Q: How does a Tesla coil work?

Physicist: Stripped down to it’s most essential parts, a Tesla coil is a wire sticking out of the ground. To get sparks to fly out of the top the rest of the machine “sloshes” electrons up and down the wire.
The picture you should have in your head is a long bathtub, open to the ocean on one end.  The machinery of the Tesla coil is like some dude in the bathtub sliding back and forth, splashing water (electrons) out of the closed end, while the tub is refilled from the ocean (ground).

The electricity in the primary coil is what’s doing the pushing, and the electricity in the secondary coil is what’s being pushed.  To understand how the driving mechanism works requires a new metaphor and some answer gravy.

Aside from inspiring fear, Tesla coils are useless. Truly, Tesla was a genius. The strange shape is an attempt to avoid arching from the torus to the primary coil, which is bad.

Answer gravy: To get sparks to really fly you need very high voltage (up to several million volts) at a fairly exact frequency.  The current that flows up and down the secondary coil, and sloshes out the top, has a high resonant frequency (~MHz, unless the coil is ridiculously huge) that you really can’t do much about.  But the current coming out of the wall has a frequency of only 60 Hz (50 Hz for our Old World readers).

One possible circuit configuration for a Tesla coil.

So how do you change frequencies?  The answer is you “pluck” the primary coil.  For example: If you pick a guitar string once a second you have a frequency of 1 Hz, but the string vibrates on its own at whatever frequency it’s made for (~10 kHz).

The AC mains have a low frequency (60 Hz) while the secondary coil needs to be driven at a high frequency (~1,000,000 Hz).  That means that the secondary will slosh back and forth thousands of times every time the current from the wall turns over just once.  Since the fast part of the circuit is so much faster than the slow part, you can just pretend that the current from the transformer is DC (direct current = 0 Hz).

The secret to plucking is to change the circuit’s “shape” using a spark gap.  Spark gaps have some pretty slick properties.  They have an essentially infinite resistance until a high enough voltage is applied across them, at which point they spark (hence the name).  The spark you see is the air being pulled apart and ionized.  Now ionized gas is a really good conductor, so a spark is like instantly closing a switch.

Also, spark gaps are the cheapest circuit element evar.  Can you cut a wire?  Now you got a gap!

Also, adding spark gaps to a device is one of the quickest ways to bridge the divide between regular and mad science.

The transformer on the left forces charge to build up in the capacitor on the top. But the voltage across a capacitor is proportional to the amount of charge it's holding, so eventually the voltage is high enough to trip the spark gap.

The only job that the slow part of the circuit has is to charge the capacitor (pull back the string).  When the spark gap sparks (pluck!) the fast part of the circuit takes over, and the slow part is essentially ignored until all the energy is exhausted by exciting the secondary coil (string vibrates and slows).

With the spark gap active the charge can flow out of the capacitor and swing back and forth many times, very fast (thousands to millions of times per second). The current through the primary coil then drives current up and down the secondary, causing electrons to "overflow" from the top of the Tesla coil. The "overflow" is a delight to children of all ages.

As current flows through the primary it creates a voltage across the secondary that’s so high that electricity actually flies out of the top of the coil, despite having nowhere in particular to go.  It generally takes at least several hundred thousand volts to make that happen.

The loop in the picture above forms an RLC circuit with a high resonant frequency (that matches the frequency dictated by the secondary).  As the energy in this system runs out the voltage needed to maintain the spark gap (which is much less than the voltage needed to start it) is lost, and the whole thing returns to the slow, charging phase.

Since the power supply oscillates at 60 Hz, the whole system briefly turns off 120 times every second (the voltage is +, 0, -, 0, +, 0, …).  For this reason Tesla coils have a very loud 120 Hz hum that sounds “staticy” and ominous, as opposed to Jacob’s ladders which are continuous, and tend to sound more like “tearing”.  Connoisseurs, I’m sure, will agree.

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38 Responses to Q: How does a Tesla coil work?

  1. nikola tesla is awesome

  2. Prabakaran Santhanam says:

    Tesla – “father of A.C”

  3. I LOVE NIKOLA TESLA SOOOOOOOO MUCH <3 <3

  4. P.S Hey Niko, lets go bowling.

  5. confused reader says:

    How is the tesla coil useless? The original design was to power the world wirelessly and for free. At his lab in Colorado Springs he used a tesla coil to power lamps 25 miles away!

  6. The Physicist The Physicist says:

    You can’t power anything for free. The energy you transmit still has to come from somewhere.
    Either way, a Tesla coil doesn’t transmit power. It a big, loud, dangerous toy.

  7. Perogie15 says:

    well, according to what i’ve reseached about tesla coils, you could theoretically creat a self sustaining generator, of course my math is probably wrong but its an interesting idea

  8. Magneticitist says:

    i just thought i should mention, you are wrong sir when you state the Tesla coil does not transmit power.. how can you deny the fact that it can, and does? would lighting an incandescent bulb via a wireless field not constitute transmission of power? i’ve done it myself. maybe you mean it does not transmit “free” power, in which case i would agree.

  9. Rob says:

    The thing that got Tesla excited about coils and power transmission wasn’t the ability to magically pull free energy of of nowhere (that can’t actually work) and more about using the magnetic fields from ridiculously high voltages to cut down on the gradually accumulating power losses you get when you pass electricity through really long power lines.

    Perogie, what you describe, the self-sustaining generator, wouldn’t work. It is a Perpetual Motion Machine of the First Kind. Tesla coils can accumulate very high voltages, from much lower ones. In exchange, however, they must slow the current down to a crawl. Power is the voltage (how much energy each electron packs) multiplied by the current (the number of electrons passing through a point each second). Your losses in current will more or less exactly cancel out your gains in voltage, for no net change in power.

    Well, technically, you’ll still lose power to inefficient transformers and any incidental wire resistance, but you see what I mean.

  10. Modularhalfpipe says:

    I built one of these with my son last year. The input
    Is a 30 ma 15kv neon transformer. There are about 12
    Winds in the primary, and 1200 in the secondary,
    Making the ratio 100:1, so the voltage is stepped up
    To 1.5 million volts. It produces a ” lighting bolt” of about
    3 feet, depending on how I “tune” the spark gap. We have
    A fan right at the spark gap to quench it, and it needs to be
    Cleaned from time to time. If its in the back yard, it will
    Light a fluorescent bulb from up to 15 feet away, and is quite
    Loud. Although Tesla’s coil is incredibly inefficient, ( think of the
    Resistance encountered when trying to “electrify the world”), loud, (probably
    About 100 dB for our medium coil), and dangerous (imagine havong a
    Pacemaker next to one of those things) it does, in fact,
    Work. Tesla is by far the most under estimated, under compensated, and
    Misunderstood inventor of all time, and my favorite by a long margine.
    I’m glad he finally got credit for the invention of radio in 1943. Too bad
    He wasnt alive to receive it.

  11. Angry Reader says:

    The Physicist is obviously NOT a physicist but instead a frod. To imply the tesla coil does not transmits power is like saying a light bulb does not emit light. Physicist either go get your degree in physics or stop being a dumb***.

  12. The Physicist The Physicist says:

    Admittedly a Tesla coil transmits power in very much the same way that a light bulb emits light energy, or a stereo (or even a ticking clock) emits sound energy. You can even harvest that energy from a distance. However, the amount of energy you can collect is tiny compared to the amount of energy that went into powering the coil. In very much the same way that the amount of energy you can collect from a light bulb using solar panels is tiny compared to the energy that goes into lighting the bulb.
    So, sure, a Tesla coil transmits power in the same sense that effectively every functioning machine transmits power. But that’s not really what it’s designed to do.

  13. Unknown Reaux says:

    It seems you’re wrong again Physicist. The Tesla Coil was designed by Tesla to be a part of his “World System” of power transmission. He wrote all about this in numerous papers as well as in his autobiography “My Inventions” which can be found at

    http://www.lucidcafe.com/library/96jul/teslaautobio.html

    I don’t know why people insist on belittling his vision and the true power of his inventions. The Tesla Coil was one part of an array of devices which would have had world-changing implications. Tesla himself states that his World System was tested, proven, and all it lacked was to be implemented. His only regret was perhaps that such great leaps in harnessing the power of Nature might lead destructive and disunited human wills to obliterate each other on massive “World” scales.

    When we look at the Tesla Coil as an independent unit of invention, we perhaps only see its independent operation. But make no mistake, Tesla knew how to use it along with his other inventions to harness the very power of the Earth and the Sun, and perform feats which many today cannot even begin to fathom.

    When primitive man first looked at the Sun, he perhaps said something like “That disc does nothing more than provide us with pretty light and warm heat.” But as we began to pierce the veil of Nature, we discovered the wonders of photosynthesis, solar winds, even gravity itself holding us in orbit, and operating our artificial means upon that discovered Nature, mankind has developed himself and every technology to its current state.

    We need some great minds like Tesla to arise in our day, to wake up to the awe in Nature as he did, and resolve to lift her veil, understand her mysteries, and through art, harness her powers towards the evolution and union of Man.

  14. Newly interested says:

    I have recently become interested in some of Tesla’s ideas. My main question is ” How plausible was his idea to transmit electricity thru the ionosphere?”

  15. The Physicist The Physicist says:

    Technically, every time you receive a radio transmission that’s been bounced off of the ionosphere it’s inducing a tiny bit electricity in your antenna. However, if you want to efficiently get electricity from one place to another, the ionosphere isn’t the way to go.

  16. Kay Gee says:

    Tesla’s theories and accomplishments are often misstated or misunderstood by both proponents and opponents regarding free-energy. Tesla never advocated “free-energy”, especially as zero cost energy formed from nothing; he did not advocate over-unity or perpetual motion; these terms differ and are not synonymous. Perpetual-motion—(hypothetical) a device of self-sustaining power and never-ending motion not possessing a necessity to create any power beyond sustaining its own motion; over-unity—(hypothetical) a device to produce more energy than it consumes.

    Free-energy (wind, water, solar, etc…), excepting its intermittency, it can be replenishable or a non-exhaustive source of energy used for the development of power. Build a dam on a river and you will have a resultant lake; the captured water is at a static potential until intentionally flowed into a turbine by means of gravitational force. Free-energy is the force of nature (the universe); capable of providing to us power in ways beyond our wildest dreams.

    Tesla’s concepts were about cleaner and more effective energy supplies . He understood that within the universe and this earth, there is non-exhaustive supplys of untapped esources. His objective was to discover methods to inexpensively obtain it, convert it into electricity, and transmit it wirelssly; so anyone within the world might receive it. The thing that keeps us from progressing in this direction is the stagnation of man’s thinking for the collective good. “Free-energy” (freedom) is a foul term to the powers that want to control it.

    Read Tesla’s Colorado Springs notes about forces acting on man relating to his own power potential vs. his future dynamics. Tesla states (paraphrased) eventually the sun and universe will fade; however, it is the direct source of all energy and life we have on this planet; and dependent on man’s directional intent and velocity, he will either retard or excel his advancement into the future.

    A “naysayer” never discovers any potential because he is more interested in thinking something will not work before ever thinking it might; it is the dreamers that are the discoverers, the visionaries and the inventors.

  17. Anti-Physicist says:

    Mr. Physicist. I might also point out that you INCORRECTLY explain the MECHANICS of the Tesla Coil. In your article above you say that “sparks to fly out of the top the rest of the machine” this could not be further from the truth. Your statement is DIAMETRICALLY opposed to the ACTUAL flow of potential through the Tesla Coil. What you fail to realize is that the EFFECT you see with your naked eye creates and optical ILLUSION. As you watch the Tesla Coil operate your eyes see what LOOKS like sparks flying OUT OF the machine but in reality (this can be and has been proven) those “SPARKS” you see that appear to FLY OUT of the top are in fact “FLYING IN TO THE TOP” of the machine. Also pure logic DICTATES that his solar antennae (which can capture as much as 110 volt of potential – maybe more) would be implemented as the source voltage supply in the Tesla Coil (how you could not fathom this is beyond even me). One more thing I might point out is that it IS possible to pull energy from the ionosphere as one inventor has recently discovered (http://www.youtube.com/watch?v=p7DaoqN8TjI). If I had to guess at your function in this article I would surmise your are one of two possible actors in the field of scientific suppression 1.) A super under educated victim of the scientific establishment or 2.) A scientific Suppression shill.

    Shill – noun
    1. a person who poses as a customer in order to decoy others into participating, as at a gambling house, auction, confidence game, etc.

    2. a person who publicizes or praises something or someone for reasons of self-interest, personal profit, or friendship or loyalty.

  18. tyra says:

    how can you accelerate a magnet with a telsa coil

  19. jack.lavald says:

    can you make a tesla coil receptor? an special antena that made to capture an exact frequency that an tesla coil may be emitting ? can you make a tesla coil work perpetually on a small energy income?, of course sacrificing range i wish to send binary code on a tesla coil to a specific receptor

  20. The Physicist The Physicist says:

    The short answer is, yes.
    What you’re talking about is exactly what a radio is. While it would be a lot easier to buy a radio, there are lots of places online where you can find out how to build one on your own, or buy a kit.

  21. Rupert says:

    I have a question that is probably very silly – please explain why the capacitor continues to charge through the negative cycle of the AC. I would have thought that the capacitor charges for half the cycle and discharges for the other half and so there is no net build up of charge. Obviously that’s not the case, since it does have a build up, but it would be great if someone explained why. Thanks in advance.

  22. OoEe says:

    Im stuck with the parallel inductor-capacitor thing. When the capacitor discharges, does its voltage goes all way down to 0-volt?

  23. rob says:

    Hi
    I red most of the comments here.
    The telsa coils we all build were teslas demonstration propose . Telsa realisation of radiant energy and theroies of plasma energy .

    Simply we all know a magnetic field a coil and a moving conduct within that field will induce flux in turn energy. Ok now telsa in my conclusion was not trying tobfind perpetual motion or free energy from the massive towers he built.
    One he had plates that where

    Simply consider this yhe earth rotation right lol the earth has two magnetic poles
    So therefore 1 you have your magnetic poles 2 rotation now you need a conductor right. Ok the hard part that everyone tried to find out how telsa got the energy
    . Ok now telsa in my conclusion was not trying tobfind perpetual motion or free energy from the massive towers he built.
    One he had plates that where positively charged to ground ( to earth ) from the primary the antenna was negatively charged and the secoundary was connected to a generator to prime a charge into the secoundary coil. Thus causing and inducance and resonance frequency. Then the generator would shut down and the earths magnetic field and resonance would induce energy. Also most likely he would built these tower along key fault lines where the magnet field are the greastest along with possibly underground water flow as water pertian minerials flow along a high magnetic field would induce a small voltage into the primary.

    So a mr pyshic says he was inputing a voltage in hes telsa coil.

    Another thoery I had was telsa was possibly using the earth as a capacitor as well. But any way. Telsa was not trying to get free energy or perpetual motion otr energy he was using the energy potential that was already there
    Earths rotation earths magnetic field the antenna (conductor) virtually he reversed the operation of the telsa coil as shown here matched the resonate frequency (unknown) transmission of energy telsa actually conducted is a story for another time. Just consider telsa coil a reverse of system of a radio transmission tower.
    Regards rob

  24. Nik Steel says:

    To get sparks to really fly you need very high voltage Could I make a small tesla coil with a 9v battery, an on/off switch (to replace the spark gap), a capacitor, and a small copper coil (primary) in series, and a large copper coil (secondary) connected to ground next to the primary? Would this make a small spark about a cm long?

  25. Kay Gee says:

    I’m not really sure what you’re trying to do…?

    In a Tesla coil, the spark gap is necessary to sustain the discharge frequency of the capacitor bank and the gap setting is critical to the overall Tesla coil performance.

    You didn’t describe the type of switch you intend to us nor its frequency of operation.

    Why not just experiment with what you want to do and tell us your results. However, from what you have described, it seems you are not wanting to build a Tesla coil at all, but something else that will produce a on-demand single-high-voltage discharge… hmm!

  26. NEW TESLA INTEREST says:

    Ok now for the serious stuff, I have to build from scrap, I have 2 capacitors 50 uF 400 to 45o volts, can I use those for a small horizontal coil?? I am working on a small self build coil trying to get parts together. please some advice. I am still looking for a high voltage transformer, maybe one from a microwave oven, or T.V.

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  29. Scott says:

    I searched “How do Tesla coils work” into google.
    I read this article.
    Still don’t know how tesla coils work. Your metaphors are bizarre.
    I am now going back on to google and changing my search to “How do Tesla Coils work guide for idiots” maybe that’ll help me.

  30. Derek says:

    The Physicist says, “Aside from inspiring fear, Tesla coils are useless”

    Know what your talking about before you criticize.

    This is NOT how a “Tesla Coil” works. It does NOT use AC Current. It uses Pulsed DC to create “Radiant Energy”. Please refer to the writings of Gerry Vassilato. Secrets of Cold War Technology chapter 1 for example. Link below:

    http://borderlandresearch.com/book/secrets-cold-war-tech/chapter-1

    For a SHORT read:

    http://diyenergyreport.com/tesla-radiant-energy/

  31. Tom says:

    I am researching building my own Tesla coil. Still going through the safety info before I attempt to electrocute myself or others…..
    I will go with two 15kV 30ma transformers, 4″ x 22″ secondary and an 18″ diameter toroid. I think the expected arc length is 30″-50″

    Anyone help me with a couple questions ?
    I am reading the varying safe distances. Is staying back twice the arc length acceptable ? that would be 100″.
    2nd question – if the arc length is up to 50″, how far away must sensitive electronics be ? cell phones, stereo equipment, etc ? Further than 100″ ?I have limited space and need to carefully find where to run this.
    3rd question – I see online pics of people running these in their garage, arcs reaching the walls and ceiling. Isn’t this a fire hazard ? I’m thinking NO to running it inside my garage and staying with outdoors.
    4th question – Neighbors are about 30 feet away. will they experience any electronic interference ?
    5th question – is someone has a pacemaker, and my arc length is up to 50″, how far back must they be ?

  32. Derek says:

    “I will go with two 15kV 30ma transformers”, so you are going to use HV rectifiers?

    Now with the safety questions, Electronics: A faraday cage could be an option (most new electronics are not shielded, they have no metal housing with ground).

    Electronic interference with Neighbors (30ft), yes/most likely.

    People: Someone with a pace maker is NOT a good idea, you might see protocol for distance from a microwave oven to get some idea. Not very many people will want to touch on this subject for obvious reasons.

    Property: Running it outdoors is my best opinion because I can’t see what you have when all is said and done.

    Hoped this helps a little bit.

    Warm regards,

    Derek

  33. Will says:

    Ok, that telsa coil is freaky! ;(. Sounds like it’s technically an unsafe weapon.

  34. Tom says:

    THanks Derek. FInally get back to this…. IMy parts supply does not have a HV rectifier. The basics are: 15KV, 60MA nst, (32) Cornell Dubillier 942C20P15K CAPS, 32 bleed resistors for the caps, spark gap components, 6ft ground rod, variac, terry filter.

    When you mention a faraday cage, are you suggesting a cage around the coil ? To allow maximum arcs to 4-5 feet and the cage will capture anything that attempts to arc past ?

  35. Derek says:

    The faraday cage is just for anything you think might need /want to protect. That’s all. Sounds like you got a nice transformer (had one like that years ago). I hope all works out well,

    Warm regards,

    Derek

  36. Abisai says:

    I recently attempted to make a tesla coil for my school project and am stuck. I get a spark at the spark gap but not at the top why is this?

  37. #loveTesla! says:

    This self-proclaimed “physicist” doesn’t know what he is talking about. Tesla was the greatest genius ever to live on this planet. Of course, this is my opinion, but it is a well founded one. Tesla’s coil isn’t useless! You know what is useless? The answer the physicist gave. I highly recommend that you leave this page at once, and quit listening to Mr. High and Mighty ramble about a topic he (or she) knows nothing about. Do not vilify Tesla! Prevaricating about Nikola Tesla does nothing to diminish his total superiority over the average human being, including this “physicist”. I demand that this obloquy stop. Nikola was a better human than most people are.

    Evil Marconi and Edison.
    And the physicist.

    And the people who chose who wins the Nobel prize. They clearly know nothing about Nikola Tesla! He invented the radio! The Tesla coil! A.C.! Remote control! the electric motor! He was a polyglot!

    Humph.

  38. Tom says:

    I’m enjoying getting to the building of my Tesla Coil and learning all aspects of it.
    I am finishing my Terry filter. I understand how the MOVs will divert a stray arc away from the NST to the earth. I have (16) 1800V MOVs, 8 each side.
    1. But I calculate that I should have 10 each side ? I have a 15k 60ma NST. I calculate 15000 * 1.414 / 1000 = 21.21 / 2 = 10.5. What I am doing wrong ?
    2. What is the purposes of the capacitors in the Terry filter ? I am thinking the MOV’s would divert the stray HV charge by themselves.

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