The coil is charged by collisions with ions in the ionosphere, which means there is a transfer of energy when the coil gains charge, this means there is a p.d. which then causes a supply current. I.e. the supply p.d. causes the supply current, therefore a p.d. always causes a current even the induced emf causes a current which both oppose the supply. Note that this electrodynamic tether can be used as a generator or motor.