Stefan's Tesla-Pages

Technological background
(or 'how to produce your own indoor lightning')


What is a Tesla Coil like?

A Tesla Coil is an air core resonant transformer for generating high frequency high voltage. It was invented by Nikola Tesla (July 10th, 1856 - January 7th, 1943) in 1891. The typical frequency range for hobby coils is from 100kHz to 1MHz with typical voltages of several 100kV and input power ranging from 200VA to some kVA.

The coilers of today experiment with these devices for two major reasons: they want to understand the techniques used by Nikola Tesla long ago or they simply want to play with high voltage and amuse by the beautiful display, the long sparks and arcs give. I myself belong to both groups: as a physicist, I'm interested in the technological and historical background of the inventions made by Nicola Tesla long ago, but I also like to build some of these unique devices and to play with them.

My current record (set in August '97) is an arc length of 143cm (56" or 4.7') with <2.6kW input power (wallplug power). However, this is nothing compared to the experiments performed by Nikola Tesla in Colorado Springs in 1899 where he shorted out the generator of Colorado Springs power company with the RF coming from his coil (he forgot the line filter!). Greg Leyh is now going to set a new record with his Advanced Lightning Facility (ALF). It is planned to produce 270feet artificial lightning between the two secondaries of the twin system at 5.1MW (yes, that's 5100kW!) input power.

The secondary coil (the 'Tesla coil' or 'secondary') is a single layer of wire wound on a form such as a plastic pipe. Tesla coils come in many sizes. They range from tiny ones about 1" in diameter and about 5" or 6" long, to monsters with a winding length of more than 12 feet. These are extremes though. The most common sizes buildt by hobby coilers are 4 or 6 (sometimes 10) inches in diameter, and about 5 to 3 times as long (respectively). Most of them have around 700-1600 turns of magnet wire. The size ranges from 26 gauge up to 18 gauge, but these numbers are of course not cast in stone.

If the capacitor ("cap") of the tank circuit is charged up to the voltage where the main system spark gap breaks through, a high frequency oscillation starts. The resonance frequency of this primary circuit (the 'tank circuit') is determined by the value of the capacitance of the cap and the inductance of the primary coil (LC-circuit). If the tank circuit is tuned to the natural frequency of the secondary coil, the energy is inductively transferred very efficient to the secondary coil. This is the moment where sparks arise from the top of the Tesla coil. These sparks go to grounded objects and will then be named arcs, or they just reach out into the air. The sparks and arcs produce quite a racket and hearing protection is required for large coils, and is highly recommended for smaller coils too. One of the main reasons that the sparks are so impressive is the compression of the energy on the time scale. The primary capacitor is charged in 0.01 seconds and this stored energy is released via the sparking secondary in only 1/100 of this time.

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What can I do with a Tesla Coil?

Well, TCs were used for several different things in the past 100 years. Nicola Tesla himself built them to transmit power without wires. He planned to load a VERY big system with such a huge toroid that spark breakout would be prevented. The power will then be transmitted through "ground currents", which means that the "common ground" we have today would not exist any longer. It is told, that during his experiments in Colorado Springs the horses threw sparks from their legs by lifting them up from the ground....

Nevertheless, Teslas inventions were the begin of wireless communication.

Tesla constructed the first apparatus (1905) for the use of high frequency currents in medical applications, a principle which is still used today. Though there have been built many quack devices in the past ("violet wand" or "violet ray machine").

Small hand held devices like these were also used for many years as "leak detectors" in vacuum systems made out of glass.

A kind of Tesla coil is in nearly every circuit for feeding the CRT (of your PC-monitor , TV, o'scope,  etc.) with high voltage.

In the industry, high frequency currents are still used for dielectric heating, e.g. in furniture production.

Many high voltage test labs used and still use TCs for high voltage testing (the Advanced Lightning Facility ALF of Greg Leyh will be the biggest one ever built).

Tesla coils have been used for special effects in films since the original Frankenstein movies with Boris Karloff (from 1931 on). The coils in those films were buildt by the greatest high voltage special effects man of all time, Mr. Kenneth Strickfadden and his life-long close friend, Mr. John Foster. Richard Gibson ('Dick') Aurandt, who was a youngster at the time of the Boris Karloff film, did become lifelong friend of "Stric" and Foster and taught people like Bill Wysock, who created all the high voltage special effects for the motion picture "The Entity," starring Barbara Hershey (approximately 300 KV at 240 Khz passing over her body, with corona streamers up to 36 inches long, read more about this at Other examples for Tesla coils in famous films are Terminator I and II (time travel scenes, for a short moment you can even see the sphere of the coil which produced the effect; Arnold Schwarzenegger), Roland Emmerichs 'Stargate' (USA 1994; Kurt Russel) and 'Tripple X' (2002).

And not to forget the lots of museums, where most of the high voltage shows content at least one TC.

And for some freaks like me and many others (look at the link section), the construction of Tesla coils is a fascinating hobby. As Fr. Tom McGahee once wrote in a posting to the Tesla coil mailing list:  
The real joy is not in having the finished Tesla coil hurling sparks around. It is the wonderful anticipation we experience as we build and experiment and tinker and learn and apply that new knowledge and come up with an even better design and then build *that* and throw the switch and cross our fingers and hope that we have got it right and we can squeeze another inch or two out of our beloved bolt hurlers.

He also wrote in another posting to the Tesla coil mailing list:
Malcolm Watts on the Tesla-2 List said: "I'm curious to know: what is/are each person's goal/s in coiling? I'd be very interested to hear. I will offer no comment unless one is requested."

I got into coiling when I saw my first Tesla Coil. I was 12 at the time. Building my first Tesla Coil was a real adventure for me. It gave me a hunger to learn more and *DO* more.

In short, it was a catalyst, a spark that has grown into a wonderful flame because it has been fanned. Among my friends I always stood out as being somewhat "peculiar" because I

(a) loved all things scientific,
(b) enjoyed READING about science and other interesting things,
(c) enjoyed DOING things scientific more than just reading about them.

The Tesla Coil is a wonderful thing. It inflames the imagination. It is fanciful and very real at the same time. It has a magic all its own. You can build a Tesla coil with little more than a few pieces of genuine junk. You can refine your understanding and your skills of workmanship, and craft one that is a gleaming thing of beauty. As you grow, so does your appreciation for all the *other* wonderful things that you have learned because of this "hobby".

Tesla Coils attract people like myself as a flame attracts moths. So I have found that many people who are somewhat like me are also attracted to this Tesla List. Through this Tesla List I have met many persons who I regard in the best sense as genuine "friends".

I have found that there is a great sense of sharing that exists among the members of this List. It is nice to have somewhere where you can ask a question (on many topics) and get answers that by and large are helpful.

It is good to share what I have learned with others. It is good to share enthusiasm and comaraderie. It is good to see that I do not have all the answers, but that together we have almost as many answers as we have questions... and that just makes the remaining questions all that more rich and tantalizing.

I have learned much from grappling with the physics of Tesla coils. I love the apparent simplicity of the device and the principles that underlie its operation. I am also humbled at how it still holds much that is beyond my current understanding. I discover something still to be learned at every turn. What a wonderful journey this Tesla Coil has taken me on!

It is a pizzazzy kind of a thing, this Tesla Coil. As close to magic as most anything we will encounter during our lifetime. And it brings out the pizzazzy in each of us who stands in awe of its magic.

Richard Hull, Malcolm Watts, Bert Hicks, Terry Fritz, Mike Hammer, Richard Quick, and a wonderful cast that includes also the likes of Gary Weaver and Bill the Arcstarter, and hopefully myself, the multitude of everyday posters, the oldies and newbies and even the unknown multitude of silent lurkers... we are in some very real way a living community of people who - at least for a moment - share something wonderful in common. And we are *all* the better for it.

I always show my high school electronics students one of my Tesla coils as part of my orientation to the course. I always hope that it will be for some of them the kind of catalyst that it was for me. All are amazed at the Tesla Coil. They all love to see it demonstrated. And there are always a few who seem attracted to it as I myself was so many years ago, in 1958 when I was 12 years old. These are the ones that begin to ask the questions. "How does it work?" "Could I build one myself?" "What does that funny looking thing over there do?" And that is always just the beginning of many questions about many things.

In my own life, when I turn to the center, where the fire of curiousity was first truly enkindled, I still see at that center the wonderful writhing arcs of the Tesla Coil beckoning me to come in closer and see more clearly the wonder and the magic of this world I live in.

My goals in coiling? To enjoy the gift of curiousity that God has so graciously given to me, and in doing so to give Him thanks and praise.

Comments graciously accepted from Malcolm and any one else who cares to comment.

Fr. Thomas McGahee

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What is the difference between an Oudin coil and a Tesla coil?

Bert Hickman wrote on this on the TCML:

Re: oudin vs. tesla coils
Date: Thu, 30 Nov 2000 04:55:27 -0700
Original poster: "Bert Hickman" <>

Chris and all,

Oudin was a Frenchman who also did HV RF research in Tesla's time. Some (such as most US scientists) regard his work as being "cloned" from Tesla's. In reality, it was more likely that Tesla and Oudin were contemporary, and mostly independent, researchers of RF/resonant transformers. Certainly the French believe this! :^)

Anyway, the main difference between a Tesla and an Oudin coil is that the Tesla Coil uses a separate primary and secondary winding and is a true double-tuned transformer using INDEPENDENT LC circuits. In the Tesla configuration, the base of the secondary is normally directly connected to ground. One end of the primary may ALSO be connected to ground (indeed, some pig coilers run with the innermost portion of the primary and base of the secondary tied to the same RF ground to reduce the potential difference between the primary and the secondary windings (I don't recommend this!)). The fact that both the primary and secondary may be tied together and the common point connected to ground does NOT mean that this is an Oudin configuration.

An Oudin coil normally connects ONLY the bottom of the primary to RF ground. The top of the primary is then connected to the base of the secondary winding. The base of the secondary is actually connected to the "hot" end of the primary(!), and the coil operates sort of like a dual tuned autotransformer, with the secondary resonant circuit sharing some of the primary circuitry as well. But, this mode of operation is significantly more dangerous than a Tesla Coil using a common ground point (above), and is definitely NOT recommended for any high-powered coils. Oudin DID obtain a patent for this configuration. Tesla also experimented with it, and this work may have been a step along the way towards his later magnifier research... (This is the way some violet wands are wired, very dangerous as described on this page)

-- Bert --

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