DC Tesla Coil - was rotary spark gap

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "David Thomson" <dwt@xxxxxxxxxxxx>

Hi Gary,

Thanks for posting your work online.  Your designs are easy to follow.

I haven't built a rotary gap yet.  But I'm moving in that direction.  My
next coil will be a small 75 watt DC coil.  I'm going to experiment with a
variometer in the primary circuit for tuning.  The variometer will be
modeled after one of Tesla's designs involving two flat spiral coils on a
sliding brass rod.  The centers of the coils will be connected through the
brass rod.  The coils will be wound with 24 gauge 1:4 ratio flattened magnet
wire.

I wound two 13" flat spiral coils on 3/16" Plexiglas, one with the windings
going the same direction and one with the windings going opposite
directions.  With just one 13" flat spiral coil, the inductance was about
4.5 mH.  With two flat spirals wound in the same direction at 3/16" apart
and connected through the center, the total inductance was just 1.11 mH for
the entire wire length.  With the same parameters but wound in opposite
directions, the total inductance was 121 mH.  The opposite wound coil has
other interesting properties as well.

I'll use the opposite wound coils on a sliding brass rod to adjust the
inductance in the primary circuit since it will give the greatest range.

I think that because I'll have so much control over the primary resonance
that I'll just use a static gap to begin with.  But since I have no
experience with rotary spark gaps I can't help but wonder if I'm missing out
on an opportunity to significantly increase the frequency and overall
performance.  What do you think?

Dave

> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Sunday, November 28, 2004 5:10 PM
> To: tesla@xxxxxxxxxx
> Subject: RE: tungsten alloys for SG electrodes?
>
> Original poster: "Lau, Gary" <gary.lau@xxxxxx>
>
> Hi Dave:
>
> I've not attempted a spherical RSG contact, but I have attempted a
> cylindrical contact, perhaps with a similar underlying motivation.  My
> goal was to use as large a ROC as possible on the electrodes to minimize
> the gap at which the RSG would fire, so as to minimize losses.  This
> attempt is illustrated on the lower half of my SRSG page at
> http://www.laushaus.com/tesla/sync_gap.htm.  I never did get that gap to
> work reliably.  I have a half-baked theory that the larger electrodes
> resulted in a longer effective dwell time, and the RSG wanted to fire
> well in advance of actual alignment.  In the end, the propeller gap with
> simple welding rod electrodes worked extremely well and better than the
> larger cylindrical electrodes.
>
> Regards, Gary Lau
> MA, USA
>
>  > Original poster: "David Thomson" <dwt@xxxxxxxxxxxx>
>  > I realize you're building a rotary spark gap, but the tungsten balls
> are
>  > excellent for a static spark gap.  I brazed the tungsten balls on the
> ends
>  > of tungsten rods and they work great.  The balls on McMaster Carr are
> grade
>  > C1/C2, which is very hard and doesn't pit.
>  >
>  > I'm now curious as to whether a spheroid contact would work with a
> rotary
>  > spark gap.  I may try to build one.
>  >
>  > Dave
>  >