Microlight
Question: When a plate with a gold decorative edge is placed in a microwave
oven, sparks appear at the gold surfaces. Why?
Answer: Any metal object acts as an antenna for electromagnetic radiation,
including microwaves. Electric currents are set up in the metal, which in turn
cause voltage differentials within and between the pieces of metal. Gold rims of
plates inevitably become scratched, and so end up with tiny gaps of less than 1
millimetre between sections of continuous gold metal.
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As a result, when the plate is subjected to microwaves the voltages generated
are applied across these gaps. Because air has quite a low breakdown voltage, it
doesn鈥檛 take much to split the molecules in the air and cause sparks.
I have to credit Eric Billet of Brunel University for this explanation.
Robert Alcock
by e-mail, no address supplied
Answer: Your microwave oven sparks around gold decorated plates because
electrical charges are attracted to sharp metallic points or edges in much the
same way as lightning is attracted to the sharp point on a lightning conductor.
The plate itself is invisible to the microwaves but the gold, being very thin,
has a sharp edge. An electric charge builds up on this edge, and you witness
lightning in your microwave oven.
George Cadigan
by e-mail, no address supplied
The same principle applies to small twists of spiky metal foil. In larger
objects with no sharp points鈥� say a rounded metal spoon鈥� eddy
currents flow in the metal to equalise the voltages, and no sparks are
蝉别别苍鈥抬诲
Answer: Microwaves are a very energetic form of radiation. Metal is receptive
to radiation because of the large number of free electrons floating in its
structure. The sparks you see are from small pieces of the metal that are
protruding or slightly separated from the mass of the gold, heating up and
exploding.
Additionally, microwaves are so useful in food preparation because the water
molecules in food are of approximately the same size as a microwave wavelength
and are easily induced to rotate back and forth, causing frictional heating of
your food.
John Dickson
by e-mail, no address supplied
Myopic mammals
Question: I would estimate that about 40 per cent of people that I know need
glasses or contact lenses for distance vision. Assuming that this sample is
typical of the human race, I would like to know why it is that eye problems
prevalent in humans such as myopia (short-sightedness) seem very rare in wild
animals?
As far as I know, myopia is a genetic condition and so is not usually
acquired by habits such as reading small print (otherwise one would expect
recovery after stopping the habit).
Obviously, it is not easy to test the eyesight of an animal, but if the
incidence of myopia is as high in wild animals as it is in humans then how can
the animals survive? (continued)
Answer: Your correspondent assumes that myopia cannot be environmental, yet
the increased prevalence of the condition in Inuit people from Greenland who
have moved to Denmark, and its increase in recent decades in Taiwan, both point
to an environmental and indeed an educational effect as the children of these
people adapt to a modern lifestyle and educational system. This may explain the
common claim by schoolchildren that their eyesight is stronger after the summer
holidays.
Roger Hitchings
London
Answer: Your correspondent is incorrect to say that the reason why myopia is
common among humans is the lack of natural selection for good eyesight.
In recent times, anthropologists have noted that Inuit had almost universally
perfect eyesight until significant numbers of them became literate. It appears
that literacy causes myopia.
The explanation is that as the eye matures throughout childhood, it is
constantly corrected so that the natural focus of the lens will be in the
position that produces the sharpest image on the retina. If one looks at distant
objects all the time, the focal length will form properly to focus clearly on
objects at long distances. If one spends a lot of time reading books, the eye
will develop in an elongated way, which means objects can be focused most easily
at shorter distances. The constant switching between long and short distances
appears to create myopia.
Patrick Draper
by e-mail, no address supplied
Conquering conkers
Question: I was once advised by a friend that the way to strengthen a conker
before a conker fight is to bake it. From my childhood I remember being told
that the way to improve your conkers was to pickle them in vinegar. Which method
produces winning conkers and why?
(continued)
Answer: I was intrigued by all the dialogue about conkers, and I gather that
they are chestnuts, somehow attached on a string. What is it you British do with
the chestnuts? I suspect it doesn鈥檛 involve consumption.
Jennifer Holtzman
North Hollywood, California
Answer: What kind of game is conkers? Is it like a pillow fight with
rock-hard chestnuts? Sometimes we yokels in the former colonies need a little
别诲颈蹿颈肠补迟颈辞苍鈥�
Jay Kangel
by e-mail, no address supplied
Conker fighting appears to be a mainly British pursuit so we realise we have
to enlighten our international readership. Conkers are the hard fruit of the
horse chestnut tree. These are collected in autumn, removed from their spiky
casing and left to mature. A hole is then drilled in the conker, and a string
threaded through. The full rules of genuine competition are complex but the game
as played by schoolchildren (and overgrown schoolchildren) is between two
opponents each with one conker. One player dangles their conker by the string,
holding it steady, while the opponent swings their conker on its string and
attempts to strike the hanging conker. Players take it in turns to do this until
one conker is so damaged that it is dislodged from its string. The winner is
obviously the player with the intact conker. Naturally, the stronger and harder
the conker, the more chance of success.
This is perhaps further proof, if it were needed, of the British obsession
for devising eccentric and meaningless methods of competition鈥擡d
This week鈥檚 question
Milking the issue: Cows eat lush grass in summer but prepared dried feed in
winter. Does the milk that I pour on my breakfast cereal differ in any way
throughout the year?
Graeme Mawson
Newcastle upon Tyne