking
Sheet of Glass
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rapidly by means of air or some other cooling medium. The resulting piece
of armourplate may be compared to a sandwich in which there are two outer
layers in compression, separated by an inside layer in tension. Its
characteristics are great strength, and the property of breaking into
fragments which are small, blunt and relatively harmless.
The following demonstrations illustrate these
properties:--
- Twisting properties. On the machine a piece of
armourplate 45×10" was twisted through an angle of 12° in each
direction at the rate of 750 reversals per minute.
- Bending properties. Equal loads give the same deflection on both
armourplate and plate glass, but the armourplate withstands a much greater
load and a correspondingly greater deflection before fracture. It was shown
how a piece of ordinary glass breaks under a given load and how much more
weight a piece of armourplate will stand.
- Impact tests. These are usually applied by dropping a steel ball
on the centre of the plate supported near its ends, and for ¼" glass
the ball can be dropped about seven times as high for armourplate as for
plate glass before fracture. A piece of ordinary plate glass 1" thick was
broken by a ball weighing 1 lb. 10 oz. dropped from a height of about 18",
while an armourplate portlight ½" thick was unbroken by the impact
of the same ball dropped from a height of 12 ft.
- Type of fracture. When the glass, which was fixed in a wooden
frame, was broken, the ease with which it could be pushed out, and the
non-cutting nature of the fragments were demonstrated.
- The Rupert's drop, the earliest form of toughened glass actually
demonstrated in this room by Faraday himself many years ago, was broken in a
jar full of water to show the effect of releasing the energy stored in the
Rupert's drop. Upon shattering the drop the jar was also broken.
In conclusion may I tell a short story? It is an
undoubted fact that very important changes have taken place been 1918 and
1933. When I
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