PMG (Plasma Motor/Generator) was an experiment that involved lowering a 500-m-long space tether from a spent Delta upper stage into the lower part of the thermosphere where atmospheric gas exists in a plasma, or ionized state. The tether consisted of a conducting wire with hollow electrodes at either end. An electric current was produced in the tether in line with expectations, demonstrating the potential of this technique to generate power that could be used by satellites or space stations in low Earth orbit.