# Why do primary coils differ in shape and location between different Tesla Coils? Answered

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I've been researching Tesla Coils, and one thing in particular strikes me, no one seems to agree on a shape and placement for the primary coil.

I've seen reverse cone, cylinder, perpendicular spiral, and just wrapped haphazard. You can see which coil is primary in the following images, as well as examples of different styles of wrapping for the coils.  I've also seen the primary coil placed at the base of the secondary, as well as in the center of the secondary.

I've gathered the following information in my research (I'm pretty sure everything is correct):
The primary coils, through induction, transfers the charge to the secondary coils.
The primary coils are much thicker to allow for the large amounts of current running through them.
For maximum induction, you want the same amount of surface area for primary and secondary coils (I've seen this written in different ways at different places).  (You don't not want Maximum Induction see: steveastrouk's comment below)
Primary coils are placed in a way to allow for....... (this is where things start to fall off and the reason for my question).

So, if your goal is to have the most charge transfered from one coil to another, why are there so many different styles? Does one style benefit over another? Is there a logical reason one would opt for one one style rather then another?

Thank you.

Image Credits:
Image 1: parts information
Image 2 & 3: cylinder and cylinder centered
Image 4: wrapped haphazard
Image 5: reverse cone
Image 6: not sure
Image 7: perpendicular spiral

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## Discussions

A flat spiral secondary is ideal because the distributed capacity is low and the tendency for streamers to break forth is suppressed, allowing you to use very high voltages with a relatively small coil. The primary is always ideally between 1-2 turns wrapped on the perimeter of the secondary. It is connected to a capacity that allows it to resonate at the same frequency as the secondary circuit. The secondary is, of course, 1/4 wavelength long and self resonant when connected to a small elevated capacity. This is the best way to build it according to the following publications by Nikola Tesla:

Colorado Springs Notes, Problem With Increasing Human Energy, and My Inventions (autobiography)

Flat spiral coils are best for high power coils (not sure why), cone primaries are best for low to moderate power coils and are apparently easier to tune, cylindrical primaries are usually lazy or used in solid state, bipolar or vacuum tube coils and wrapped hazardously is just lazy.
The hight and position of the primary is due to coupling, if the coupling is too low you move the primary up if it is too high you'll move it down and as Steve said you don't want maximum coupling only around 20%.

It is the focal point center differently with each design i guess

No, you DO NOT want "maximum induction", or coupling between the coils !!

This is operates by being a fairly high Q resonant system, and you don't want too tight a coupling between the coils.

So the art of design is to get enough, but not too much coupling, and that depends a lot on materials of construction, and the proportions used.

The actual voltage transfer ratio of Tesla coils is the square root of the ratio of their inductances if I remember right.

Steve

Cool. I've changed that part of the question a bit so as not to confuse anyone. Thank you. But I'm still unclear as to why there are so many variations of the shape of the Primary Coils. I feel that if one style worked, there should only really be that one style being used for larger coils. Am I missing something?

I am not a great Tesla Coil Guru, but the exact field configuration around your coil affects the nature of the discharges, and will throw them in different directions etc depending on the aesthetic you are after perhaps.

Steve

A lot of the primary coils are also  constructed to avoid being sparked by the
secondary Note the strike ring on your first pic.
My very best favorite Tesla site simply winds his primary on what looks
like an electrical pvc pipe.

A