Nikola Tesla U.S. Patent 725,605 - System of Signaling Patent Wrapper Page 56

670 23,847, 3. In a wireless telegraph system, the combination of two or more surfaces differing with respect of one of their electrical constants, means for electrically charging and discharging said surfaces, and an indicating mechanism operative only by conjoint action of two or more currents or voltages generated by waves from sending station, substantially as set forth. G.C. Klean Examiner, Div. 16.