Tesla List wrote:

> Would like to build a coil and I have a few questions.
> 1.  I have a cap rated at 1 uf  -at- 25kv.  cap says its DC but there is no
> polarity marking on the cap.  can I use it ???

The rating is high for a TC. Most small coils use .01 uF or lower. You could
build a coil that could tune to it. It may not be worth the effort unless
it's a good cap though. If it's not polyethene or polyproplene I wouldn't
bother since a TC will eat most other types for breakfast. ;) I'd just hold
onto it and use it in something else. But that's just me. For a first coil
performance isn't critical so you may want to just go for it. Just keep the
runtimes short when you start and check it for heating. If it gets hot make
sure to let it cool before running the coil again, or you risk blowing it.

> 2. I have seen pictures of the primary coil wound  vertical,
> horizontal  and conical...what are the differences in results.

The different forms change the "coupling". I'm sure someone else can give a
better explination of coupling. But basicly it describes how the primary and
secondary react to one another. For smaller coils the conical form seems to
be recomended, changing to the flat spiral for the larger coils. The vertical
(hellical) primary seems to be used only on magnifier designs for the most

> 3. I have a small dc motor that I can control the speed ....I am thinking
> of using it to power a Rotary spark gap.  Cant I tune the primary by
> changing the speed of the motor??

If you are new to coiling I'd recomend you go with the static gap to start
with. There is a lot to rotary gaps. Check out the RQ gap design. It's easy,
cheap, and performs well. Save the rotary for later. You can tune the coil
with the speed of a rotary, but I believe there are some limitations to this.
I have not done any work with rotary gaps yet, so perhaps someone who has can
comment more on it. I think you still have to tune the taps on the primary or
the size of the tank cap for best performance.