How Things Work DEMONSTRATIONS - How Everything (142 pages)

tempered glass - a bologna bottle • tempered glass - rupert drops • glass fibers

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HOW THINGS WORK: DEMONSTRATIONS

135

Supplies:

1 bottle of salad oil (Wesson works well)
1 Pyrex or Kimax flask or beaker
1 clear container

Procedure:

Put the flask or beaker in the container and

observe that it's plainly visible. Now pour the salad oil
into the container and into the flask or beaker. The flask
or beaker will become essentially invisible.

Explanation:

The indices of refraction of the salad oil

and borosilicate glasses are almost identical. With no
change of speed upon entry or exit from the flask or
beaker, light doesn't refract or reflect, and you can't tell
that the flask or beaker is there.

355. Tempered Glass - A Bologna Bottle

Description:

You use a peculiar glass bottle to pound

in a nail. You then drop a tiny chip of sharp crystal into
the bottle and it falls apart.

Purpose:

To show that the surface stresses experienced

by glass determine its resistance to tearing and break-
age.

Supplies:

1 bologna bottle (available from a scientific sup-

ply company, at non-negligible expense. Sar-
gent-Welch charged $41 for them recently.
Still, they are remarkable.)

1 piece of wood
1 nail with a large head (just to be safe)
safety glasses

Procedure:

Hold the neck of the bologna bottle and tap

the nail into the wood with the side of the round bottle.
Having demonstrated that the outside of the bottle is
extremely tough, hold the bottle upright over a garbage
can and drop the crystal chip that came with the bottle
into the neck of the bottle. When this chip hits the in-
side bottom of the bottle, the bottle will tear itself apart
and its pieces will drop into the garbage can.

Explanation:

The bottle is tempered in such a way that

the outside surface is experiencing compression and
the inside surface is experiencing tensile stress. Since
it's very hard to start a tear in a layer that is being com-
pressed, it's hard to tear the outside of the bologna bot-
tle. But the inside is under tension and the slightest in-
jury to it will cause the surface to tear itself to shreds.

356. Tempered Glass - Rupert Drops

Description:

When you break the tail of a small glass

drop, the drop crumbles into dust.

Purpose:

To show that tempered glass exhibits dicing

fracture when its compressed outer skin is broken.

Supplies:

2 or 3 Rupert drops (available from a scientific

supply company)

1 needle-nosed pliers
cloth gloves
safety glasses

Procedure:

Hold a Rupert drop in your gloved hand

and break off its tail with the pliers. If the drop has
been properly tempered (I've had mixed luck), it will
tear itself to powder. You may have to try more than
one to observe this self-destruction.

Explanation:

The Rupert drops are tempered glass—

their outer surfaces are under compression while their
insides are under tension. When you break through the
compressed surface layer and expose the tense inner
portion of the drop, it tears itself apart.

357. Glass Fibers

Description:

You heat the middle of a glass rod until it

softens and then pull its ends away from one another.
A glass fiber forms in between the ends. This fiber is
relatively flexible and extremely strong for its size.

Purpose:

To show how glass fibers are formed.

Supplies:

1 glass rod
1 gas burner
matches
safety glasses

Procedure:

Light the burner and hold the middle of the

glass rod over the flame. When the glass has softened
significantly, pull the two ends of the rod away from
one another in a smooth and steady motion. Stop when
you have stretched the rod to about 1 m long. Allow
the pieces to cool briefly. Show that the glass fiber is
flexible (don't bend it too far or it will break!). Be care-
ful with the hot ends of the glass until they've had
enough time to cool completely. Be careful with eyes.

Explanation:

The glass fiber's strength comes in part

because of its relative lack of defects on its surface.
With so little surface on any given length of fiber, there

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