Wiki article on electrodynamic tethers....<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>Voltage and current<br /><br />When a tether is moved at a velocity (v) at right angles to the Earth's magnetic field (B), an electric field is observed in the tether's frame of reference. This can be stated as:<br /><br /> E = v * B = vB<br /><br />The direction of the electric field (E) is at right angles to both the tether's velocity (v) and magnetic field (B). If the tether is a conductor, then the electric field leads to the displacement of charges along the tether. Note that the velocity used in this equation is the orbital velocity of the tether. The rate of rotation of the Earth, or of its core, is not relevant. In this regard, see also homopolar generator.<br /><br />NASA has conducted several experiments with Plasma Motor Generator (PMG) tethers in space. An early experiment used a 500 meter conducting tether. In 1996, NASA conducted an experiment with a 20,000-meter conducting tether. When the tether was fully deployed during this test, the orbiting tether generated a potential of 3,500 volts. This conducting single-line tether was severed after five hours of deployment. It is believed that the failure was caused by an electric arc generated by the conductive tether's movement through the Earth's magnetic field.<br /> /><br />Surges<br /><br />As mentioned earlier, conductive tethers have failed from unexpected current surges. Unexpected electrostatic discharges have cut tethers (eg. see Tethered Satellite System Reflight (TSS-1R) on STS-75), damaged electronics, and welded tether handling machinery. <font color="yellow">It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed.</font>p><hr /></p></blockquote> <br />Understatement of the decade <img src="/images/icons/tongue.gif" /> <div class="Discussion_UserSignature"> </div>