The electrodynamic tether is an aluminum wire orbiting the Earth. Electrons are extracted from the adjacent ionosphere, enter the wire at one end, and leave it at the other end. The Earth magnetic field generates voltage in the wire. The electrodynamic tether is similar to an electric motor; it can either convert its orbital energy to electric energy (generator), or it can convert electric energy to orbital energy (propulsion). It is the most efficient when it operates in the area of the highest conductivity of the ionosphere: the altitude of 300 km and the sunny side of the Earth. The electrodynamic tether is unstable, unless it spins or is attached to a rigid structure with both ends.
Roy R. I. Samanta, D. E. Hastings, and E. Ahedo, "Systems Analysis of Electrodynamic Tethers," Journal of Spacecraft and Rockets, Vol. 29, No. 3, 1992, pp. 415-424.
Vladimir Vasilievich Beletskii and Evgenii M. Levin, "Dynamics of Space Tether Systems," Advances in the Astronautical Sciences, Vol. 83, Univelt 1993, pp. 267-322.