Tesla Coils

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 terminal 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. A good analogy is that of a bell. To get the bell to ring, you need to tap it with a hammer. If you tap too hard, you can crack the bell. If you tap and hold the hammer on the bell too long, you don't get a clean, pure tone out of the bell.

Energy pulses come from the primary circuit. This circuit is made up of (1) the high-voltage transformer, (2) the primary 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 inductor. The primary inductor 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 inductor. 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.

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 secondary coil. Much like a laser, this energy grows and amplifies itself until there is an incredible voltage built up at the top of the coil, which dissipates into the air in the form of electrical sparks.

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