英语听力：自然百科 地球力场 Earth's Force Field—4
时间：2014-05-26 07:03:19
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（单词翻译）
Scientists believe that the South Atlantic Anomaly, is a sign of a global decline in the magnetic field. Evidence of this comes from a surprising source.
Dr. Mimi Hill uses
pottery1 to uncover the secrets of earth's magnetic past. Ancient pots tell us about our distant ancestors, but they also tell us how our magnetosphere has changed over thousands of years.
I'm very interested in ancient pottery because it's a really a good recorder of what the earth's magnetic field has done at the time when the pot was fired.
Clay contains tiny particles of magnetic iron-based mineral called magnetite. Normally, these are
randomly2 orientated3. When clay is fired at high temperatures, they start to move around.
All the little magnetic minerals will
align4 up with the earth’s magnetic field, when it cools down, it's locked in.
As the clay cools and hardens, these minerals become re-magnetized by the earth's magnetic field, locking in a precise record of the strength of the field at that moment.
What we are doing is, taking a tiny little core from an ancient pot and put this small core into a microwave air archetype system.
Hill measures the magnetite from an ancient pot. She uses this measurement to calculate the extract strength of earth's magnetic field at the time the pot was fired.
And, yes, we'll go on to microwaving.
She compares this to samples taken from several pots over the last 400 years.
We know this is dated about 1640 to 1670 and can tell from the experiment that the magnetic field strength was stronger by about 10 percent.
Hill's research indicates that our magnetic protection is fading at an alarming rate.
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