Jearl Walker www.flyingcircusofphysics.com
May 2015 The Tesla coil designed by Nikola Tesla in 1891 was meant to be dramatic, even frightening. Long sparks would leap from the coil and through the air, zapping anything in their way. The coil was charged to such a large electric potential that the electric field in the surrounding air could rip electrons from the air molecules. That left conducting ionized paths, along which the freed electrons could move and recombine with the molecules. The conduction of current along the ionized paths heated the path so rapidly that the air suddenly expanded, sending out an acoustic shock wave. The recombination of the freed electrons with the air molecules produced light. So, Tesla had a splendid light and sound show, one that could even kill you if you wandered close enough to catch one of the discharges.
Modern tesla coil shows still have the sound, light, and dramatic discharges, but they now are controlled by computers so that their sound and light accompany music. Here are two examples:
https://www.youtube.com/watch?v=hgll-XTqcS4 The House of the Rising Sun
The larger tesla-coil shows also include performers who purposely intercept the discharges. Indeed, the presence of a performer makes a discharge in the performer’s direction more probable.
http://www.wimp.com/irontesla/ Iron Man
https://www.youtube.com/watch?v=PdrqdW4Miao Doctor who theme song
The performer is not injured or killed by the intercepted discharges because of a conducting suit that acts like a Faraday cage. The current stays on the outside of a Faraday cage instead of penetrating through the conducting walls to the interior. Metallic cars and airplanes act as a Faraday cage when struck by lightning; the passengers probably will not even know about the lightning strike. Similar conducting suits are worn by power-line workers when they must repair a live, high-voltage power line.
http://www.youtube.com/watch?v=B1BPL9b97UI man in Faraday cage, big discharges
The trick of a tesla coil is to convert the low voltage (about 100 or 200 volts) available in the usual wall AC power supply (mains) to tens of thousands of volts or more. Links below describe in more detail how this done but the main idea is that a series of coils are used to increase the voltage. Oscillations of current at a low voltage in one coil can produce oscillations in a second coil but at a lower current and greater voltage. (The voltage increases at the cost of the current.) The connection between the two coils is by the oscillating magnetic field that the oscillating current in the first coil. The main tower that you see in a tesla coil contains the final coil with a great many turns. It is excited by a flat coil with far fewer turns at its base.
The operation of tesla coil is actually tricky because the oscillations in the various stages must be in synch (that is, in resonance) in order for the power to be efficiently transferred from the wall supply to the final coil.
https://www.youtube.com/watch?v=al_nqxSgIz4 big, really big coil
When a tesla coil is played with music, a computer system must control the onset of the oscillations in the coils, in order to properly time the sound and light in the sparking.
Information about tesla coil circuitry:
https://www.youtube.com/watch?v=v3D_uG2ET2s&feature=youtube_gdata_player Tesla music
http://wimp.com/teslacoil/ tesla coil music, man in suit
https://www.youtube.com/watch?v=Amd9l6T1jEc Doctor Who theme, different video than above
https://www.youtube.com/watch?v=3FpjcOWwiI4 two coils, two men, long, poorly shot
Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
· Kelley, J. B., and L. Dunbar Sr., “The tesla coil,” American Journal of Physics, 20, 32-35 (1952)
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