If ever we can ascertain at what period the earth's charge, when disturbed, oscillates, with respect to an oppositely charged system or known circuit, we shall know a fact possibly of the greatest importance to the welfare of the human race. I propose to seek for the period by means of an electrical oscillator or a source of alternating currents. (Nikola Tesla in "On Light and Other High Frequency Phenomena," February and March, 1893.)