Re: Dimensions of my flat spiral coil

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hello David,

On 15 Feb 2002, at 10:47, Tesla list wrote:

> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>
> 
> Hi Malcolm,
> 
> I have wound a total of four flat spiral coils so far (I'm winding two more
> tomorrow.)  The experiments that I have described on this list were
> performed on two particular coils.
> 
> Before I go on, I want to share that large is not necessarily better for
> transformers of 15KV 60mA or less.  The inductance of my 29" coil was so
> strong, or there is a strange effect on the voltage, that my LCR meter could
> not take a reading.  Also, no matter what capacitance I used in the primary,
> 8 turns on the primary with a 15KV 60mA transformer could not start
> oscillating.

The high self-capacitance might have been interfering with the 
measurement. I'd look for simple answers before diving into complex 
ones.

> My 13" coils have provided ample flat spiral action and can be put into
> action with 3.5 turns on the primary and .026uF of capacitance in the
> primary circuit.  I could not get a 15KV 60mA NST to start oscillations in a
> flat spiral coil at the measured resonant frequency.  I've had to operate my
> coils at about 3 times the resonant frequency to get any action.

That explains to me why the spark results are so poor. Something is 
wrong with the method somewhere I'd say.

> The first 13" diameter coil is made from 21 gage magnet wire and wound from
> the center out.  There is no inner radius other than the width of the 1/4"
> brass machine screw.  The coil is single layer and there are no cross over
> wires.  The finished coil has a slight oblong shape due to the difficulties
> of winding.  It has a measured inductance of about 35mH.  The calculated
> inductance was 5.7mH!

When a discrepancy like that rears its head, it is wise to start 
looking for a reason rather than immediately believe that something 
anomalous is happening. Was your formula correct? You've previously 
stated that your instrument had trouble reading one coil also.

> The second coil was 13.75" diameter and also made from 21 gage magnet wire.
> The coil started with three independent strands connected to the 1/4" center
> post.  All three were wound parallel in a single flat layer.  There were no
> crossed over wires in this coil either.  It was actually easier to wind this
> "wye" coil as I call it as three wires required 1/3 the number of turns to
> complete the winding.  On this coil I ended the three wires at 120 degrees
> (more or less) from each other.  This resulted in three different
> inductances in each of the three wires.  The inductances were 6.20mH,
> 6.23mH, and 6.25mH.  Oddly enough, each of the three wires had pretty close
> to the calculated inductance!

Oddly? I would have expected them to be roughly the same.
 
> The purpose I originally intended for the wye coil was to feed it with three
> phase alternator current.  Had I realized the inductance difference between
> the three wires I would have had all three wires end at the same degree.
> 
> Even without running the above coils, just the fact of the inductance
> measurements is odd in itself.  A single wound coil yielded, in this case, a
> 7 to 1 gain over the calculated inductance.  In the wye coil, each wire
> measured close to the calculated inductance, but there were three of them.

A single wound coil with the same inner and outer diameter *should* 
have a much higher inductance than one with the same geometry and 
wound with three wires in parallel. There will be three times the 
number of turns contributing to the inductance.
 
> I am very interested to hear the explanations for the discrepancies between
> actual design and calculations from the skeptics.  If everything we need to
> know about coils is already known, what is the simple answer to these
> measurements?

See above. 
 
> When I characterized the frequency of the single wound coil, the resonant
> frequency band was from 50KHz to 100KHz.  But I can't make the coil operate
> at that frequency.
> 
> I look forward to your report as to the characteristics of your coil.

I will let you know when I have wound one. *I* do not expect to find 
anything unusual.

Regards,
Malcolm