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In 1996, NASA experimented with such a thing. It used a tethered satellite (the tether containing metal and being 12.5 miles long), moving through the magnetosphere. It worked under the same principle as a dynamo, where a moving wire in a magnetic field would create electricity. One part of the circuit was the tether itself, where the other was the ionosphere, the distant, ionized region of our atmosphere. This effect is not without cost, however, as the magnetic field opposes the motion of the system, thus lessening its kinetic energy and degrading its orbit. The tether exploited about 1 ampere at 3500 volts of electricity.
See more about this experiment at:
Tethered Satellite System (TSS-1R) by NASA
'When I examine myself and my methods of thought, I come to the conclusion
that the gift of fantasy has meant more to me than my talent for absorbing
positive knowledge.'