Nikola Tesla in a famous double exposure from Colorado Springs. The
Tesla coil shown is the world's largest functioning one to date.
The coil at left is actually a tertiary coil, or "magnifier," while the
primary and secondary coils are supported on the surrounding building and
circular "fence."
Over the years, technology has evolved and so have Tesla coils. Builders have found what makes the best coils, and although Tesla took plenty of secrets to the grave, we can make a better Tesla coil out of modern components than even Tesla himself could in his time. Additionally, the vacuum tube and transistor have opened new courses of Tesla coil innovation. There is still much to be learned about Tesla coils, particularly the theory of operation. There are big differences in performance between tube and transistor-driven coils and their disruptive counterparts that are not fully understood.
Tesla coils have historically had no application in mainstream technology,
although the concepts of resonance and high-frequency power that Tesla
pioneered have a myriad of applications, from television and radio transmitters
to switching power supplies that are in the computer you are using now.
However, Tesla coils are perhaps the easiest way to get dazzling high-voltage
effects for the entertainment industry, and Tesla coils are frequently
used in that setting now. And due largely to the resources of the
internet, "Tesla coiling" has become a popular international hobby.
LC resonance is the key to the Tesla coil-
pumping energy into the secondary system at its resonant frequency allows
for a buildup of energy there, developing high potentials and long sparks.
Depending on the way a coil is built, transmission-line or antenna phenomena
have something to do with operation as well as lumped LC resonance, and
there is still a good bit of disagreement about how much this contributes
(open quarter-wave transmission lines have a standing-wave pattern, with
a voltage antinode at the open end and a current antinode at the closed
end). Tesla came up with a quarter-wave formula for the amount of
wire to wind on the secondary, and that has been a standard rule in coil
building for many years. But the well-performing modern coils use
rather large terminal capacitance (big toroids) and it has been shown that
large current flows throughout the whole secondary, not just the base,
in these coils- and furthermore, the current into the toroid appears in
phase with that in the base lead. The quarter-wave idea would predict
low current at the top of the coil and large current at the bottom, and
also a phase difference between top and bottom. If you follow old
'40's and 50's directions, and wind a really long amount of wire onto the
secondary, and you don't use a topload, you may see a standing wave pattern
by holding a fluorescent lamp near the secondary. It will light up
every odd quarter wavelength at the antinodes, clearly showing an antenna
effect predominating.
There has to be some sort of circuit to drive
the Tesla coil at resonance- you need a means of generating oscillations
at the right frequency. In most coils, the primary coil and the primary
capacitor are connected through a spark gap to make a disruptive oscillator.
The circuit produces a damped RF wave at the resonant frequency whenever
the gap fires. More recently, vacuum tube and transistor oscillators
have been applied to do the job. Tesla said that the spark gap was
a necessary evil and in his day it was. They are noisy, make ultraviolet
light, eat electrodes, and frequently involve moving parts. Tube and transistor
circuits don't have this element. These circuits take feedback from
the primary tank circuit in the proper phase and amplify it to generate
continuous undamped oscillations. But they don't give the same performance
as their more traditional spark-gap cousins- in particular, they have much
poorer efficiency in making long sparks. The reasons why are not
too clear to me. Maybe it has to do with driving wave shape and pulse
frequency, or with primary impedance differences between tube / transistor
coils and disruptive coils. Recent developments to improve efficiency
of tube coils include "sputter" or "staccato" modes -terms are due to J.
Freau- which are pulsed modes of operation where the oscillator operates
for a short time and is then off for an interval.
