The air-ground system evolved from Tesla’s one-wire method of energy transmission over a circuit that was not closed “in the ordinary acceptance of the term.” While there is no physically solid second conductor connected back to the generator, the capacitor plates at both ends of the “one-wire” circuit couple to its counterpart or opposite member directly or through ground, depending upon the distance between the two terminals, thus constituting the return circuit.
This, as well as the description “air-ground method” implies that the system depends upon the existence of a connection between the elevated terminals. Additional support to this assertion is found in the Electrical Transformer patent covering the Tesla coil, which describes electrical power transmission through a single wire with ground for return.
In this figure Tesla shows two additional examples of one-wire transmission. In the arrangement labeled I above, his intention is to show the effect of resonance in promoting the movement of energy along conductor L. Arrangement II diagrams a self-induction coil with a tap near one end, effectively dividing the coil primary and secondary sections. It shows one-wire transmission from the transformer’s free terminal to a single terminal lamp. In both cases, conductor L1 constitutes a part of the return circuit. Also notice the two vertical lines to the extreme left and right in the illustration. These appear to represent the walls of an enclosed space, or, perhaps, nearby parts of the general environment.
Tesla’s 1897 patent drawing showing the transmission of electrical energy through one wire with ground for return (US593,138 - Electrical Transformer).
Also, the apparatus used in the 1898 Patent Office demonstration at the Houston St. lab involved the transmission of electrical energy in industrial amounts through a rarified medium with ground for return.
The figure above showed the transmission of electrical energy through a rarefied atmosphere with ground for return. This is a diagram representing the arrangement of apparatus as in a practical experiment which I performed before G.D. Seeley, Examiner in Chief, U.S. Patent Office, on the 23rd of January, 1898. This experiment illustrates a great departure I had made a little prior to that date. [Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, p. 125]
Tesla’s demonstration for the U.S. Patent Office, on January 23, 1898, at his Houston St. laboratory in New York City, was to show of the practicability of transmission of electrical energy in industrial amounts by the method and apparatus described in the patents US645,576 - “System of Transmission of Electrical Energy” - March 20, 1900 and US649,621 - “Apparatus for Transmission of Electrical Energy” - May 15, 1900. The applications for both patents were filed September 2, 1897. [These are the initial patents specifically covering Tesla’s wireless system.]
In 1898 I made certain demonstrations before the Examiner-in-Chief of the Patent Office, Mr. Seeley, and it was upon showing him the practicability of the transmission that patents were granted to me. [Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, p. 27]
It might be argued the Colorado Springs experiments also served to demonstrate the technology in advance of patent issuance. The above-mentioned patents are dated March 20 and May 15, 1900, about 7 months after Tesla’s return to New York. The Wardenclyffe project served to further demonstrate and refine the method and apparatus, as described in the patent US787,412 - “ Art of Transmitting Electrical Energy Through the Natural Mediums” - April 18, 1905 and US1,119,732 - “Apparatus for Transmitting Electrical Energy” - December 1, 1914.
Tesla’s 1900 patent drawing showing a system for the wireless transmission and reception of electrical energy through the earth’s rarefied upper atmosphere with ground for return. [Transmitter type-one, C/S #1]
"My experiments . . . in Colorado showed that at a height of 1 mile it is plenty enough rarefied to break down under the stress and conduct the current to the distant points. . . . My patent says that I break down the atmosphere "at or near" the terminal. If my conducting atmosphere is 2 or 3 miles above the plant, I consider this very near the terminal as compared to the distance of my receiving terminal, which may be across the Pacific. . . . I have constructed and patented a form of apparatus which, with a moderate elevation of a few hundred feet, can break the air stratum down. You will then see something like an aurora borealis across the sky, and the energy will go to the distant place. . . . An apparatus which permits displacing a certain quantity of electricity in the terminal—we shall say so many units—will produce an electric potential at a distance of 5 miles, and the fall of electric potential per centimeter will be equal to the quantity of electricity divided by the square of the distance. . . . Now, I have satisfied myself that I can construct plants in which I may produce, per kilometer of the atmosphere, electric differences of potential of something like 50,000 or 60,000 volts, and at 50,000 or 60,000 volts that atmosphere must break down and will become conductive". [NTAC]
Furthermore, Tesla made the following statement regarding his theory and technique of energy transmission.
"The earth is 4,000 miles radius. Around this conducting earth is an atmosphere. The earth is a conductor; the atmosphere above is a conductor, only there is a little stratum between the conducting atmosphere and the conducting earth which is insulating. . . . Now, you realize right away that if you set up differences of potential at one point, say, you will create in the media corresponding fluctuations of potential. But, since the distance from the earth's surface to the conducting atmosphere is minute, as compared with the distance of the receiver at 4,000 miles, say, you can readily see that the energy cannot travel along this curve and get there, but will be immediately transformed into conduction currents, and these currents will travel like currents over a wire with a return. The energy will be recovered in the circuit, not by a beam that passes along this curve and is reflected and absorbed, . . . but it will travel by conduction and will be recovered in this way". [NTAC]
Tesla’s diagram explanatory of the transmission of electrical energy by the ground air method. This was first put before Lord Kelvin in the Houston Street laboratory in September 1897.
It should be noted that in describing the “atmosphere above” as being conducting he roughly predicted the existence of the ionosphere and the earth-ionosphere cavity.