Grey matter
Question: The surfaces of the incandescent light bulbs where I work become
progressively greyer over time. Why?
Answer: The greying of the inner surfaces of incandescent bulbs is the result
of gradual evaporation of tungsten from the filament while the light is on. This
evaporation eventually makes the filament so thin it burns out.
Various methods have been developed to reduce greying. Filaments of the first
incandescent lamps burnt in a vacuum, but it was soon found that introducing
inert gas to the bulb reduced the rate of greying. A mixture of nitrogen and
argon is used today. In addition, 鈥済etters鈥濃�攔eactive metals such as
tantalum and titanium鈥攃an be placed near the filament to attract the
tungsten so that it is not deposited on the glass. Alternatively, a small amount
of abrasive tungsten powder can be placed in the bulb. Shaking it occasionally
will remove the grey coating from the surface of the glass.
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Greying can be almost eliminated by introducing a small amount of the
halogens iodine and bromine. As tungsten evaporates from the filament, it reacts
with the halogens which then redeposit the tungsten on the filament. This keeps
the bulb wall clean. To prevent the tungsten halides from condensing on the bulb
and breaking the cycle, the temperature of the bulb wall must be at least 500
掳C. This is too hot for glass bulbs, which normally operate at about 150
掳C, so fused quartz (silicon dioxide) must be used instead.
Compared with ordinary incandescent lamps, quartz-halogen lamps have longer
lives and maintain their light output over time. For example, a quartz-halogen
lamp with a 2000-hour life will have dimmed by less than 5 per cent by the time
it burns out. When an incandescent lamp with a 1000-hour life burns out, it will
have dimmed by more than 15 per cent.
Ross Firestone
Winnetka, Illinois
Answer: This can be explained by the fact that lights work not by emitting
light but by sucking dark. 鈥淒ark sucker鈥� theory is too complex to be described
here in detail, but it proves the existence of dark, that dark is heavier than
light, that dark is coloured, and that it travels faster than light.
To answer your question, a bulb becomes darker over time because of all the
dark it has sucked in. Similarly, a candle, which is a primitive type of dark
sucker, has a white wick when new and this becomes black when used, due to all
the dark which has been sucked into it.
Ken Walke
Wigan, Lancashire
Readers should note that the revolutionary 鈥渄ark sucker鈥� theory has yet to
win widespread support from the scientific community鈥擡d
Christmas cracker 1
Question: During Christmas lunch, an argument began over whether a water jug
forming the container for a water filtration system was made of glass or plastic
(or even some combination). When struck, the jug did not 鈥渞ing鈥� like glass and
some of its curves were suspiciously smooth. But in other respects the jug felt
exactly like glass. How could the argument be settled without smashing the
jug?
Answer: You could first weigh the jug in question and then immerse it in
water to find its volume by measuring the amount of liquid it displaces. From
this you can calculate its density by dividing its mass by its volume. Compare
this density with the densities of glass, which lie between 2.4 and 2.7 grams
per cubic centimetre (g/cm3), and plastic, which has a density between 0.9 and
1.2 g/cm3.
Any clear moulded plastic used in a water-filter jug would most likely be
polycarbonate, which has a density of 1.2 g/cm3. Polycarbonate can be made as
clear as glass, with high resistance to scratching and impact damage. Today鈥檚
sophisticated moulding techniques can virtually eliminate telltale mould-split
lines and injection points.
Howard Fulford
Malmesbury, Wiltshire
Answer: Bulk physical properties will distinguish glass from plastic. The
most obvious differences are the lower density and thermal conductivity of
plastics compared with glass. Many plastics float in water, others sink only
slowly when submerged. In contrast, a glass jug of a similar size and shape
would sink rapidly. This ought to settle it, short of doing the classical
Archimedean determination of density.
The lower thermal conductivity of plastics means that their surfaces more
rapidly approach the temperature of a hand that is in contact with them. If you
pick up plastic and glass objects that have been cooled in a fridge, the glass
object will feel much colder. An extreme example of this effect is that foamed
polystyrene cups feel warm, even without any hot coffee in them.
John Smith
Cranleigh, Surrey
Answer: If it is a clear jug it is likely to be glass, acrylic or
polycarbonate. You could heat a needle over a candle flame until it is red hot
and then try to push it into the surface of the jug (in an inconspicuous place).
If it is plastic it will penetrate the surface, but be careful not to go right
through.
John Scheirs
Casula, New South Wales
Answer: You might try placing the offending object between two Polaroid
filters鈥攁 good pair of Polaroid sunglasses or photographic polarisers will
do. Because glass is not a true crystalline solid, it will have no polarising
effect on the light. So when you place the jug between the two filters and
rotate one of them by 90掳, they will eliminate all of the light. Most
plastics, however, will display a riot of colour when treated similarly. The
plastic, being crystalline, messes with the polarised light passing through it
and generates quite a light show.
With the help of a polarising microscope you can use the same technique to
find out whether your insulation is fibreglass or asbestos.
Daniel Goodale
Chemical Teams Chief
US Army
Answer: Place your thumbs together against one of the walls of the jug and
push inwards. If you can feel the wall flex it鈥檚 almost certainly plastic.
Paul Covacich
New Zealand
Answer: Teeth are the best way to distinguish between glass and plastic,
using a combination of feel and sound. Gently knock the object against an
incisor. Plastic produces a dull thud, while glass gives a high-pitched clink.
Wristwatches are a good test for this theory because some have a glass front and
others plastic.
Ben Wright
Sydney