Tesla Coil

Age

Elementary School, Middle School, High School

Format

Stage Show

Materials

   Tesla Coil
   Tesla wand (with copper ball on end and grounding wire)
   Metal coil tip
   Transformer box
   Two green power cables to connect Tesla Coil to transformer box
   Screwdriver
   Sandpaper
   Power strip with on/off switch 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. The Tesla Coil is harmless if used correctly, but can be EXTREMELY dangerous if misused! Only a trained volunteer should go near the Tesla Coil. Extreme caution should be exercised while the devise is turned on. Both the coil itself and the transformer box may carry very harmful levels of voltage. Beware that the Tesla coil will turn on as soon as it is plugged in - use a power strip with an on/off switch to help you control the power flow.

   The device should not be turned on unless the plastic transformer box is closed.
   No one should ever touch the device while turned on except with the grounding wand.
   You must sweep the coil with the grounding wand after using the coil to eliminate all excess charge.
   Do not perform the Tesla coil near any metal or electronic devices - empty your pockets before the demonstration.
   You must warn the audience that anyone with a pacemaker or metal implant should move away from the stage. 

Preparation

Start off by hooking everything up. Connect the two green power cords to the coil itself and to the transformer box. Connect the black power cord attached to the grounding wand to one of the three grounding outlets on the transformer box. Make sure the plastic lid is screwed into the transformer box. Place the metal tip on top of the Tesla coil. Finally, plug the black power cord into the transformer box and into the power strip - make sure the power strip is turned off until you're ready for it (or simply leave it unplugged at first)!

There is a spark gap inside the transformer box that moderates the power output of the box (see diagram). You may need to sand down the metal tips on this gap to remove deposits that will inhibit the sparking. To do this, make sure the coil is unplugged and remove the transformer box's plastic lid. Then simply rub the ends of the metal on each side with some sandpaper.

Demonstration

First warn the audience that this demonstration will generate large electromagnetic fields that may be dangerous to anyone with a pacemaker or metal implant - anyone with such devices should move very far away for safety. Make sure EVERYONE in the audience is at least 15 ft away from the Tesla Coil no matter what.

Start off by grounding the coil by placing the copper head of the grounding wand up against the coil tip. Then plug in and turn on the coil and you should see the spark gap in the transformer gap light up with a spark. Now you can move the wand away from the coil tip and you should see a spark bridge between them.

You can change the spark gap width during the demonstration. Making this gap smaller will make it easier to produce a spark on the coil, but will reduce the power of the spark. You may want to start off with this gap small so you can get a spark going, then increase the gap as much as you can to make the spark more dramatic. You can adjust the gap by inserting a flathead screwdriver in the hole in the transformer box and using it to turn the dial inside.

You can turn the coil on and off a few times if it's having trouble starting up - sometimes this will help to get it going. Usually, if it's not working, you just need to reduce the spark gap width.

What to Say

We all grew up with electric lightbulbs, cell phones, computers, and tons of other electronic devices that we rely on evry day. Did you know that just a little more than a hundred years ago, none of these electronic devices existed? At the turn of the nineteenth century, a great inventor named Nikola Tesla was one of a handful of scientists working to unlock the secrets of electricity. Among the many things that he did, he wanted to build a device to generate huge voltages to help him study x-rays, radio, the alternating current, power transmission, and other phenomenon. We have here a Tesla Coil, very similar to the one Tesla himself invented, so we can show you what it means to generate a huge voltage.

How do you get a huge voltage? High voltages are created by a concentration of charges. Electrons are subatomic particles that carry charge - so if we can squeeze enough electrons together, we can produce a large voltage. The Tesla coil works by forcing as many electrons as it can into the very tip on the coil, so we get a huge concentration of charge and a huge voltage.

What does a huge voltage look like? What does it do? We can show you using the Tesla coil!

(Perform the demonstration)

So what did you guys see? You saw a spark jumping from the wand to the tip! An electrical spark is a conduction path that forms in the air - its sort of like a wire that forms spontaneously in the air, but instead of copper, this wire is made of superheated gas ions! The conduction path allows electrons to flow from the wand head to the tip.

Now, you don't normally see sparks in the air, do you? Even when there are small voltage differences, like those created by the wires in your house, you normally don't get sparks. The reason is that, while metal wires are conductors that make it easy for electrons to flow, the air is an insulator. The air resists the flow of electrons, so we normally don't see electrons flowing through it. If you get a sufficient voltage difference though (like the one between the high voltage metal tip and the 0-volt grounded wand), you get an electric field strong enough to force the electrons on the tip to travel over to the wand. The air is not conducting electricity - no longer insulating it! We call this effect dielectric breakdown. This produces a cascade of moving electrons, ionizes and superheats the air molecules in between, sets up that plasma conduction path, and produces the brilliant glowing spark that we see.

So it looks like this demonstration is all about electricity - but it also has a lot to do with magnetism as well. Have you ever wondered why a refrigerator or bar magnet is attracted to some metal objects? The magnet seems to be pulled towards that metal, even though there's nothing reaching out to grab it - the force is invisible. The phenomenon responsible for producing this force is a magnetic field. The Tesla coil relies on a huge magnetic field just like the one from the bar magnet, but much stronger. Take a closer look at the Tesla Coil - the metal tip is actually not in any way connected to the wall outlet, so how does all these electrons get transferred to the tip? Well, the power from the wall supplies the energy to set up a current (a flow of electrons) in the primary coil. The primary coil isn't attached to the secondary coil or the tip at all, but these moving electrons from the primary coil current create a large magnetic field right in the center of the device. When each turn of wire in the secondary coil is exposed to this magnetic field, it causes the electrons in that secondary wire to start flowing. So the current in the primary wire actually induces a current in the secondary wire by a magnetic field. Once the current is set up in the secondary coil, electrons start spiraling up the wire towards the tip and are forced together up there, producing the high voltage.

Why It Is

The information in the "What to Say" section above should tell you all you need to know about the science behind this demonstration, but here are a few more details just for fun. The Tesla Coil is a very complicated instrument and you can find much more information online.

The magnetic field interaction between the primary and secondary coils is a phenomenon called induction, the same concept at play in a transformer. A huge current is sent through the primary coil, which has only a few turns of wire. This induces a huge voltage (but lesser current) in the secondary coil, which has many, many turns of wire. A Tesla coil differs from a traditional iron-core transformer in that the details of the alternating current at work in its design produce an even greater stepped-up voltage than in the traditional transformer - the stepped up voltage will go as the square of the ratio of turns rather than the ratio itself.

What is the voltage produced by the Tesla Coil? You can get an idea based on how long a spark you can produce. The dielectric strength of air is about 3 million volts per meter. In other words, it takes about 30,000 Volts for each centimeter's worth of spark that you are able to produce. Of course, a current will only flow when a voltage difference exists, so we introduce the grounded wand (fixed at 0 Volts by definition) to provide a lower potential for the electrons to "fall" into. So, if you have a spark that will extend to at most 2 cm, you can estimate that the metal tip is at 60,000 V and the grounded wand is at 0 V.

Note that there are also a variety of alternating current concepts at work in the Tesla Coil, although we have not emphasized them here.

Real Life Examples

The spark produced by the Tesla coil operates on the same principal as lightning - dielectric breakdown.

Modified version 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 principles of electronics that the Tesla coil illustrates - current, voltages, induction - are the foundation of all the modern electronic devices that you use every day!