Re: Tuning Isis and Osiris

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Nicholas Field by way of Terry Fritz <twftesla-at-qwest-dot-net>" <nick.field-at-hvfx.co.uk>

> Thanks for the good info. Now you tell me.  :-))
> I thought that splitting the 48 inch horizontal bipolar into a bipolar
twin
> would be mere tea and crumpets, basically just a spatial reconfiguration
> with little or nothing changed electrically. Indeedy, the twin has already
> produced
> five feet of white hot spark with the tune points not very much changed
from
> the original settings. I thot that
> the voltages at the base of each resonator would still sum to zero since
the
> bases
> are connected with a piece of wire. But now there is arcing between the
> primary
> and the grounded base and I don't know why.

I too have observed this.  It is only occasional, and easily solved
by connecting the midpoint to a reference ground.  My explanation
is that if other factors (proximity capacitance effects, mainly) alter
the capacitance of one of the two toroids significantly then this 'pushes'
the 0 Volt node away from the ground point.  This could be tuned out
of the system by adding capacitance to the other terminal, but for
reasons of practicality connecting up a reference ground is what we
tend to do.  It doesn't have to be a 'heavy' ground, a structural girder
or even water pipe will do.

> If we can use John F's equation for spark length, your 96 inches shows
> a multitippyplier of 1.85. Can I expect a higher multiplier than 1.7 for a
> well tuned
> coil? I figure Isis and Osiris should spark up at better than 80 inches.

I would imagine so.  On Isis the limiting factor is in fact the vertical
distance from the breakout points to the transmission lines.  For rental
use we don't let it arc longer than 8 feet, but with very slightly increased
power input it can hit 10.  However xmission line arcs and power arcs
to the floor become quite frequent. As we frequently operate on staging
where one can never be sure exactly what  is on the other side of the stage
we prefer to errr on the side of conservatism.  I would far rather a couple
of feet less arc than discover (!!!) that there's an unshielded audio or
lighting circuit running under the stage.

> I had again measured F sec for each secondary after spltting the bipolar.
> Each coil is exactly 1/2 L as the full coil. So nothing changed. Tuning
with
> the
> sig gen and an oscilloscope is nice but I also have a HP ACVTVM. I have an
> antenna connected to each; you can't beat an analogue meter for tuning
> purposes.

Remember to take into account the off axis inductance of the xmission lines.
Breakout points also seem to be fairly crucial to twin coils.  Without a
breakout point, and particularly with very smooth spun toroids, the two
resonators can become imbalanced, one breaking out before the other.
The base impedance of the streamer loaded resonator then goes sky high,
which is transformed into the primary.   This imbalance in the impedance
ratio of the two primaries means that far less power is available to the
non-sparking resonator, may not even get to break out, or will produce
much shorter sparks than the other.  Then the gap quenches and the whole
cycle starts again in 2ms time. So to the human eye this effect is not
apparent, one simply sees a poorly performing coil.

Safe Coiling
Nick

_______________
Nicholas Field
Director,  HVFX
www.hvfx.co.uk