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Why did Tesla make his coil in the first place? What was
it that he was trying to accomplish? Other than just the fun of
making one, do they have any practical purposes?
 Tesla
built his first oscillator as a tool for studying high-frequency
electrical phenomena. He also constructed high-frequency
electrical alternators for the same purpose. When it comes to
describing practical applications for this type of electrical power
supply, top on my list is high-frequency electric lighting.
Tesla's investigations in the area of high-voltage
RF power processing techniques resulted in the very first high-efficiency, high-frequency lighting ballasts.
He experimented with both fluorescent- and incandescent-type lamps, as
well as high-frequency arc lighting. Some of the modern electronic
fluorescent ballasts which first appeared in the late 1980s have great similarities to those
he developed in the 1890s, the principal difference being replacement of
the mechanical switching mechanism with a solid-state device.
 Early
on in the course of his research Tesla made another remarkable
discovery. Using the principle of electrical resonance he found
that it is possible to eliminate one of the two conductors normally used
to carry current from the power supply to the electrical load. He
created a light bulb specifically designed to be used in this manner,
called the carbon button lamp It was generally based upon the
dual-terminal tubes developed by William Crooks. He also developed
a single-wire electric motor that could be operated in much the same
manner. This involves attaching a metal plate to one of the
high-voltage power supply terminals in place of one of the connecting
wires. The electrical capacitance at the load, the refractory
button or in the case of the motor a second metal plate, serves to
complete the circuit through "electrostatic induction," sometimes
referred to as 'capacitive coupling.' [The Inventions, Researches and Writings of Nikola Tesla]
 In operating various devices with his high-frequency power supply using only one connecting wire he realized
the load can placed at some distance from the power supply and still
function properly. This is what Tesla called the transmission of
electrical energy through one wire without return. While a
load connected to only one of the high-potential terminals does not form
a closed circuit, —in the ordinary acceptance of the term— the
circuit is closed in the sense that a return path is established
back to the secondary by so called displacement currents. This return
path is directly between the two metal plates. At greater
distances the capacitive coupling to ground predominates at both the
transmitter and the receiver ends of the circuit. Instead of using
individual capacitor plates at the transmitting and receiving ends it is
also possible to make the connection directly to ground, completing the circuit entirely through the earth.
The
accompanying illustration of a one-wire power transmission system is from Tesla's U.S. Patent No. 593,138 titled
"Electrical Transformer," covering the Tesla coil resonance
transformer.
 His
work with the single terminal incandescent lamp led him to investigate
other phenomena associated with vacuum tubes including the generation
and physical properties of X rays. Tesla was one of the first
researchers to alert the scientific community to some of the biological
hazards associated with X-ray exposure. Tesla's more advanced
tubes differ from typical X-ray tubes in that they have no target
electrode and produce X rays through vacuum
high-field emission and a process known as Bremsstrahlung. His work in this
area set the stage for the development of high-energy particle
accelerators. [Nikola Tesla: Lecture
Before The New York Academy of Sciences, April 6, 1897]
 When standing in the same room as an operating Tesla coil, one might
observe the presence of the aromatic gas ozone. Tesla took
advantage of this operating characteristic by designing a device for the
production of ozone in unlimited amount (U.S. Pat. #568,177, Sept. 22,
1896). Tesla observed that with some additional energy and certain
modifications it is possible to efficiently induce a reaction between oxygen
and relatively inert nitrogen. In the industrial processes for the
fixation of atmospheric nitrogen, the resulting gaseous oxides are reacted with
other substances to form solid nitrogen compounds. [Dr.
Nikola Tesla — Complete Patents]
 As Tesla's research progressed, he placed additional emphasis on finding a
solution to one of the most vexing problems of his era—the development
of a practical system for wireless telegraphy and telephony. Some
preliminary experiments with high-frequency alternators connected
to an aerial and ground showed promising results towards this end.
He next replaced the alternator with his resonance transformer and achieved
even better results. When used as a radio transmitter it was
demonstrated that Tesla's oscillator, with its tuned primary and secondary
circuits, was able to produce radio waves thousands of times more powerful than
the simple spark-coil transmitter used by Heinrich Hertz just a few
years before hand—a practical application indeed! [Nikola Tesla On His
Work With Alternating Currents and Their Application to Wireless
Telegraphy, Telephony, and Transmission of Power]
 In spite of this
initial success Tesla was clearly not satisfied with his results.
Further improvements included the development of techniques for obtaining frequency
stability, selectivity, immunity to interference, and security of
communications. It is a footnote to history that a portion of this
work with high-power radio-frequency oscillators involved the first
electronic implementation of the AND logic function. The resulting
U.S. Patents # 723,188 & # 725,605 for wireless signaling cover
logic gates in general and also describe the basic principles of
frequency-hopping and frequency-division multiplexing in wireless spread
spectrum telecommunications. See Wireless Remote Control and the
Electronic Computer Logic Gate for an exhibit in the U.S. patent
interference "Nikola Tesla vs. Reginald A. Fessenden, Interference
No. 21,701, Systems of Signaling" showing a circuit diagram of an
electronic AND logic gate used by Tesla in 1899. [Nikola Tesla: Guided
Weapons & Computer Technology]
While many of Tesla's
innovations are embodied in our present apparatus for wireless
telecommunications there are certain elements which have not been
incorporated. In Tesla's own words, "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." [The True Wireless]
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