# Re: K calculations

```Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 2/15/01 10:14:07 AM Eastern Standard Time,
tesla-at-pupman-dot-com writes:

>
> Original poster: "R Lunsford by way of Terry Fritz <twftesla-at-uswest-dot-net>" <
> Millipede-at-carolina.rr-dot-com>
>
> Ok, here is a question that requies a lot more theory than I have at my
> command...
> Why are the majority of primary coils built flat or conical in relation to
> the secondary? Wouldn't a cylindrical primary work best?
> _Robert Lunsford

Hi Robert,
A number of "professional" coils are built with cylindrical
(helical) primaries. The problems with helical primaries as I understand
them, stem from the shape and distribution of the electrical field around the
coils.
1) A helix  produces a tight, intense field at one end of the secondary, but
only at one end; the spiral produces a less intense but more uniform field.
2) Because of the proximity to the secondary, changing the tap-point on a
helix also changes the coupling and the field shape more radically than with
a flat or conical spiral.
3) A helical coil is much more tightly coupled to the secondary, which makes
inefficient and potentially destructive reflection back into the primary more
likely in TC applications, where discharges into the air is the aim.
4) The closer the end of the primary is to the top load, the better target it
becomes for streamers and arc-over. The potential difference between the
outer edge (tap point) of a spiral and the closest point of the secondary is
at most a few kV/inch, while with a helix, the difference at the top of the
primary could be 50-100KV/inch or more.
If your objective were high voltage output without arcs (e.g., using a
TC to power an X-ray tube, etc.) a well insulated, helical primary of large
diameter might be a good choice, but for amateur "thunder and lightning" the
spiral presents fewer problems of construction, smoother operation, and