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Hot
Rocks: A Natural Source of Electricity
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The nearer you get to the Earth’s
centre the hotter is becomes. Nuclear reactions, caused
by the decay of radioactive materials, constantly heat
the molten core to 7200oF (400oC). Because of this geothermal
energy of a mine than it is at the top.
In some places hot rocks lie quite near the surface, causing
hot springs, geysers or steam to rise out of the ground.
These can be used to produce electricity.
The first geothermal power station was built in 1904,
at Larderello in northern Italy, where steam was coming
out of the ground at temperatures between 280oF and 500oF
(140oC and 260oC). The steam was piped to turbines which
powered generators.
In New Zealand, the Philippines, California and exico,
power stations have been built where the Earth’s
heat reaches the surface naturally. But in most places
geothermal energy has to be tapped by drilling. In some
cases there may be no water present at all, just dry hot
rocks, whose heat can only be used if water is pumped
down to them and then recovered as steam. The steam is
then used to drive turbines and generate electricity.
The granites of Cornwall are a source of geothermal energy
that has recently been tested. Some 650ft (1980m) beneath
Camborne in Cornwall, the rocks reach temperatures of
about 158oF (70oC).
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To extract
energy two boreholes would have to be drilled, cold
water pumped down one and pressurized hot water
returned up the other. The water would flow from
one borehole to the other through fissures in the
rock created by blasting it with explosive. Although
the water is at 390oF (200oC), the pressure it is
under prevents it from boiling. But when it is returned
to normal atmospheric pressure at the surface, it
instantly ‘flashes’ into steam, ready
to drive the turbines.
Like other sites where geothermal energy could be
tapped, Camborne has several problems. Minerals
will have to be removed from the hot water, otherwise
they could fur up pipes and corrode turbines. Tests
have also shown that only one-third of the water
pumped down finds its way back to the surface, the
rest is lost. The third problem will be drilling
deep enough.
If all these problems can be solved, the potential
is enormous. It has been calculated that the Cornish
granites alone contain as much energy as the whole
of Britain’s coal reserves.
More and more countries are looking into geothermal
energy as an alternative to fossil fuels. A major
powers station has been started in New Mexico, and
a joint French and German project is being carried
out near Strasbourg.
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| How
High-Speed Photographs are Taken |
To freeze the beating of an insect’s wing needs
a far shorter exposure than an ordinary camera can
manage. Even at 1/1000th of a second the wings are
a blur. Exposures ten or twenty times shorter are
needed.
The British photographic pioneer W.H. Fox Talbot
also pioneered high-speed photography as long go
as 1851. He attached a copy of The Times newspaper
to a wheel, rotated it rapidly, and succeeded in
taking a clear picture by illuminating the wheel
very briefly with an intense spark of light which
lasted only 1/100,000 of a second. If this technique
is used in a blacked-out room, the camera shutter
can be left open, and the film is exposed for an
instant when the spark goes off.
The greatest difficulty is to arrange for the flash
to go off when the subject is in exactly the right
position. Often the best way is to make the subject,
such as a bullet spending through an apple, trigger
the shutter or flash (or both) itself, by breaking
a fine infrared beam or light beam that is focused
on a reactive cell, for example.
A series of flashes may be used, with the film moving
between each one. This technique was pioneered by
an American, Harold Edgerton, in the 1930s. by using
ten flashes a second and superimposing all the images
on the same frame, he was able to show a drop of
milk splashing into a bowl. |
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