Nikola Tesla patented the Tesla coil circuit on April 25, 1891.Denicolai, 2001, Tesla Transformer for Experimentation and Research, Ch.1, p. 1-6U.S. Patent No. 454,622, Nikola Tesla, SYSTEM OF ELECTRIC LIGHTING, filed 25 April 1891; granted 23 June 1891 and first publicly demonstrated it May 20, 1891 in his lecture "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination" before the American Institute of Electrical Engineers at Columbia College, New York.The lecture "Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination" is reprinted in The Tesla coil circuit is shown p. 193, fig. 127The lecture is reprinted in The Tesla coil illustration is shown p. 103, fig. 32, archive Although Tesla patented many similar circuits during this period, this was the first that contained all the elements of the Tesla coil: high voltage primary transformer, capacitor, spark gap, and air core "oscillation transformer".

From Tesla's time until the 1930s Tesla coils were widely used in radio transmitters, quack electrotherapy, and experiments in wireless power transmission, and more recently in movies and show business.

Invention

During the Industrial Revolution the electrical industry exploited direct current (DC) and low frequency alternating current (AC), but not much was known about frequencies above 20 kHz, what are now called radio frequencies. In 1887, four years previously, Heinrich Hertz had discovered Hertzian waves (radio waves), electromagnetic waves which oscillated at very high frequencies. This attracted much attention, and a number of researchers began experimenting with high frequency currents.

Tesla's background was in the new field of alternating current power systems, so he understood transformers and resonance. In 1888 he decided that high frequencies were the most promising field for research, and set up a laboratory at 33 South Fifth Avenue, New York for researching them, initially repeating Hertz's experiments.

He first developed alternators as sources of high frequency current, but by 1890 found they were limited to frequencies of about 20 kHz. In search of higher frequencies he turned to spark-excited resonant circuits. Tesla's innovation was in applying resonance to transformers."Tesla is entitled to either distinct priority or independent discovery of" three concepts in wireless theory: "(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit" Transformers functioned differently at high frequencies than at the low frequencies used in power systems; the iron core in low frequency transformers caused energy losses due to eddy currents and hysteresis. Tesla p. 196-197 and fig. 2: Tesla describes the steps in his invention of the high frequency transformer. and Elihu Thomson independently developed a new type of transformer without an iron core, the "oscillation transformer", and the Tesla coil circuit to drive it to produce high voltages.

Tesla invented the Tesla coil during efforts to develop a "wireless" lighting system, with gas discharge light bulbs that would glow in an oscillating electric field from a high voltage, high frequency power source. For a high frequency source Tesla powered a Ruhmkorff coil (induction coil) with his high frequency alternator. He found that the core losses due to the high frequency current overheated the iron core in the Ruhmkorff coil and melted the insulation between the primary and secondary windings. To fix this problem Tesla changed the design so that there was an air gap instead of insulating material between the windings, and made the iron core adjustable so it could be moved in or out of the coilW. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 122 He eventually found the highest voltages could be produced when the iron core was omitted. Tesla also found he needed to put the capacitor normally used in the Ruhmkorff circuit between his alternator and the coil's primary winding to avoid burning out the coil. By adjusting the coil and capacitor Tesla found he could take advantage of the resonance set up between the two to achieve even higher frequencies.W. Bernard Carlson, Tesla: Inventor of the Electrical Age, Princeton University Press - 2013, page 124 He found that the highest voltages were generated when the "closed" primary circuit with the capacitor was in resonance with the "open" secondary winding.

Tesla was not the first to invent this circuit. Henry Rowland built a spark-excited resonant transformer circuit (above) in 1889 and Elihu Thomson had experimented with similar circuits in 1890, including one which could produce 64 inch (1.6 m) sparks, In this letter Thomson lists papers he published in technical journals which support his claim to priority in inventing the "Tesla coil" resonant transformer circuit and other sources confirm Tesla was not the first. However he was the first to see practical applications for it and patent it. Tesla did not perform detailed mathematical analyses of the circuit, relying instead on trial and error and his intuitive understanding of resonance. He even realized that the secondary coil functioned as a quarter-wave resonator; he specified the length of the wire in the secondary coil must be a quarter wavelength at the resonant frequency."The length of the...coil in each transformer should be approximately one quarter of the wave length of the electric disturbance in the circuit, this estimate being based on the velocity of propagation of the disturbance through the coil itself..." US Patent No. 645576, Nikola Tesla, System of transmission of electrical energy, filed September 2, 1897; granted March 20, 1900 The first mathematical analyses of the circuit were done by Anton Oberbeck (1895) and Paul Drude (1904)., English translation

Tesla's demonstrations

A charismatic showman and self-promoter, in 1891-1893 Tesla used the Tesla coil in dramatic public lectures demonstrating the new science of high voltage, high frequency electricity. The radio frequency AC electric currents produced by a Tesla coil did not behave like the DC or low frequency AC current scientists of the time were familiar with. In lectures at Columbia College May 20, 1891, scientific societies in Britain and France during an 1892 European speaking tour,Thomas Cummerford Martin 1894 The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed., p. 198-293 the Franklin Institute, Philadelphia in February 1893, and the National Electric Light Association, St. Louis in March 1893,"On light and other high frequency phenomena", Thomas Cummerford Martin 1894 The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed., p. 294-373 he impressed audiences with spectacular brush discharges and streamers, heated iron by induction heating, showed RF current could pass through insulators and be conducted by a single wire without a return path, and powered light bulbs and motors without wires. He demonstrated that high frequency currents often did not cause the sensation of electric shock, applying hundreds of thousands of volts to his own body, causing his body to light up with a glowing corona discharge in the darkened room. These lectures introduced the "Tesla oscillator" to the scientific community, and made Tesla internationally famous.

Wireless power experiments

Tesla employed the Tesla coil in his efforts to achieve wireless power transmission, his lifelong dream. In the period 1891 to 1900 he used it to perform some of the first experiments in wireless power, transmitting radio frequency power across short distances by inductive coupling between coils of wire.Tesla, Nikola (May 20, 1891) Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination, lecture before the American Inst. of Electrical Engineers, Columbia College, New York. Reprinted as a In his early 1890s demonstrations such as those before the American Institute of Electrical Engineers and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a room. He found he could increase the distance by using a receiving LC circuit tuned to resonance with the Tesla coil's LC circuit, transferring energy by resonant inductive coupling. At his Colorado Springs laboratory during 1899–1900, by using voltages of the order of 10 million volts generated by his enormous magnifying transmitter coil (described below), he was able to light three incandescent lamps at a distance of about .The longest Tesla wireless power transmission for which there is credible evidence is probably his 1899 picture of a receiving coil with a 10 watt light bulb lit by power transmitted from his 300,000 watt magnifying transmitter. Tesla did not give the distance, but Marincic has claimed Tesla's lab notes indicate it was at a distance of 1,938 feet (591 m) from the transmitter. This represents a transmission efficiency of only 0.0033%.Tesla was notoriously secretive about the distance he could transmit power. One of his few disclosures of details was in the caption of fig. 7 of his noted magazine article: The Problem of Increasing Human Energy, Century magazine, June 1900. The caption reads: "EXPERIMENT TO ILLUSTRATE AN INDUCTIVE EFFECT OF AN ELECTRICAL OSCILLATOR OF GREAT POWER - The photograph shows three ordinary incandescent lamps lighted to full candle-power by currents induced in a local loop consisting of a single wire forming a square of fifty feet each side, which includes the lamps, and which is at a distance of one hundred feet from the primary circuit energized by the oscillator. The loop likewise includes an electrical condenser, and is exactly attuned to the vibrations of the oscillator, which is worked at less than five percent of its total capacity." Today the resonant inductive coupling discovered by Tesla is a familiar concept in electronics, widely used in IF transformers and short range wireless power transmission systems such as cellphone charging pads.