Tesla Coil

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Physics, Engineering: Electricity, Circuits
Grade Range: Middle School, High School
Format: Stage

The Tesla Coil is one of our most impressive and awe-inspiring demonstrations. This demonstration will generate a large and powerful bolt for students to see, and it might generate some surprised screams as well!



  • Tesla Coil (Inner Coil, Outer Coil, Stand)
  • Grounding Rod
  • Metal Tip
  • Circuit Box with Outlet Cable
  • Power Cables (2 Green, 1 Red)
  • Long Flathead Screwdriver
  • Sandpaper
  • Power Strip with On/Off Switch

Safety Precautions

The Tesla Coil is safe if used correctly, but can be EXTREMELY dangerous if misused! Only a trained volunteer can be near or use the Tesla Coil. Extreme caution should be exercised while the device is turned on. Both the coil itself and the circuit box may carry harmful levels of voltage, so ground the Tesla Coil after every use. Follow the guidelines below in order to safely use the Tesla Coil:

  • Use a power strip with an on/off switch. This will provide you with a safe way to turn the coil on and off.
  • The Tesla Coil should not be turned on until all the cords are in place, the lid is on the circuit box, the grounding rod is plugged in and held near the tip, and the audience is at least six feet away.
  • No one should touch or handle any part of the system, excluding the grounding rod and the screwdriver, while the power is on. Upon turning off the Tesla Coil, the grounding rod must be swept along opposite sides of the coils to remove any excess charge, as well as the tip.
  • Do not perform this demonstration until you remove all metal objects and electronic devices from your person, such as necklaces, rings, bracelets, phones, keys and wallets.
  • You must warn the audience that anyone with a pacemaker or metal implant should move away from the stage, and should be at least 15 feet away from this demonstration to prevent any risk of damage to their device.
  • Please see the Demonstration Safety Page for additional General Safety Precautions, and read the Loud Demonstration Safety section.


Preparation: Place the small coil on the stand, aligning the black-spot leg with the black-spot hole. put the large coil inside of it and onto the stand, aligning the port at the bottom of it with the ports on the small coil. Place the metal tip on top of the coil. Plug the red power cable into the port on the large coil, and connect it to one of the three vertical grounding ports on the circuit box. Plug the green cables into the small coil, and connect them to the two horizontal ports on the circuit box. Plug the grounding rod into one of the three grounding ports. Open the circuit box, and check the spark gap to make sure it is clear of rust or buildup. If there is any, use the sandpaper to clean off the tips. Close the circuit box, and use the screwdriver to close the spark gap fully. Now, plug the power strip in, keeping the switch in the OFF position. Plug the outlet cable into the circuit box, and place the other end next to the power strip. DO NOT PLUG IT IN YET.

  1. Explain to the audience that you will be showing them the Tesla Coil, a device first invented by Nikola Tesla. Alert the audience that it will be a loud demonstration, and that any individuals with implanted devices should stay at least 15 feet away for this presentation.
  2. Explain to the audience the different parts of the circuit: Transformer, Capacitor, Coils, Spark Gap. you can use the coils as an example of a transformer. State that the spark gap is the limiting factor in this system; If the spark gap stays small, the bolt will stay small, and if it gets bigger, the bolt will as well!
  3. Put on safety glasses and pick up the grounding rod. Explain the purpose of the grounding rod.
  4. Have your co-presenter sit next to the circuit box, away from the coil and within reach of the power strip. explain that they will be controlling the spark gap, and therefore the size of the bolt. Have them plug in the circuit box to the power strip while still in the OFF position.
  5. When ready, hold the ball on the end of the grounding rod against the tip of the Tesla Coil. Give your co-presenter the OK, and after a brief countdown have them turn on the coil.
    • When first turned on, there will be a small spark or no visible spark, depending on how open the spark gap is. Have your co-presenter open up the spark gap with the screwdriver slowly, allowing the bolt to gradually grow larger and larger. As this happens, slowly move the grounding rod further from the coil, so the audience can see how big the bolt is getting.
    • When the bolt starts to get bigger than 6in, do not let the cord of the grounding rod be closer to the coil than the ball on the end. The discharge from the coil is powerful enough to forgo the insulation of the wire. DO NOT TOUCH THIS WIRE, OR ANY OTHER PART OF THE COIL, WHILE IT IS ON.
    • At some point, the spark gap will get too big and the tips will get too oxidized, and the power will cut out. Have your co-presenter slowly close the spark gap. It will jump across again, but the longer it runs the more corroded the tips will get. Have them slowly close up the gap.
    • Once they close the gap, turn off the power. Ground the coil, and have them unplug it from the power strip.
  6. After presenting, ask the audience the following questions:
    1. What color was the bolt? (Blue/Purple) What causes the color of the bolt? (Oxygen)
    2. What state of matter was the bolt? (Plasma)
    3. Can anyone smell the odor coming from the demo? If so, what is that odor? (Ozone)

Why This Works

The Tesla Coil was invented by Nikola Tesla, a scientist in the late 19th and early 20th centuries. The device is a way to see how a charge can be built up and made larger by use of what we call Transformers. A transformer is pretty simple overall: You set up two coils, with one inside the other, like the Tesla Coil. Then, you run a charge through one of the coils. The size of the coils affect the outcome you get; That is, if one coil is bigger than the other, then you will get a different charge out of the second coil. In this case, we run a charge through the smaller coil, which means the larger one will generate a larger charge. This is known as a Step-up Transformer, since we are "stepping up" the voltage. Likewise, if we ran the charge through the big coil instead, the small coil would give a smaller charge, resulting in a Step-down Transformer.

In our circuit, we start with the voltage from a wall outlet and send it through a step-up transformer, which boosts the voltage by a factor of 50. It then sends the charge to the capacitors in the circuit. Capacitors are, simply put, a set of metal plates that are really close to each other, with only a tiny amount of space between them. These gaps will hold a charge, making a capacitor something similar to a battery. Unlike a battery, a capacitor doesn't slowly discharge; it empties all the charge it has at once, which gives a boost to the voltage by a factor of 10. From here, the circuit goes into our Tesla coil's outer, smaller coil, which acts like a second step-up transformer, which boosts the voltage by a factor of 100. In the end, the entire system will take in the wall voltage and can multiply it by as much as 50,000!

The output of the system is controlled by what is called a Spark Gap. The spark gap is our adjustable resistance for the system, and controls how big the bolt can get. If we leave it closed, the bolt cannot get big, and might not even be visible. As we widen the spark gap, the bolt will grow bigger, with a ratio of about 10:1. That is, for every inch the spark gap opens, the bolt can fire an extra 10 inches! The maximum possible output of the system is controlled by the spark gap, which greatly limits the actual output of the system. Since it is open to the air, the copper tips of the spark gap quickly oxidize while the Tesla coil is running. This rapid oxidation makes it harder for the spark gap to conduct, and needs to be removed between runs to allow the coil to run.

When we see the bolt, we are actually looking at Plasma, the fourth state of matter. We don't often encounter plasma in our day-to-day lives, but it is the most common state of matter in our universe! Plasma is when you excite a gas so much that it gets super hot, and the electrons start flying around it really fast, no longer staying with their original atoms or molecules. The color of the bolt, the blue-purple color, is because of the gas in the air being so excited. Notably, it is oxygen gas, since nitrogen gives off a white light when excited. Oxygen gives off a blue-purple color when it is excited. This is a different reason for why the sky is blue, however, since our sky and air is all gases, and not made of plasma! Lastly, the smell you might notice is a result of this oxygen plasma. When the oxygen atoms got all excited, some of them split and recombine not as O2, the oxygen we breathe, but as O3, which is known as ozone.

Tips and Tricks

  • Modified versions of the Tesla coil have been put to use to generate high voltages for radio transmission, x-ray generators, fluorescent devices, and a variety of other applications.
  • The Tesla Coil is a part of the Quantum Mechanics Show and the Electromagnetism Show.
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