"I have just returned from Colorado, where I have been carrying on some
experiments since a few months past. The success has been even greater than I anticipated,
and among other things I have absolutely demonstrated the practicability of the
establishment of telegraphic communication to any point on the globe by the help of the
machinery I have perfected."
The experiments Tesla referred to were preliminary steps taken toward the construction of a prototype global communications facility on eastern Long
Island—a place known as Wardenclyffe. They centered around a unique form of electrical oscillator that Tesla had developed for the production of high voltage, high frequency alternating electrical currents, an invention which his contemporaries dubbed the "Tesla Coil." Also known as the disruptive discharge coil, these devices initially served as power supplies for various experimental high frequency electric lamps and other high frequency apparatus. The Tesla coil was subsequently adapted to the generation of radio frequency currents for Tesla's then decade-long investigation into wireless transmission, offering a superior alternative to the high frequency electrical alternators that he had also constructed for the same purpose.
The main component of the classic Tesla coil is an air core electrical step-up
transformer—an assembly of two concentric wire coils positioned in fairly close proximity to each other. When an alternating current passes through the first coil or primary winding a time varying magnetic field is established which allows the transfer of energy by magnetic induction to the second coil or secondary winding. The transformer's primary is excited by the rapid discharge of a high voltage capacitor through a high speed switching device known as a break or circuit controller. The potential which appears at the secondary's high voltage terminal is developed through a process known as resonant rise. It can greatly exceed the voltage that would be expected from a conventional iron core transformer, using a simple calculation based upon the ratio of primary to secondary turns, that is to say, ratio of transformation. While in operation, the oscillator's primary capacitor is continuously recharged by a regular high voltage transformer, allowing for an uninterrupted flow of radio frequency current in the primary and secondary circuits.
Credit: Tesla Said, John T. Ratzlaff
Nikola Tesla's historic laboratory and wireless
communications facility known as Wardenclyffe, located about 65 miles east of New York
City on the North Shore of Long Island. It was here this creative genius worked out the
final details related to his "World System" for ground-based global
communications. The distinctive 187 foot tall tower was demolished in 1917, but the sturdy
94 foot square building still remains standing in silent testimony to Tesla's unfulfilled
dream.
The magnifying transmitter, which was the focus of Tesla's investigations in Colorado, is an advanced form of radio frequency oscillator specifically designed for wireless transmission. In addition to the primary and secondary inductors which made up the classic Tesla coil, Tesla added a third inductor, actually a helical resonator, known as the extra coil. Power from the grounded primary / secondary combination, now known as the master oscillator, was fed to the lower end of the extra coil helical resonator through a heavy electrical conductor. This centrally located extra coil was separated by a wide space from the other two coils, which comprised the master oscillator section. (Such spacing minimizes inductive coupling between the extra coil and the master oscillator, preventing, for the most part, a portion of the energy that is continuously flowing into the resonator from passing backward through the system and becoming lost.) In addition to maximizing the efficiency of the system, allowing development of the highest possible power output for power consumed, the extra coil also served as the device's main transmitting element.
While Tesla's wireless transmitter and present day radio transmitters are fundamentally the same, the method in which Tesla preferred to use his apparatus was radically different from that which is employed in present day radio systems. Conventional radio transmitters are set up so as to maximize the amount of power radiated from the antenna. When used for long range transmission, such equipment must process tremendous amounts of power in order to counteract the reduction in field strength (P = 1/R2) encountered as the signal radiates outward from its point of origin. In contrast, Tesla's magnifying transmitter was configured so as to minimize the power which was radiated out into space in the form of radio waves. Instead of being connected to a radio antenna, Tesla's prototype wireless transmitter had a large tower-mounted metal structure called an elevated terminal capacitance, positioned directly above the extra coil. (The elevated terminal can be viewed as the smaller of the two plates of an immense
asymmetrical capacitor. The opposite plate is the earth itself and every conducting object on its surface.) There was also a substantial ground terminal consisting of a 300-foot long iron pipe extending downward from the bottom of a 120-foot deep shaft sunk below the tower's base. This structure provided a connection to the earth through which a powerful oscillating electrical current would flow.
Unlike a conventional radio transmitter with an antenna that radiates dissipating electromagnetic waves out into space, the magnifying transmitter's extra coil excites a low frequency ground wave called the Zenneck surface wave. In this case the propagating energy does not radiate into space but is concentrated near the earth's surface. Furthermore, Tesla asserted that it is possible to periodically disturb the equilibrium of the earth's electrical charge and cause it to oscillate with his apparatus. This would be accomplished by superposing an extra low frequency (ELF) signal on the somewhat higher frequency signal coursing through the resonator—the low frequency current in the presence of an enveloping corona-induced plasma of free charge carriers produced by the oscillator in effect "pumping" the earth's charge. It is believed the resulting ground current with the associated Zenneck surface wave complex would have propagated wireless transmissions to any distance around the earth with as little as 5% loss due to radiation. Using a global array of these magnifying transmitters, it was Tesla's plan to establish what he called the World System, providing multi-channel global broadcasting, an array of secure wireless telecommunications services, and a long range aid to navigation, including means for the precise synchronization of clocks. In a more highly developed state he envisioned the World System could expand to include the wireless transmission of electrical power.
