My first foray into Tesla Coil building has been selected to be displayed in the Power House Museum, Sydney. I won't be getting it back until June 1997. So I won't be able to put up pictures of it. But I've put up some info on my coil up soon, for now here's a run-down on how they work.

TESLA COIL THEORY

Tesla coils are air-core, resonant transformers. Working at high frequencies, they can generate tremendous voltages with spectacular lightning-like discharges. The most popular use for them has been in the film industry, where they are used whenever lightning or electrical arcs are required. Perhaps the best use of Tesla coil effect s can be seen in "The Entity". In this film, the actor actually has lightning bolts radiating from her body, and this scene was not faked. (William Wysock provided the effects.) Another film was "Terminator II", specifically the scene where the 'bad' Terminator arrives from the future in the first part of the film. (the scene with all the trucks.).

The picture of the man on the right-hand side is of Nikola Tesla. The man whose invention bears his name. The greatest electrical inventor of modern times. For more info on him click on his picture or goto my Nikola Tesla Bio.

How They Work

As mentioned, Tesla coils are resonant transformers. This implies that there is a specific frequency at which they operate - the resonant frequency. There is no "special" universal Tesla coil frequency - rather, you either target a frequency in the design, or tune a coil into whatever frequency it happens to be happy with.

What determines this resonant frequency is the secondary coil - a complex LCR network. The inductive (L) component is the physical coil itself, and is based upon the number of turns, the diameter and length of the coil. The capacitive (C) component is comprised of several isotropic values; the surface of the secondary wire and the discharge electrode. (Isotropic capacitance in essence is 'virtual' capacitance - there is a capacitive effect even though it appears like there isn't any physical plates to create the capacitor.) The resistive (R) component consists of the wire itself, and identifies the physical resistance of the secondary coil at the resonant frequency.

To get the secondary to resonate, pulses of energy have to be fed at just the right rate and frequency through the primary to enable energy transfer through the air. A good example of this can be obtained with two identical tuning forks. Strike one tuning fork then hold near the second, after 15s stop the first one from vibrating. The second tuning fork will still be vibrating because of the influence of the first fork, which has the same resonant frequency. Energy is transferred easily through nearby air molecules which are vibrating at the frequecies of the forks.

Energy pulses come from the primary circuit. This circuit is made up of (1) the high-voltage transformer, (2) high-voltage capacitor, (3) the spark gap and (4) the primary coil. Together, these parts form a crude type of oscillator. What happens is thus: the transformer charges the capacitor up until there is a high enough voltage across the spark gap to jump the air gap. When this spark occurs, the energy stored in the capacitor is 'dumped' into the primary coil. The primary coil then builds a magnetic field as the capacitor's energy flows through it. The magnetic field will eventually collapse, and will in turn dump what energy is left back into the capacitor. This see-saw activity continues until there isn't enough voltage left to jump the spark gap.

The oscillation frequency is determined by the value of the primary capacitor and the primary coil. Together, they form what is called a parallel-resonant circuit. In typical Tesla coil designs, the frequency is adjusted by altering the primary coil's inductance, by changing the number of turns on the primary coil.

If the energy bursts are of the same frequency as the secondary, the energy transferred by the primary's magnetic field will start to build up in the (5) secondary coil through resonance. Much like a laser, this energy grows and amplifies itself until there is an incredible voltage built up on the (6) discharge electrode at the top of the coil. Which dissipates into the air in the form of electrical sparks.

Information on Tesla Coil theory from:

Brent’s High Voltage Page:
(see below for URL)

Information on Resonance obtained from:

Bunn, Dean J.
"Physics for a Modern World"
The Jacaranda Press, 1990.

- These pictures are from Jimmy's Tesla Coil Page.