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Nikola Tesla World System for Wireless Energy Transmission

Nikola Tesla’s Transmission Systems

My previous writings have laid the foundation needed to understand this article, in which I will give a brief overview of how the current power grid evolved, and how Tesla planned to radically change this.

Evolution of electric power transmission

In the beginning there was direct current, or DC. However, there was not one single universal power line as we know it today, since DC is not so easy to “transform” into different voltage ratings. So instead, there were different lines for different purposes, since arc lights needed around 10kV, while street cars used 500V for example. Another major downside was that DC, especially at lower voltages, could not be transmitted very far or the losses became too great, and so the generating plant usually had to be within 1 mile from the point of consumption.

Nevertheless, Thomas Edison, who invented a reliable incandescent light bulb in 1879, wanted to replace gas lamps and candles in people’s homes by electric light. On Thursday, June 8, 1882, the Edison Electric Company lit up their first house: the house of banker J. P. Morgan. Edison then continued to launch the first electric utility company that same year, with 85 launching customers. It was an impressive feat, although there were still some teething problems to overcome. It is also interesting to note that Edison used a 3-wire system, so he could transmit power at 110V as well as 220V, since not all motors required the same voltage.

On the other side of the ocean, the focus was instead on alternating current (AC), since the consensus seemed to be that Edison’s low voltage DC system was highly impractical for long-distance power transmission. To distribute power efficiently, high voltages were required, which in turn called for electrical transformers, and economically viable transformers only existed for AC. In 1886, George Westinghouse and others used this affordable step-up / step-down technology to create the basis for the modern power grid.

However, Westinghouse and his companions had one major flaw in their system: there was no motor in the world that could run on their beloved AC power! This meant AC could be used to light lamps, but not used to power machines or street cars, while DC could. Luckily, their problems were solved when in 1887 Nikola Tesla invented his revolutionary induction motor, for which he received a patent in 1888 1. In fact, Tesla was granted 7 patents on May 1st 1888 2, which did not just cover the electric motor, but also a way to use this invention in the electrical transmission of power. This technology was licensed by Westinghouse in that same year.

Plaque at the Niagara Falls hydroelectric power plant, showing 9 Tesla patents
Plaque at the Niagara Falls hydroelectric power plant, showing 9 Tesla patents

The Tesla / Westinghouse AC system was used in 1893 to power the entire Chicago World Fair, and in 1895 this dynamic duo created the first large-scale hydroelectric power plant in the US at Niagara Falls. These were decisive blows against DC.

In the end, the War of the Currents was won by AC because it was more efficient, could be transmitted over larger distances, could be served from a central power plant instead of requiring a power plant per square mile, could easily be transformed into different voltages, and ultimately because it was a heck of a lot cheaper due to economies of scale and more affordable apparatus.

More than 100 years later, our modern power grid still uses the same principles to transmit power, except that the voltage at which power is transmitted has increased significantly, from 4000V on the first AC power line in the US, to 110kV or sometimes even 765kV on today’s long-distance power lines.

This was of course a very brief summary, and many more people were involved in the development of electrical power transmission, but for the sake of this article, the above introduction should suffice. For a more detailed history, I highly recommend you read Empires of Light by Jill Jonnes.

Fresh ideas

So not much has fundamentally changed over the past 100 years when it comes to long-distance power transmission, but this does not mean that no radical improvements were envisioned during that period. In fact, 10 years after coming up with his AC motor, Nikola Tesla made several ingenious inventions capable of revolutionizing the industry once more. These inventions made his earlier groundbreaking work on AC pale in comparison, but were never implemented for a variety of reasons.

The most important reason being that the whole industry had just spent over a decade investing in AC power lines, and so they did not really feel like starting from scratch again. Also, Tesla was a great inventor, but a poor business man, and was not able to bring his inventions to market in an economically viable manner. His AC inventions were only commercialized properly thanks to George Westinghouse.

We will now have a look at the radical ideas Tesla had regarding the transmission of electrical energy.

Wireless light

During his lecture back in 1891 titled “Experiments With Alternate Currents of Very High Frequency And Their Application To Methods of Artificial Illumination“, Tesla displayed, amongst other spectacular phenomena, that he could light up an “exhausted tube” without having any wires connected to the lamp. The following diagram shows one of these setups, in which two metal plates were connected to the terminals of a Tesla Coil, creating a high-voltage, high-frequency electrostatic field in the room which excited the molecules in the exhausted tubes, causing them to light up.

Original caption: "Ideal method of lighting a room. Tubes devoid of any electrodes rendered brilliant in an electrostatic field.
Original caption: “Ideal method of lighting a room. Tubes devoid of any electrodes rendered brilliant in an electrostatic field.”

There are several famous pictures of Tesla holding a wirelessly illuminated lamp in its hand, like the one below.

Nikola Tesla holding a wireless light bulb.
Nikola Tesla holding a wireless light bulb.

Lighting fluorescent tubes by holding them close to a Tesla Coil is still something many people enjoy doing, and it is a cool effect for sure. However, some believe that this was Tesla’s big “wireless energy” idea, and then say that this would never have worked over long distances. Indeed, the field created by Tesla’s coils does not reach very far, which is exactly why he never intended to use this method as a way to transmit energy over long distances, but only to light up rooms. In fact, Tesla believed sending waves through the air was a losing strategy, since power will not get very far due to the inverse-square law.

“Through ages past man has attempted to project in some way or other energy into space. In all his attempts, no matter what agent he employed, he was hampered by the inexorable law of nature which says every effect diminishes with distance, generally as the square of the same, sometimes more rapidly.” 3

Tesla often negatively referred to this type of energy transmission as Hertz wave radiation, which he in fact saw as an energy loss in his own system, and therefore tried to minimize. Yes, Tesla wanted as little energy as possible to “escape” into the air.

“It will be of interest to compare my system as first described in a Belgian patent of 1897 with the Hertz-wave system of that period. The significant differences between them will be observed at a glance. The first enables us to transmit economically energy to any distance and is of inestimable value; the latter is capable of a radius of only a few miles and is worthless… the amount of energy which may be transmitted [with my system] is billions of times greater than with the Hertzian.” 4

In the rather blunt statement above, Tesla clearly states that he devised a method of energy transmission far superior to sending electromagnetic waves through the air. So what exactly was this superior system he is referring to?

Single wire energy transmission

We see the first version of Tesla’s unique transmission system in an 1897 “Electrical Transformer” patent, in which Tesla describes his invention as follows:

“What I claim as my invention is a system for the conversion and transmission of electrical energy, the combination of two transformers, one for raising, the other for lowering, the potential of the currents, the said transformers having one terminal of the longer or fine-wire coils connected to line, and the other terminals adjacent to the shorter coils electrically connected therewith and to earth.” 5

This sounds an awful lot like the AC transmission system used by Westinghouse, so what’s so radical about this invention? The accompanying diagram depicted below might make the difference clear.

Single wire energy transmission system using two iron core Tesla Coils
First single wire energy transmission patent (1897): “a diagram illustrating the plan of winding and connection which I employ in constructing my improved coils and the manner of using them for the transmission of energy over long distances.”

What we see here are two flat spiral coils connected by a single wire, without a return! Yes, no return wire is necessary to transmit power. This is the radical improvement Tesla made over traditional 2-wire energy transmission, drastically reducing the costs of constructing long-distance transmission lines.

The transmitter coil is a step-up transformer of a special kind, invented by Tesla to reach higher voltages than were possible with traditional transformers of the day:

“The improvement involves a novel form of transformer or induction-coil and a system for the transmission of electrical energy by means of the same in which the energy of the source is raised to a much higher potential for transmission over the line than has ever been practically employed heretofore.” 6

The receiver is an identical coil, which functions as a step-down transformer, to which lamps H and motors K are connected. The high voltage allowed energy to be transmitted with more efficiency, and, more importantly, this new type of coil enabled energy to be transmitted over a single wire, without a return. 6 years earlier, Tesla already used these coils to light incandescent bulbs with a single wire:

“I have discovered that if I connect to either of the terminals of the secondary coil or source of current of high potential the leading-in wires of such a device, for example, as an ordinary incandescent lamp, the carbon may be brought to and maintained at incandescence, or, in general, that any body capable of conducting the high-tension current described and properly enclosed in a rarefied or exhausted receiver may be rendered luminous or incandescent, either when connected directly with one terminal of the secondary source of energy or placed in the vicinity of such terminals so as to be acted upon inductively.” 7

And in 1898 he said:

“Among the various noteworthy features of these currents there is one which lends itself especially to many valuable uses. It is the facility which they afford for conveying large amounts of electrical energy to a body entirely insulated in space. The practicability of this method of energy transmission, which is already receiving useful applications and promises to become of great importance in the near future, has helped to dispel the old notion assuming the necessity of a return circuit for the conveyance of electrical energy in any considerable amount.” 8

Unfortunately, today, over 100 years later, electrical engineers still believe the “old notion” of a return wire is a necessity, even though it is extremely simple to connect two small Tesla Coils and replicate Tesla’s results. I have transmitted power over a single wire, and so have thousands of others around the world, and it works. It’s time electrical engineering courses start including this knowledge into their curriculums, for it is a great shame that this promising branch of research has been completely neglected by Universities and engineers for over a century now. The cost savings from requiring only a single wire could finally make it economically viable to connect millions of people in rural areas to the grid who have so far had to do without electricity.

One final thing to note about the transmission system described in the patent above, is that the transmission line is above ground, and mounted on tall, insulated poles to contain the high voltage current:

“The line-wire should be supported in such manner as to avoid loss by the current jumping from line to objects in its vicinity and in contact with earth—as, for example, by means of long insulators, mounted, preferably, on metal poles, so that in case of leakage from the line it will pass harmlessly to earth.” 9

This setup would change very soon, for instead of bringing this innovative invention to market, it was later that same year, 1897, that Tesla made yet another major improvement to his transmission system, even more radical than single wire energy transmission, which aimed to do away with transmission lines altogether…

“Wireless” energy

While single wire energy transmission already goes way beyond traditional electrical engineering practices, Tesla had bigger plans. He wanted to remove transmission lines from the equation by using the “natural medium” as a conductor instead. He had two different mediums in mind for this.

Atmospheric currents

Tesla’s brainwave was that the transmission line could be made obsolete if current was transmitted through the atmosphere instead of a wire, which he believed was possible if the voltage was high enough and the coils were properly tuned. He filed for two patents on this idea in 1897, which were granted in 1900 and numbered 649,621 and 645,576.

“The invention which forms the subject of my present application comprises a transmitting coil or conductor in which electrical currents or oscillations are produced and which is arranged to cause such currents or oscillations to be propagated by conduction through the natural medium from one point to another remote therefrom and a receiving coil or conductor at such distant point adapted to be excited by the oscillations or currents propagated from the transmitter.” 10

First wireless transmission system, using flat spiral coils as both a transmitter and a receiver, and the earth as a conductor
First wireless transmission system, using flat spiral coils as both a transmitter and a receiver, and the atmosphere used as a conductor

In the image above we see a circuit similar to the single wire transmission circuit discussed earlier, with one big difference: there is no wire connecting the transmitter and the receiver coils! Instead of a power line, the voltage would be pushed so high that a conducting path between the two elevated terminal was established in the upper air strata!

“If the potential be sufficiently high and the terminals of the coils be maintained at the proper elevation where the atmosphere is rarefied the stratum of air will serve as a conducting medium for the current produced and the latter will be transmitted through the air, with, it may be, even less resistance than through an ordinary conductor.” 11

To make this arrangement work, the transmitter and receiver coil needed to be tuned to the same resonant frequency.

And “the best conditions for resonance between the transmitting and receiving circuits are attained [when] the points of highest potential are made to coincide with the elevated terminals D D‘” 12. In other words, “the capacity and inductance in each of the circuits [need to] have such values as to secure the most perfect condition of synchronism with the impressed oscillations”. So these coils were tuned resonant transformers.

Later in his life, Tesla shared the setup he used to proof to the patent examiner that this mode of energy transmission actually worked:

Experimental demonstration in the Houston Street laboratory, before G.D. Seeley, Examiner in Chief, U.S. Patent Office, January 23, 1898, of the practicability of transmission of electrical energy in industrial amounts by the method and apparatus described in U.S. Patents No. 645,576 and No. 649,621. Applications filed September 2, 1897.
Experimental demonstration in the Houston Street laboratory, before G.D. Seeley, Examiner in Chief, U.S. Patent Office, January 23, 1898, of the practicability of transmission of electrical energy in industrial amounts by the method and apparatus described in U.S. Patents No. 645,576 and No. 649,621. Applications filed September 2, 1897.

Tesla had the following to say about this:

“In experimenting with these high potential discharges which I was always producing, I discovered a wonderful thing. I found, namely, that the air, which had been behaving before like an insulator, suddenly became like a conductor; that is, when subjected to these great electrical stresses, it broke down and I obtained discharges which were not accountable for by the theory that the air was an insulator.  When I calculated the effects, I concluded that this must be due to the potential gradient at a distance from the electrified body, and subsequently I came to the conviction that it would be ultimately possible, without any elevated antenna — with very small elevation — to break down the upper stratum of the air and transmit the current by conduction.

…I took a tube 50 feet long, in which I established conditions such as would exist in the atmosphere at a height of about 4 1/2 miles, a height which could be reached in a commercial enterprise.

…I used that coil which is shown in my patent application of September 2, 1897 (Patent No. 645,576 of March 20, 1900), the primary as described, the receiving circuit, and lamps in the secondary transforming circuit, exactly as illustrated there.

And when I turned on the current, I showed that through a stratum of air at a pressure of 135 millimeters, when my four circuits were tuned, several incandescent lamps were lighted.” 13

The patent examiner must have been impressed by the demonstration, since the patents were granted.

It is once more important to note that Tesla was not “radiating” energy into the air, but was actually establishing a conducting path between the transmitter and the receiver:

“It is to be noted that the phenomenon here involved in the transmission of electrical energy is one of true conduction and is not to be confounded with the phenomena of electrical radiation which have heretofore been observed and which from the very nature and mode of propagation would render practically impossible the transmission of any appreciable amount of energy to such distances as are of practical importance.” 14

Earth currents

While atmospheric currents were one option, Tesla had in fact a more promising candidate lined up: earth currents. In a February 1901 Collier’s Weekly article titled “Talking With The Planets” Tesla described his “system of energy transmission and of telegraphy without the use of wires” as:

“(using) the Earth itself as the medium for conducting the currents, thus dispensing with wires and all other artificial conductors … a machine which, to explain its operation in plain language, resembled a pump in its action, drawing electricity from the Earth and driving it back into the same at an enormous rate, thus creating ripples or disturbances which, spreading through the Earth as through a wire, could be detected at great distances by carefully attuned receiving circuits. In this manner I was able to transmit to a distance, not only feeble effects for the purposes of signaling, but considerable amounts of energy, and later discoveries I made convinced me that I shall ultimately succeed in conveying power without wires, for industrial purposes, with high economy, and to any distance, however great.”

In the following statement it is made clear that ever since his 1893 lecture, Tesla had been thinking about the possibility of transmitting signals and power using the earth as a conductor:

“I do firmly believe that it is practicable to disturb by means of powerful machines the electrostatic condition of the earth and thus transmit intelligible signals and perhaps power. In fact, what is there against the carrying out of such a scheme? We now know that electric vibration may be transmitted through a single conductor. Why then not try to avail ourselves of the earth for this purpose?” 15

In the same article, Tesla foresees that the earth and the atmosphere could function as a giant condenser, in which case he expects that the earth’s “period of vibration might be very low”, which later turned out to be true. He proposed “to seek for the period by means of an electrical oscillator”, which he eventually did, and which led Tesla to the conclusion that the earth’s resonant frequency was around 11.78Hz, as mentioned in his Canadian patent from 1906 (calculated by using his .084840 seconds for an electric pulse to travel the earth’s diameter and back). He also noted already that “in order to displace a considerable quantity [of electricity], the potential of the source must be excessive.”, which is why he pushed his Magnifying Transmitter to ultimately generate 30 million volts!

What is a Magnifying Transmitter you might ask? I’ll let Tesla describe his self-declared “best invention” to you:

Patent drawing of the Tesla Magnifying Transmitter
Patent drawing of the Tesla Magnifying Transmitter

“It is a resonant transformer with a secondary in which the parts, charged to a high potential, are of considerable area and arranged in space along ideal enveloping surfaces of very large radii of curvature, and at proper distances from one another thereby insuring a small electric surface density everywhere so that no leak can occur even if the conductor is bare.

… this wireless transmitter is one in which the Hertz-wave radiation is an entirely negligible quantity as compared with the whole energy, under which condition the damping factor is extremely small and an enormous charge is stored in the elevated capacity. Such a circuit may then be excited with impulses of any kind, even of low frequency and it will yield sinusoidal and continuous oscillations like those of an alternator.

… Taken in the narrowest significance of the term, however, it is a resonant transformer which, besides possessing these qualities, is accurately proportioned to fit the globe and its electrical constants and properties, by virtue of which design it becomes highly efficient and effective in the wireless transmission of energy. Distance is then absolutely eliminated, there being no diminution in the intensity of the transmitted impulses. It is even possible to make the actions increase with the distance from the plant according to an exact mathematical law.” 16

In short, Tesla’s idea was to determine the resonant frequency of the earth, then create a powerful oscillator, the Magnifying Transmitter, which pushed enormous potentials into the earth at exactly the right frequency in order to create electrical standing waves inside the earth, which then allowed energy to be received at the nodal points of the standing waves, with very little transmission loss. Power would be pushed into the earth through the ground connection at the transmitter, conducted through the earth, and collected again from the earth at the receiver end.

A lot of misunderstanding exists regarding the resonant frequency of the earth Tesla was referring to. The reason is that when you Google for “earth resonant frequency”, the first articles all mention the Schumann resonance of 7.83Hz, which then leads people to conclude that Tesla’s calculations were way off. However, the Schumann resonance is not the frequency of interest here, as my friend Simon from Tesla Scientific once explained to me:

“The original frequency [of the Tesla Magnifying Transmitter] is around 45 kHz, not the Schumann frequency. Tesla’s idea was to use harmonic frequencies or maybe “beat frequencies”, otherwise the coil would have to be ¼ the size of the earth! But the frequency that’s of interest is the earth’s frequency itself, not the Schumann resonance, since the Schumann resonance deals with the cavity that exists between the earth’s surface and the ionosphere, but the energy travels through the earth, and the resonant frequency of the earth is around 11.78Hz.”

Everyone I tell about this bold plan to transmit energy through the earth mentions how much resistance the earth has compared to a normal conductor, but Tesla disagrees and believed the earth is an “ideal conductor”:

“It is difficult for a layman to grasp how an electric current can be propagated to distances of thousands of miles without diminution of intention. But it is simple after all. Distance is only a relative conception, a reflection in the mind of physical limitation. A view of electrical phenomena must be free of this delusive impression. However surprising, it is a fact that a sphere of the size of a little marble offers a greater impediment to the passage of a current than the whole earth. Every experiment, then, which can be performed with such a small sphere can likewise be carried out, and much more perfectly, with the immense globe on which we live. This is not merely a theory, but a truth established in numerous and carefully conducted experiments… This mode of conveying electrical energy to a distance is not ‘wireless’ in the popular sense, but a transmission through a conductor, and one which is incomparably more perfect than any artificial one. All impediments of conduction arise from confinement of the electric and magnetic fluxes to narrow channels. The globe is free of such cramping and hinderment. It is an ideal conductor because of its immensity, isolation in space, and geometrical form.” 17

Of course it is correct to say that a handful of soil has more electrical resistance than a copper wire, which is why Tesla had to use millions of volts and had to create an extremely elaborate grounding system:

“In this system that I have invented it is necessary for the machine to get a grip of the earth, otherwise it cannot shake the earth. It has to have a grip on the earth so that the whole of this globe can quiver.” 18

For a detailed analysis of the grounding system Tesla employed, I can recommend reading this article on Open Tesla Research.

Semi-finished Wardenclyffe Tower in Shoreham, New York
Semi-finished Wardenclyffe Tower before it was torn down

An experimental setup of the Magnifying Transmitter was erected in Tesla’s Colorado Spring laboratory between 1899 and 1900. Tesla then continued to develop the first commercial power station based on his earth transmission system in Shoreham, New York, called the Wardenclyffe Tower. In his autobiography, Tesla referred to his global energy transmission scheme as his “World System”. The Wardenclyffe project would have been the crown on Tesla’s work, but was unfortunately never finished due to a myriad of reasons, and Tesla never really managed to recover from this blow (read more about this in my previous post on the history of the Tesla Coil). In 1917 the tower was demolished, since the US government suspected it was being used by German spies for radio transmission 19.

Conclusion

The goal of this article was to clear up the confusion surrounding Tesla’s “wireless” energy. It turned out he meant “conduction without wires” instead of “wirelessly radiating through the air”. There were a few ingenious transmission systems Tesla experimented with, in chronological order:

  1. Wireless light (room-scale)
  2. Single wire energy transmission
  3. Atmospheric currents
  4. Earth currents

The only serious effort I know of at this moment to develop wireless power transmission is the company WiTricity, but they work on a variation of #1 above and are therefore limited to very small distances. Earth currents were dubbed by Tesla to be his “best invention”, but we might never know if his World System would have worked in practice, since modern Electrical Engineers do not spend any time researching this fascinating technology, and because I am quite certain that with all the sensitive electronic equipment in use today it will not be allowed to push millions of volts into the earth on the off-chance that all those devices will be fried.

However, even if transmission through the earth on a global scale is out of the question, or you believe it would not work for any number of reasons, then one question still remains:

Why are we not researching single wire energy transmission?

Tesla showed in countless easy-to-replicate experiments that energy can be transmitted over a single wire without a return, so why are we still using two wires more than 100 years later?

Let’s park the tricky earth currents for now, start with the simple basics, and build our knowledge from there. Single wire energy transmission deserves to become an active research field, since it promises to cut the money spent on electric wiring in half, and could therefore finally make it economically viable to connect “the last billion” to the electricity grid. It seems wrong not to pursue this noble task, especially since Nikola Tesla has already done most of the hard work for us. If only we tried…

History of the Tesla Coil and its geometries

History of the Tesla Coil and its Geometries

The first time I ever heard about a Tesla Coil, was actually while playing the video game Red Alert during my teenage years. In the game, the coil was a powerful weapon which shot massive bolts of lightning at the enemy. Shooting big sparks is actually the main association people have with the Tesla Coil, since that is what modern “coilers” use it for.

Modern Tesla Coil (left) versus Tesla's original Colorado Springs "Magnifying Transmitter" coil (right)
Figure 1. Modern table top Tesla Coil (left) versus Tesla’s original Colorado Springs “Magnifying Transmitter” coil (right)

However, Nikola Tesla, it’s original inventor, had many practical uses for these high voltage, high frequency coils besides shooting sparks, and it actually came in many shapes and forms, all with their own characteristics and use cases. This article shows how the Tesla Coil evolved over time, and what it was used for.

Where it all began

In 1887, Heinrich Hertz created the first experimental proof of the electromagnetic waves predicted by Maxwell’s theory. This created a lot of buzz around high frequency currents and sparked Tesla’s curiosity as well, who had up till then mainly been working on his AC motors and transmission system, as is evident from his patent history.

Today we are used to hearing “gigahertz” being thrown around, which is equal to 1 BILLION oscillations per second, while in Tesla’s time 20 to 30 THOUSAND oscillations was considered a very high frequency. The reason is that we have transistors now, which switch on an atomic scale, while bulky mechanical oscillators were used in the late 1800’s, and there was just no good way to make them run much faster.

However, Tesla wanted to push beyond the 30 kHz limitation, and in 1891 he patented his Method and Apparatus for Electrical Conversion and Distribution, in which Tesla described how he created rapidly oscillating currents without a mechanical oscillator:

“I have thus succeeded in producing a system or method of conversion radically different from what has been done heretofore—first, with respect to the number of impulses, alternations, or oscillations of current per unit of time, and, second, with respect to the manner in which the impulses are obtained. To express this result, I define the working current as one of an excessively small period or of an excessively large number of impulses or alternation or oscillations per unit of time, by which I mean not a thousand or even twenty or thirty thousand per second, but many times that number, and one which is made intermittent, alternating, or oscillating of itself without the employment of mechanical devices.” 1

So how did Tesla achieve this impressive feat?

“I employ a generator, preferably, of very high tension and capable of yielding either direct or alternating currents. This generator I connect up with a condenser or conductor of some capacity and discharge the accumulated electrical energy disruptively through an air-space or otherwise into a working circuit containing translating devices.” 2

In other words, Tesla found practical use for Lord Kelvin’s condenser discharge – who he regularly credited 3 – and patented it. Tesla now had a way to create high frequency currents.

The first application of this high frequency setup is found in another patent granted to Tesla in 1891, titled System of Electric Lighting 4. In this key patent, we see the first ever diagram and description of a complete Tesla Coil circuit, and the text contains a revealing passage about the reason why this device was invented:

“For a better understanding of the invention it may be stated, first, that heretofore I have produced and employed currents for very high frequency for operating translating devices, such as electric lamps, and, second, that currents of high potential have also been produced and employed for obtaining luminous effects, and this, in a broad sense, may be regarded for purposes of this case as the prior state of the art; but I have discovered that results of the most useful character may be secured under entirely practicable conditions by means of electric currents in which both the above-described conditions of high frequency and great difference of potential are present. In other words, I have made the discovery that an electrical current of an excessively small period and very high potential may be utilized economically and practicably to great advantage for the production of light.” 5

So the very first Tesla Coil ever was used to produce light using not just high frequency currents, not just high voltage currents, but both high frequency and high voltage at the same time! The Tesla Coil essentially expands the high frequency circuit mentioned earlier with a high voltage step up transformer to achieve “enormous frequency and excessively high potential”.

Another telling passage from the patent text mentions how Tesla noticed that a single wire could be used to light a bulb:

“I have discovered that if I connect to either of the terminals of the secondary coil or source of current of high potential the leading-in wires of such a device, for example, as an ordinary incandescent lamp, the carbon may be brought to and maintained at incandescence, or, in general, that any body capable of conducting the high-tension current described and properly enclosed in a rarefied or exhausted receiver may be rendered luminous or incandescent, either when connected directly with one terminal of the secondary source of energy or placed in the vicinity of such terminals so as to be acted upon inductively.” 6

The diagram in the patent shows two bulbs connected by a single wire (the other end of the coil connected to a conducting wall), and Tesla even shows two custom single wire bulbs which he used in these lighting experiments. All very interesting stuff, and no mention of large sparks yet! So what did this first coil look like? And did it have other uses besides lighting?

Bipolar coil (1891)

The first drawing of a Tesla Coil are found in Tesla’s 1891 lecture before the American Institute of Electrical Engineers, in which various experiments are shown utilizing the coil, including an extensive study of “discharge phenomena”, or sparks.

Study of discharge phenomena using a bipolar Tesla Coil during a lecture in 1891
Figure 2. Study of discharge phenomena using a bipolar Tesla Coil during a lecture in 1891

Something that stands out immediately is that this coil is placed horizontally, not vertically like “modern” Tesla Coils. Also, there is no “terminal capacitance” on one end, and a ground connection on the other, like you usually see in a Tesla Coil. This setup is what is now known as a “Bipolar” Tesla Coil.

Another interesting fact is that the primary coil is actually placed inside the secondary coil, as can be seen in the image below. Tesla instructed to “introduce it from one side of the tube, until the streams begin to appear.” 7

Bipolar Tesla Coil with primary located inside the secondary
Figure 3. Bipolar Tesla Coil with primary located inside the secondary

Modern Tesla Coils have an air core, but these early models had their primary wound on top of an iron core:

“In many of these experiments, when powerful effects are wanted for a short time, it is advantageous to use iron cores with the primaries.” 8

Iron cores increase magnetic fields, resulting in more powerful induction effects. However, these iron cores had a major drawback:

“In these experiments if an iron core is used it should be carefully watched, as it is apt to get excessively hot in an incredibly short time.” 9

And at the end of the lecture Tesla already predicted that using iron cores would become a thing of the past:

“In the present systems of electrical distribution, the employment of the iron with its wonderful magnetic properties allows us to reduce considerably the size of the apparatus; but, in spite of this, it is still very cumbersome.  The more we progress in the study of electric and magnetic phenomena, the more we become convinced that the present methods will be short-lived.” 10

In the lecture, Tesla describes how he used his coil to study, amongst other things:

  1. Discharge phenomena
  2. Single wire & wireless light
  3. HF / HV electrical insulation
  4. Impedance phenomena

It is interesting to read that Tesla did not have any math at hand to calculate the ideal primary length and capacitance, he simply said that “the length of the primary should be determined by experiment.”11 Also, no mention of resonance between the primary and secondary is mentioned in the lecture, so it seems like Tesla’s 1891 coil was still a tightly coupled, “normal” induction coil.

Now, was this bipolar coil used often, or only during the 1891 lecture? Well, years later, in an article in the Electrical Experimenter of July 1919, Tesla shared a whole range of bipolar coils and the things he used them for.

Tesla Oscillators from the Electrical Experimenter from July 1919
Figure 4. Tesla Oscillators from the Electrical Experimenter from July 1919

It is clear from the picture above that Tesla used this type of coil extensively, and came up with all sorts of ingenious improvement. Further, it is funny to note that Tesla sometimes calls this device a “Tesla Coil”, and other times a “Tesla Transformer” or even a “Tesla Oscillator”.

Some of the uses of these coils mentioned in the article:

A very versatile device!

After reading all this info, I wondered how big these coils actually were, since size is hard to gauge from the pictures and drawings above. Luckily I found an image of an original coil displayed in the Tesla museum in Belgrade, powering an original Tesla light tube, and which has someone standing next to it, providing a good estimate of the size.

Original Tesla Transformer as displayed in the Tesla museum
Figure 5. Original Tesla Transformer as displayed in the Tesla museum

Conical coil (1894)

After the 1891 lecture, Tesla gave several other lectures on high frequency, high voltage devices, and in 1893 Tesla got involved in developing the world’s first hydroelectric power plant at Niagara Falls, together with George Westinghouse. We see that during that period Tesla filed for several generator, motor, and power transmission patents. However, on the side he kept developing his coil, resulting in the massive conical Tesla Coil shown below.

Conical Tesla Coil
Figure 6. Conical Tesla Coil from 1894

“This coil, which I have subsequently shown in my patents Nos. 645,576 and 649,621, in the form of a spiral, was, as you see, [earlier] in the form of a cone.” 12

The cone shape was chosen to minimize the potential difference between the turns near the top of the coil, so there was less risk of damaging the insulation of the wires. In his comment, Tesla references two of his most famous wireless energy transmission patents, saying that he first used a conical geometry, and later a spiral one, but that in essence they were one and the same device.

Inspired to “develop electric forces of the order of those in nature”, this conical coil was Tesla’s first to reach 1 million volts:

“The first gratifying result was obtained in the spring of the succeeding year when I reached tensions of about 1,000,000 volts with my conical coil. That was not much in the light of the present art, but it was then considered a feat.” 13

This cone shaped coil was a very significant step in the development of the Tesla Coil, since we now for the first time see an upright coil with one end connected to ground. This is what Tesla had to say about this coil:

“This [Figure 6] shows the first single step I made toward the evolution of an apparatus which, given primary oscillations, will transform them into oscillations capable of penetrating the medium. That experiment, which was marvelous at the time it was performed, was shown for the first time in 1894.” 14

So Tesla was figuring out a way to create waves that could penetrate the surrounding medium, and his setup was as follows:

“The idea was to put the coil, with reference to the primary, in an inductive connection which was not close—we call it now a loose coupling—but free to permit a great resonant rise. That was the first single step, as I say, toward the evolution of an invention which I have called my “magnifying transmitter.” That means, a circuit connected to ground and to the antenna, of a tremendous electromagnetic momentum and small damping factor, with all the conditions so determined that an immense accumulation of electrical energy can take place. It was along this line that I finally arrived at the results described in my article in the Century Magazine of June 1900.” 15

According to Tesla’s words, this coil was the first of his coils to implement a loose coupling between the primary and the secondary, and the reason for this was to allow the secondary to vibrate more freely without being weighed down by the primary, thus allowing for much greater voltages at the top of the coil, resulting in beautiful discharges.

“The discharge there was 5 or 6 feet, comparatively small to what I subsequently obtained.” 16

We encounter this conical coil once more in an 1897 “Electrical Transformer” patent.

Conical Tesla Coil from 1897 patent
Figure 7. Conical Tesla Coil from 1897 patent

Tesla mentions how different shapes can be used for the same purpose:

“Instead of winding the coils in the form of a flat spiral the secondary may be wound on a support in the shape of a frustum of a cone and the primary wound around its base, as shown in Fig. [7].” 17

In another drawing from the same patent, it becomes clear how Tesla intended to use these coils to transmit energy at extremely high voltages over a single wire, stepping it up at the transmission end, and stepping it down at the receiver end to power loads.

Single wire energy transmission system using two iron core Tesla Coils
Figure 8. Single wire energy transmission system using two iron core Tesla Coils

In this drawing we also see that a magnetic core is still part of the design.

Flat spiral coil (1897)

In the same patent discussed above, Tesla mentions that around 1897 he usually worked with flat spiral coils:

“The type of coil in which the last-named features are present is the flat spiral, and this form I generally employ, winding the primary on the outside of the secondary and taking off the current from the latter at the center or inner end of the spiral. I may depart from or vary this form, however.” 18

So Tesla generally used a flat spiral coil, but said other geometries worked as well.

In one of the most famous pictures ever made of Nikola Tesla, we can see the inventor sitting in front of a humongous flat spiral coil in his lab on Houston Street, New York
Figure 9. In one of the most famous pictures ever made of Nikola Tesla, we can see the inventor sitting in front of a humongous flat spiral coil in his lab on Houston Street, New York

The main reason Tesla started using flat spiral coils was because they better suppressed the sparks, which were essentially losses in the circuit, allowing him to achieve higher voltages:

“In carrying on tests with a secondary in the form of a flat spiral, as illustrated in my patents, the absence of streamers surprised me, and it was not long before I discovered that this was due to the position of the turns and their mutual action.” 19

Another major benefit of using flat coils is that they are relatively safe, since the highest potential terminal is at the center, out of reach:

“Those portions of the wire or apparatus which are highly charged will be out of reach, while those parts of the same which are liable to be approached, touched, or handled will be at or nearly the same potential as the adjacent portions of the ground, this insuring, both in the transmitting and receiving apparatus and regardless of the magnitude of the electrical pressure used, perfect personal safety, which is best evidenced by the fact that although such extreme pressures of many millions of volts have been for a number of years continuously experimented with no injury has been sustained neither by myself or any of my assistants.” 20

The “transmitting and receiving apparatus” referred to in the quote above are from two patents filed in 1897 for a revolutionary “wireless” energy transmission system, which planned to use the earth as a conductor.

First wireless transmission system patent, using flat spiral coils as both a transmitter and a receiver, and the earth as a conductor
Figure 10. First wireless transmission system patent, using flat spiral coils as both a transmitter and a receiver, and the earth as a conductor

From the patent drawing it is clear that iron cores were not longer being used at this point. We also see a new part being introduced: an “elevated terminal… preferably of large surface”. This terminal acted as a capacitance, and Tesla tweaked his circuit so that the point of highest potential would coincide with these elevated terminals:

“It will be readily understood that when the above-prescribed relations exist the best conditions for resonance between the transmitting and receiving circuits are attained, and owing to the fact that the points of highest potential in the coils or conductors A A are coincident with the elevated terminals the maximum flow of current will take place in the two coils, and this, further, necessarily implies that the capacity and inductance in each of the circuits have such values as to secure the most perfect condition of synchronism with the impressed oscillations.” 21

Here we see clearly that the Tesla Coil has officially evolved from a traditional, magnetically coupled transformer, into a resonant transformer, based on resonant inductive coupling. This is a crucial point in which Tesla Coils differ from regular transformers. It’s all about tuning the primary circuit to the secondary, and the receiver to the transmitter.

Bifilar spiral coil

Tesla's bifilar flat pancake coil from 1893
Figure 11. Tesla’s bifilar pancake coil from 1893

As early as 1893, Tesla already patented a very unique flat spiral coil design, now commonly known as the “bifilar pancake coil”. This coil was wound using two parallel wires, and then connecting the end of one to the beginning of the other wire. This setup resulted in a larger difference in potential between the turns, and therefore a much larger self-capacitance in the coil, with the goal to do away with expensive capacitors altogether, and let the coil itself contain all the capacitance it needs to oscillate at a certain frequency:

“My present invention has for its object to avoid the employment of condensers which are expensive, cumbersome and difficult to maintain in perfect condition, and to so construct the coils themselves as to accomplish the same ultimate object.” 22

However, Tesla kept using capacitors, mainly Leyden jars, in many of his later experiments. So even though the idea behind the bifilar coil is brilliant, it seems like Tesla did not find it useful enough in practise.

Helical coil (1899)

It’s interesting that we’ve already discussed 3 different coil geometries, none of which resembles the “modern” helical Tesla Coil. So did Tesla himself even use this shape at all? Yes, he sure did, as you can see from the beautifully preserved original below, as displayed in the Tesla museum in Belgrade.

Original 500KV helical Tesla Coil in the Tesla Museum in Belgrade
Figure 12. Original 500KV helical Tesla Coil inside the Tesla Museum in Belgrade

In the picture above we see the familiar thin helical coil shape with a toroid as terminal capacitance, as well as three 10-turn primaries, which could most likely be connected in series to vary the operating frequency of the coil.

While this is the shape that first comes to mind for most people when they think of a Tesla Coil, Tesla did not actually write much or anything about this geometry in particular. However, we do see this coil shape return on multiple occasions in pictures taken around 1899 inside Tesla’s Houston Street and Colorado Springs laboratories. These helical coils are sometimes wide and short instead of thin and tall.

Houston Street lab, showing a spiral coil on the wall and a wide, short helical coil in the foreground
Figure 13. Houston Street lab, showing a spiral coil on the wall and a wide, short helical coil in the foreground
Nikola Tesla holding a helical receiver coil
Figure 14. Nikola Tesla holding a helical receiver coil
Wide helical receiver coil tuned to receive electrical energy from a transmitter elsewhere in the lab
Figure 15. Wide helical receiver coil tuned to receive electrical energy from a transmitter elsewhere in the lab, showing sparks between two condenser plates
Helical coils bombarded with sparks from the Colorado Springs Magnifying Transmitter
Figure 16. Helical coils bombarded with sparks from the Colorado Springs Magnifying Transmitter
Helical receiver coil outside of the Colorado Springs lab, tuned to the Magnifying Transmitter, lighting a single bulb
Figure 17. Helical receiver coil outside of the Colorado Springs lab, tuned to the Magnifying Transmitter,”wirelessly” lighting a single bulb
More helical Tesla Coils used in Colorado Springs
Figure 18. More helical Tesla Coils used in Colorado Springs

From the pictures above, and especially the Colorado Springs pictures, it seems like Tesla used a wide selection of helical receiver coils for experimentation purposes. Many of them even come without a primary and terminal capacitance, which suggests that this was not the “final form”, but merely a simple-to-put-together version of a coil used for testing purposes. We also see that the Houston Street coils were wide and short, while the Colorado Springs coils were of the thin and tall variety.

Magnifying Transmitter (> 1899)

Patent drawing of the Tesla Magnifying Transmitter
Figure 19. Patent drawing of the Tesla Magnifying Transmitter

We have now arrived at the final and most advanced Tesla Coil ever created: the Tesla Magnifying Transmitter (TMT). This coil, developed in Colorado Springs and patented in 1902, was intended to transmit ultra high voltage electrical energy around the globe, by using the earth as a conductor. In his autobiography, Tesla explained the TMT in the following way:

“It is a resonant transformer with a secondary in which the parts, charged to a high potential, are of considerable area and arranged in space along ideal enveloping surfaces of very large radii of curvature, and at proper distances from one another thereby insuring a small electric surface density everywhere so that no leak can occur even if the conductor is bare.

… this wireless transmitter is one in which the Hertz-wave radiation is an entirely negligible quantity as compared with the whole energy, under which condition the damping factor is extremely small and an enormous charge is stored in the elevated capacity. Such a circuit may then be excited with impulses of any kind, even of low frequency and it will yield sinusoidal and continuous oscillations like those of an alternator.

… Taken in the narrowest significance of the term, however, it is a resonant transformer which, besides possessing these qualities, is accurately proportioned to fit the globe and its electrical constants and properties, by virtue of which design it becomes highly efficient and effective in the wireless transmission of energy. Distance is then absolutely eliminated, there being no diminution in the intensity of the transmitted impulses. It is even possible to make the actions increase with the distance from the plant according to an exact mathematical law.” 23

As you can read in the above excerpt, the TMT was created to reach as high of a voltage as possible by spacing the conductors appropriately and in large circles, an insight he gleaned from working with his flat spiral coils 24, whereas on a modern Tesla Coil secondary the conductors are always wound close together. Also, Tesla’s intention was to minimize “Hertz-wave radiation” (electromagnetic radio waves), because he wanted to focus all the energy of the coil into the earth for his “wireless” energy transmission scheme. This massive coil’s dimensions were such that it would oscillate with the earth’s natural electric charge to enable this energy transmission, as Tesla explains in more detail in a short 1908 article:

“When the earth is struck mechanically, as is the case in some powerful terrestrial upheaval, it vibrates like a bell, its period being measured in hours. When it is struck electrically, the charge oscillates, approximately, twelve times a second. By impressing upon it current waves of certain lengths, definitely related to its diameter, the globe is thrown into resonant vibration like a wire, forming stationary waves.” 25

Extra coil

But was the only difference between a “regular” Tesla Coil and a Magnifying Transmitter the fact that the conductors were properly spaced? Not exactly. The biggest difference in fact is the addition of another coil into the circuit. Whereas a Tesla Coil has a primary and a secondary coil, the Magnifying Transmitter has a third, freely oscillating coil, which Tesla simply called the “extra coil” (in the patent drawing above).

Tesla in front of his Extra Coil in Colorado Springs
Figure 20. Tesla in front of his Extra Coil in Colorado Springs

Tesla explains the purpose of the extra coil as follows in his Colorado Spring Notes:

“It is a notable observation that these “extra coils” with one of the terminals free, enable the obtainment of practically any e.m.f. the limits being so far remote, that I would not hesitate in undertaking to produce sparks of thousands of feet in length in this manner.

Owing to this feature I expect that this method of raising the e.m.f. with an open coil will be recognized later as a material and beautiful advance in the art.” 26

So the extra coil is what allowed the TMT to reach such immense voltages, but how? The primary coil and secondary coil oscillate at an identical frequency and are therefore resonantly coupled. While this coupling is key to induce current from the primary into the secondary, it also weighs the oscillations down. It adds a certain damping factor, since the secondary coil is not 100% free to oscillate as it pleases. Tesla’s brilliant insight here was that he would have a resonantly coupled primary and secondary, but then add an extra coil which is not tuned to the same exact frequency and can therefore oscillate more freely, allowing the voltage to rise to even greater heights.

Extra Coil circuit diagram
Figure 21. Extra Coil circuit diagram from Tesla’s Colorado Spring notes

“It is found best to make [the] extra coil 3/4 wave length and the secondary 1/4 for obvious reasons.” 27

Since we’re talking about geometries in this article, it is also crucial to note that the extra coil had a 1:1 length to diameter ratio. The reason is that a coil’s self-capacitance is at its lowest at a 1:1 ratio, as was later determined through research by Medhurst 28

Table of H values for the self-capacitance formula Capacitance in pF = H x Diameter, as measured by Medhurst. Self-capacitance is at its lowest at H = 1, where H is length / diameter.
Figure 22. Table of H values for the self-capacitance formula Capacitance in pF = H x Diameter, as measured by Medhurst. Self-capacitance is at its lowest at H = 1, where H is length / diameter. Source: Radiotron Designers Handbook 

Anyone who has ever experimented with Tesla Coils know how sensitive the circuit is. For example, if you put your hand close to the coil, the frequency immediately drops due to the extra capacitance your body adds to the circuit, without even touching it! So the lower the self-capacitance, the less the circuit is weighed down, and the higher the voltage rise that is attainable.

Wardenclyffe

Semi-finished Wardenclyffe Tower in Shoreham, New York
Figure 23. Semi-finished Wardenclyffe Tower before it was torn down

Once Tesla finished up his research in Colorado Spring, he started working on his first full-scale transmitting tower in Shoreham, New York, called the Wardenclyffe Tower. Construction was never completed, and only scattered notebook drawings remain, so we have no idea if Tesla’s groundbreaking wireless transmission system would have worked in practise.

Many “conspiracies” have popped up over time that big industry and banking powers did not want Tesla to transmit “free” energy all over the globe and therefore shut the project down. You often read that J.P. Morgan, who financed the initial construction of the Wardenclyffe tower, said something along the lines of “you can’t put a meter on that”, and therefore cut the funding. In fact, reading this very statement fueled my quest into the world of Nikola Tesla. However, Tesla himself felt that Morgan lived up to all his promises and was not to blame:

“I would further add, in view of various rumors which have reached me, that Mr. J. Pierpont Morgan did not interest himself with me in a business way but in the same large spirit in which he has assisted many other pioneers. He carried out his generous promise to the letter and it would have been most unreasonable to expect from him anything more. He had the highest regard for my attainments and gave me every evidence of his complete faith in my ability to ultimately achieve what I had set out to do. I am unwilling to accord to some small-minded and jealous individuals the satisfaction of having thwarted my efforts. These men are to me nothing more than microbes of a nasty disease. My project was retarded by laws of nature. The world was not prepared for it. It was too far ahead of time. But the same laws will prevail in the end and make it a triumphal success.” 29

However, when one reads the letters between Tesla and J.P. Morgan from that time, it does appear that J.P. Morgan might not have been as helpful as he could have been, and that in the above statement Tesla simply did not wish to speak ill of him in public.

Despite that, Tesla’s energy was not “free”, as some claim. Tesla had to put in thousands of horsepower to kick his transmitter into motion, and even fried the power station near his Colorado lab once because he was using so much energy. However, his system being an open system, like a windmill or solar panel, rather than a traditional closed circuit, it is not impossible to conceive that energy already present in the earth or the ambient medium, which oscillated at the same frequency, could be resonantly coupled in to reinforce his input.

Also, once an oscillating current was established in the earth, a lot less energy would be required to keep it going, just like how you need a lot of power to push someone on a swing, but once they are going you only have to give a tiny push to keep them swinging about.

Magnifying Transmitter Design Sheet
The TMT is a complex device and therefore harder to replicate than a Tesla Coil, which is why I was ecstatic to find a Colorado Spring Magnifying Transmitter Design Sheet, created by Simon from Tesla Scientific. There are designs for several operating frequencies, and you can even request a custom frequency design. Click here to check it out.

Modern coils

Modern Tesla Coil
Figure 24. Modern Tesla Coil

After discussing the Magnifying Transmitter and its bold purpose to transmit power across the globe, it feels like a gigantic step backward to now talk about how modern “Coilers” apply some of Tesla’s ideas, simply to create big sparks and light fluorescent tubes in their hands. However, this is the current state of the Tesla Coil, and the reason why many people are unaware of the original grand purpose of these coils.

On a positive note, thanks to the active coiling community, modern Tesla Coil calculators like TeslaMap have emerged, which make coil calculations a whole lot easier. These coils are almost exclusively helical coils with one terminal grounded and a toroidal terminal capacitance on the top end.

Another very interesting development is that today’s Tesla Coils are not always powered by spark gaps anymore, like in Tesla’s time, but often by electronics. Such coils are called Solid State Tesla Coils (SSTC), or Dual Resonant Solid State Tesla Coils (DRSSTC), and their circuit diagrams are mental, but work very well.

DRSSTC circuit diagram
Figure 25. DRSSTC circuit diagram

The only thing I find funny is that the community’s general solution to creating larger sparks is to add more turns on the secondary and push more power through the circuit, while a much smaller coil, designed according to the Magnifying Transmitter specifications, would result in way larger discharges.

Closing thoughts

The goal of this article was  to clear up some of the confusion surrounding the use cases for these high frequency, high voltage coils, as well as to show the various stages of development they went through and the different shapes they came in, as each of the geometries had their own pros and cons. Tesla himself called his Magnifying Transmitter his best invention, and solely based on that fact it is worth researching in more detail, which I plan to do in the future.

I hope you enjoyed this little journey through time! In my next article I will dive deeper into the various electrical transmission systems Tesla devised over his lifetime.