The benefits of wireless power transfer have not changed since originally described by Tesla in 1900  and 1904 . A cheap, efficient means of distributing energy would revolutionize development and improve access to new energy sources. Energy could be coupled into the cavity at the source, eliminating the need for the costly and time-consuming process of constructing and maintaining power transmission lines. The system would enable better utilization of remote sources of energy and would facilitate power transfer to remote users worldwide. While Tesla primarily proposed supplying power for lighting in conjunction with his high frequency single electrode lighting systems, he also envisioned ":..energy of a waterfall made available for supplying light, heat and motive power anywhere - on sea, or land or high in the air... ". Of course, the economic viability of such a system depends upon either 1) a technical means for controlling/measuring the supply and use of wireless power around the world, or 2) a very low cost source energy.
Nikola Tesla was a prolific inventor best known for the AC induction motor and AC polyphase distribution system which are the basis for our present AC power system. His other inventions include the Tesla coil, high frequency generators, the Tesla Magnifyomg Transmitter, key elements of radio, single electrode high frequency, the single electrode x-ray tube, a viscous turbine, and remote control. Following his developments in low frequency AC machines and power distribution systems , Tesla experimented with single electrode, high frequency, high voltage lamps utilizing rarefied gases, the forerunner of present fluorescent lights. Initially he utilized patented high frequency alternators with 384 poles to produce the necessary 20 KHz power, but subsequently invented the disruptive discharge high voltage transformer, a.k.a. Tesla Coil, in 1891 .
In a Tesla Coil, low frequency AC power is amplified in voltage with a conventional transformer. The output of this transformer feeds the Tesla Coils' resonant LC primary circuit through a spark gap. The spark gap creates a broad spectrum of energy, components which resonate the primary and secondary circuits of the Tesla coil. The secondary of the Tesla Coil is tuned to be electrically 1/4 wavelength long, with one terminal grounded, and acts as a "slow wave" device to resonantly amplify the voltage further.
Tesla found that the high frequency output from the Tesla coil could readily power lights and motors utilizing a single wire with a ground return. Tesla presented these results in this lecture to the IEE in London in 1892 . Following the work of Kelvin and Crookes, Tesla also noted that slightly rarefied gases were excellent conductors, leading him to propose a system for "... transmitting intelligence or perhaps power, to any distance through the earth or environing medium".  In February 1893, at his lecture on high frequency currents before the Franklin Institute of Philadelphia (repeated in March in St Louis.) Tesla proposed to determine the capacitance of the earth and the period of oscillations resulting from a disturbance of the earth's charge . After subsequent patented improvements to the Tesla Coil Tesla patented the single wire power distribution system in March., 1897,  and patented the wireless power distribution 6 month later [36,37]. In the wireless system , the single wire conductor was replaced by a conductive path through a slightly rarefied gas coupled to bodies of large surface area, or open capacitors, connected to the high tension terminals of the transmitter and receiver, thus forming an open resonator circuit between the body and the earth. In his patent, Tesla claimed the use of the conductive layers in the upper atmosphere as the conductive path.
in the 1892 lecture in London, Tesla noted that " It is quite possible, however, that such 'no wire' motors, as they might be called, could be operated by conduction through the rarefied air at considerable distances. Alternate currents, especially of high frequencies, pass with astonishing freedom trough even slightly rarefied gases. The upper strata of the air are of difficulties of a merely mechanical nature. There is no doubt that with the enormous potential is obtainable by the use of high frequencies and oil insulation, luminous discharges might be passed through many miles of rarefied air, and that by thus directing the energy of many hundreds of thousands of horsepower, motors or lamps might be operated at considerable distances from stationary sources. But such schemes are mentioned merely as possibilities. We shall have no need to transmit powers in this way. We shall have no need to transmit powers at all. Ere many generations pass, our machinery will be driven by a power obtainable at any point of the universe... " Tesla demonstrated plasma conduction in a glass tube with rarefied air surrounding a central axial platinum electrode, he observed that the wire was heated only at the ends, and not in the middle. He also observed that the pressure at which the gas becomes conducting is directly related to the applied voltage.
Tesla moved to Colorado Springs in May 1899, after reaching the limits of his New York Laboatory with Tesla Coils operating at 4 million volts. The dry, electrostatic filled air at the 2000 m facility in Colorado Springs facilitated his developments. His primary and secondary coils w e r e 5 1 ft. in diameter, and it was here that he developed the concept of an extra coil placed in series with the secondary but with loose inductive coupling so as to enable large resonant amplification of voltage. In addition to the development and improvement of the high power Tesla coil, Tesla concentrated on the development of sensitive receivers necessary for detecting communication signals. On July 3, 1899, using these devices, Tesla monitored the progression of a passing thunderstorm, observing electrical standing waves which he attributed to the storm's disturbance of the earth's electrical charge and a corresponding propagation of this disturbance around the conductive globe. Tesla also experimented with his single electrode x-ray tubes. The oscillator reportedly operated at frequencies between 45KHz and 150 KHz, at voltages between 12 MV and 18MV, and with secondary currents as high as 1100A [1,12,12,39].
Funded principally by J.P. Morgan, Tesla proceeded with the construction of a system of "World Telegraphy" at Wardenclyffe on Long Island upon his return from Colorado Springs in 1900 [12,13]. While he intended to use the facility publicly for communications, Tesla's secret aim was to implement wireless power transfer. The facility featured at 187 ft. wooden tower designed to support a 68 ft. diameter copper hemisphere, which was not completed because of Tesla's difficulty in obtaining funding following Marconi's success in demonstrating transoceanic wireless communication with much simpler equipment (albeit using Tesla's patents in the process) The transmitted was to have operated at 30 MV, which Tesla claimed was sufficient for worldwide power distribution; however, the transmitter was designed to handle up to 100MV. Aside from its toroidal elevated capacitor, patent 1,119,732  filed in 1902 shows the Wardenclyffe configuration of the transmitter, which incorporated the 'extra coil" from the Colorado Springs experiments.
Tesla's Concept of Worldwide Wireless Power Transfer
Tesla outlined the requirements for wireless powcr distribution in patent 787.412, describing the earth as " behaving like a perfectly smooth or polished conductor of inappreciable resistance with capacity and self-induction uniformly distributed along the axis of symmetry of wave propagation". He described reflections of signals from antipodes, the points on the globe diametrically opposite from the transmitter, as being similar to those from the end of a conducting wire, thus creating stationary waves on the conductive surface. He provided three requirements for resonance: 1) the earth's diameter should be equal to an odd number of quarter wavelengths, 2) the frequency should be less than 20 KHz to minimize Hertzian radiation; and 3) most critical, the wave train should continue for a minimal period which he estimates to be 1/12 second, and which represents the period of time for a wave to propagate from and return to the source at a mean speed of 471,240 Km/sec. Tesla conceived the wave as propagating through the earth along a straight line path, the effect on the outside surface being that of concentric rings expanding to the equator and then contracting until reaching the opposite pole. Tesla also applied a fluid analogy to the earth and the water level representing the earth's state of charge at any given point. While his earlier work emphasized ground currents as the mechanism for transferring power, he later indicated that he had conclusively demonstrated that "... with two terminals maintained at an elevation of not more than thirty thousand to thirty five thousand feet above sea level, and with an electrical pressure of fifteen to twenty million volts, the energy of thousand of horse-power can be transmitted over distances which may be hundreds, and, if necessary, thousands of miles. In am hopeful, however that I may be able to reduce very considerably the elevation of the terminals now required... ".
Summary of Tesla's Proof of Concept
Tesla claimed to have observed the effects of the Colorado Springs transmitter at a distance of up to 600 miles. An advertising brochure for the World Telegraphy system claims the transmission of power around the globe in sufficient quantity to light incandescent lamps (50watts). Others report that a bank of 200 watt lamps, 50 watts each, were lit at a distance pf 26 miles [12,13]. The article in Century magazine shows photographs of an isolated extra coil powering and incandescent lamp as evidence of "...electrical vibrations transmitted to it through the ground from the oscillator..." . However, this extra coil was most likely within the inductive field of primary transmitter, with the ground serving as a return path.
Rationale for a Renewed Interest in Wireless Power Transfer
Given Tesla's firm and unending belief in the feasibility of wireless power transfer, yet his inability, after considerable expenditure of time and money, to conclusively demonstrate its viability, the reader may question why there is a renewed interest in demonstrating the feasibility of wireless power transfer. Aside from the benefits outlined initially, the best reason probably lies in both 1) the legacy of Tesla himself, and 2) the fact that because of insufficient funding, Tesla was never able to teat a facility that had been developed strictly for power transfer, and thus hi wireless power transfer concept remains to be proven.
The legacy of Tesla speaks for itself in terms of his many and varied significant inventions, his insightful pioneering understanding of physics and electrical engineering, his tremendous drive and creative energy enabling him to constructively, work long hours on a protracted basis guided by a keen sense of vision, his ability to visualize and test concepts in his mind enabling him to achieve good results with little trail and error, and his genuine concern for improving the condition of humanity. The breath of his accomplishments at Colorado Springs with less than 8 months exemplifies these. The Colorado Springs experiments focused primarily in the development of wireless communications, i.e., radio rather than wireless power transfer. As indicated by Marincic , 56% of his time was spent in developing the Tesla Coil, 21% on receivers for small signals, 16% on measuring the capacity of the vertical antenna, and 6% on miscellaneous other research, including fireballs. Wireless power transmission experiments were limited to small distances. Tesla shared much with the world in the form of his patents, publications, lectures, he was also a very secretive person, and never fully documented his intended configuration for the wireless power system, even though he was confident there would be a workable solution. He believed that that his Magnifying Transmitter (Tesla Coil w/extra coil designed to excite the earth) would ultimately be recognized as his greatest invention , and felt that there would be no problem in wireless disturbing the earth's energy. He also believed the universe to be so full of energy that, ultimately, wireless distribution would not be necessary. Modern day researchers attempting to follow his path, must also be part detective. Tesla's belief and confidence in wireless power transfer is clear, however, so too was Edison's belief in magnetic ore separation, which, like Tesla's experience with Wardenclyffe, left him in deep financial debt. 
In recent years, there has been a renewed interest in Tesla's work on high voltage, high frequency phenomena. Beginning in 1968, R. Golka formed Project Tesla to measure, under Air Force Contract , aircraft susceptibility to lighting discharge and to repeat Telsa's ball lighting experiments for application to laser fusion, hi the process, he replicated Tesla's Colorado Springs transmitter and succeeded in operating it at twice Tesla's original power levels [25,26]. In 1986, Golka and Grotz proposed the application of this device to artificially resonating the earth-ionosphere waveguide .
Cheney reports on wireless power projects that had been planned and some partially implemented circa 1977-1980 in Canada, Central Minnesota and Southern California.  Wait indicated how Tesla's early wireless experiments were the forerunner of modern developments in ELF. He observed that Tesla's fluid analogy for the process is faulty in its assumption that all of the signal energy would propagate through the fluid medium, i.e. the earth. Also faulty was Tesla's notion that energy propages to the antipode via the center of the earth, although it is not known if Tesla had viewed this as a conceptual model as opposed to a physical model as presently interpreted.
Marincic, in his annotations of Tesla's Colorado Springs Notes [1,2] and his excellent review of Tesla's wireless work  applies results from recent ELF experimental data to show that the transfer of power via ELF radiation would be extremely inefficient. He indicates that for a typical gridded ELF antenna, 106 m. total length, that the antenna operating efficiency would be only 0.026% and for both receiving and transmitting antennas, the total efficiency would be (0.026%), not to mention the path loses, which are as low as 0.25 dB/Mm at 10Hz and 0.8dB/Mm at 50Hz. For a fixed size antenna, efficiency increases with operating frequency, but so do path losses, so that for long distance power transfer, the overall efficiency of a radiation-based system will be low.
Corum and Corum [27-31] also replicated some of Tesla's Colorado Springs fireball experiments but with much smaller scale equipment. This work extended to a critical engineering evaluation of Tesla's wireless power concept. [20-23], showing how the current moment in the tower of Tesla's transmitter could be used to excite the Shummann resonances in the earth-ionosphere cavity. They also hypothesized that Tesla intended to use hit single electrode x-ray to both ionize a current path to the sphere of elevated capacitance and to rectify the RF energy enabling the sphere to be electrostatically charged at RF rates [20,21] The sphere would then be discharged to ground, either naturally or via a second x-ray device, at a Schumman resonance frequency. Corum and Corum have also verified that Tesla's electrical measurements such as the attenuation constant, phase velocity, cavity resonant frequency and Q are consistent with modem measurements  and that the loses due to glow discharge around the transmitter would be small .
J. F. Corum patented a toroidal helical antenna [46,47] one of whose applications could be a waveguide probe for either ELF communications or wireless power transfer. This antenna is physically small while reportedly possessing good radiating efficiencies with vertical polarization. Since the propagating Schumman modes are primarily vertically polarized, a vertically polarized antenna would have a distinct advantage over the horizontally polarized example presented by Marincic. However, in applying Corum's design formula to the 8 Hz example presented in his patent, one finds that an antenna with a 6 Km major radius (0.0002) free space wavelengths) would require a virtual continuum of 43, 200 semicircular loops each 600 m in diameter, with a total conductor length equivalent to half the circumference of the earth.
The Q of the earth-ionosphere cavity is generally reported to be about 6-8 but Corum and Spaniol  indicate that a low Q cavity does not necessarily limit the practicability of wireless power. However, Sutton and Spaniol  found that the previously measured Q values were limited by instrument noise and using modern equipment they measured levels as high as 1000, which they say were also confirmed by others. .
In 1986-1988, Nash, Smith, Craven and Corum of WVU utilized a 1/4 wave coaxial resonator to develop a high frequency "Tesla Coil" and proposed coupling this device to a Tesla single electrode x-ray tube to generate ionizing radiation with possible application to wireless power transfer .
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It seems like the efforts to find the best alternative energy sources are seriously being looked into by lots of countries including most US cities. One proof is the signing of the Kyoto Treaty. The main aim of the concerned group and individuals is to lessen the greenhouse gases and pollutants.