In 1919 Nikola Tesla wrote,
The popular impression is that my wireless work was begun in 1893, but as a matter of fact I spent the two preceding years in investigations, employing forms of apparatus, some of which were almost like those of today. It was clear to me from the very start that the successful consummation could only be brought about by a number of radical improvements. Suitable high frequency generators and electrical oscillators had first to be produced. The energy of these had to be transformed in effective transmitters and collected at a distance in proper receivers. Such a system would be manifestly circumscribed in its usefulness if all extraneous interference were not prevented and exclusiveness secured. In time, however, I recognized that devices of this kind, to be most effective and efficient, should be designed with due regard to the physical properties of this planet and the electrical conditions obtaining on the same. I will briefly touch upon the salient advances as they were made in the gradual development of the system. . . .
Fig. 7. Transmission of Electrical Energy Thru the Earth as Illustrated in Tesla's Lectures Before the Franklin Institute and Electric Light Association in February and March, 1893, and Mechanical Analog of the Same.
My original plan of transmitting energy without wires is shown in the upper diagram of Fig. 7, while the lower one illustrates its mechanical analog, first published in my article in the Century Magazine of June, 1900. An alternator, preferably of high tension, has one of its terminals connected to the ground and the other to an elevated capacity and impresses its oscillations upon the earth. At a distant point a receiving circuit, likewise connected to ground and to an elevated capacity, collects some of the energy and actuates a suitable device. I suggested a multiplication of such units in order to intensify the effects, an idea which may yet prove valuable. In the analog two tuning forks are provided, one at the sending and the other at the receiving station, each having attached to its lower prong a piston fitting in a cylinder. The two cylinders communicate with a large elastic reservoir filled with an incompressible fluid. The vibrations transmitted to either of the tuning forks excite them by resonance and, thru electrical contacts or otherwise, bring about the desired result. This, I may say, was not a mere mechanical illustration, but a simple representation of my apparatus for submarine signaling, perfected by me in 1892, but not appreciated at that time, altho more efficient than the instruments now in use.
Fig. 8. Tesla's System of Wireless Transmission Thru the Earth as Actually Exposed In His Lectures Before the Franklin Institute and Electric Light Association in February and March, 1893.The electric diagram in Fig. 7, which was reproduced from my lecture, was meant only for the exposition of the principle. The arrangement, as I described it in detail, is shown in Fig. 8. In this case an alternator energizes the primary of a transformer, the high tension secondary of which is connected to the ground and an elevated capacity and tuned to the imprest oscillations. The receiving circuit consists of an inductance connected to the ground and to an elevated terminal without break and is resonantly responsive to the transmitted oscillations. A specific form of receiving device was not mentioned, but I had in mind to transform the received currents and thus make their volume and tension suitable for any purpose. This, in substance, is the system of today and I am not aware of a singe authenticated instance of successful transmission at considerable distance by different instrumentalities. It might, perhaps, not be clear to those who have perused my first description of these improvements that, besides making known new and efficient types of apparatus, I gave to the world a wireless system of potentialities far beyond anything before conceived. I made explicit and repeated statements that I contemplated transmission, absolutely unlimited as to terrestrial distance and amount of energy. But, altho I have overcome all obstacles which seemed in the beginning unsurmountable and found elegant solutions of all the problems which confronted me, yet, even at this very day, the majority of experts are still blind to the possibilities which are within easy attainment.
— "The True Wireless," Nikola Tesla, Electrical Experimenter, May 1919. . . In my patents, frequencies were specified ranging all the way from about 1,000 per second up to 200,000 or 250,000, as I have set forth in the patents; but I have also indicated that the frequency will depend on what use the apparatus is to be put to. In the apparatus patent, I have stated that there were instrumentalities with which you can generate, if you like, electromagnetic waves. It will give you a very high pressure and a very big current. It is an ideal apparatus for the production of these electromagnetic waves. If you want to transmit electromagnetic waves, use a frequency of 500,000 if you choose; but, if you want to transmit the way I transmit [1], you will use a very low frequency so that the loss in these electromagnetic waves . . . should be minimized. . . .
. . . the apparatus which I devised was an apparatus enabling one to produce tremendous differences of potential and currents in an antenna circuit. These requirements must be fulfilled, whether you transmit by currents of conduction, or whether you transmit by electromagnetic waves. You want high potential currents, you want a great amount of vibratory energy; but you can graduate this vibratory energy. By proper design and choice of wave lengths, you can arrange it so that you get, for instance, 5 percent in these electromagnetic waves and 95 percent in the current that goes through the earth. That is what I am doing. Or, you can get, as these radio men, 95 percent in the energy of electromagnetic waves and only 5 percent in the energy of the current. Then you are wondering why you do not get good results. I know why I do not get good results in that way. The apparatus is suitable for one or the other method. I am not producing radiation in my system; I am suppressing electromagnetic waves. But, on the other hand, my apparatus can be used effectively with electromagnetic waves. The apparatus has nothing to do with this new method except that it is the only means to practice it. So that in my system, you should free yourself of the idea that there is radiation, that energy is radiated. It is not radiated; it is conserved. . . . — Nikola Tesla On His Work With Alternating Currents, 1916
Diagram of Tesla's wireless system drawn by
himself for R.U.J. [Robert Underwood Johnson] Aug. 14, 1914.
1 "As it turns out, the Zenneck wave is generally difficult to excite with a realistic source [a short vertical monopole] because it has a rather slow decay with height above the earth's surface. But there is still an open question whether other types of sources may not be more favorable. . . . An infinite vertical aperture with a height variation corresponding to that of the Zenneck wave will excite only the Zenneck surface wave with no radiation field. . . . The infinite Zenneck aperture excites no radiation field and the pure Zenneck surface wave is the expected result. . . .
"A layered or slightly rough surface which results in a more inductive surface impedance could yield a more advantageous situation. A much smaller vertical aperture would then be sufficient. . . . " — Hill, D. A., and J. R. Wait, "Excitation of the Zenneck Surface Wave by a Vertical Aperture (on a Planer Earth)," Radio Science, Vol. 136, No.6, 1978, pp. 967-977.
