# Re: 2 layer primary puzzle/Parallel-Series as Parametric Oscillation

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

Hi Harvey,
This seems like an incredibly complicated reply to a
rather simple problem ;) :).
FWIW, I routinely used stacked spiral primaries, mainly because
it costs a lot less for copper pipe to score the same inductance as a
single spiral. It is also far more compact. The next primary I build
for an impending 12" coil will have three layers. Thanks to mutual
inductance between the layers, you can score around 70% more
inductance using two layers using the same length of pipe and
allowing for some clearance between the layers (e.g. an acrylic sheet
with a centre hole). The plumbing suppliers obligingly supply neat
two-layer coiled pipe in plastic bags here and all one has to do is
thread half of it through the centre hole in the sheet and hot melt
it to both sides.

Regards,
Malcolm

On 26 Sep 2001, at 8:16, Tesla list wrote:

> Original poster: "harvey norris by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <harvich-at-yahoo-dot-com>
>
>
> --- Tesla list <tesla-at-pupman-dot-com> wrote:
> > Original poster: "James T by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <jamest2000-at-att-dot-net>
> >
> > Hi Everyone,
> >  I will keep this brief. I am working on an 8" coil
> > with a 2 layer primary.
> > I have constructed the primary and
> > it is not behaving as I expected. I measured the
> > inductance of the primary
> > by itself.
> >   What happens is, I measure from the outside of the
> > top coil to the inside
> > and the inductance goes up as
> > expected. Then I drop down to the bottom layer and
> > measure from the
> > top/outside to the bottom/turn, oops, the
> > inductance goes down as I go out. The inductance of
> > the bottom coil seems
> > to cancel the inductance of the top
> > coil. Please set me straight on this.
> > 1.understanding/myth - the winding direction between
> > the 2 coils is irrelevant.
> > 2. understanding/myth - the "mutual inductance" of
> > the 2 coils increases
> > inductance, not cancels out the
> > others inductance.
> > 3. 2 layer primary's are a practical approach. I
> > seem to recall some posts
> > on this, and it was considered
> > reasonable. Brilliant to ask now that it is done!
> > Any comments will help. Thanks,
> >  James Cart
> >
> Oops,you said any,so heres a possible scenario for
> making a parametric oscillation.
>
> The above description does not sound right at all. If
> you had used magnetic cancellation the series
> connection between identically wound spirals should be
> on the inside, leaving two outside ends for inductance
> measurement. The inductance should then be reduced by
> the mutual inductance consideration. The opposite
> approach could also be TWO options, where identical
> spirals are NOT used, or rerouting the series
> connection back to the outside spiral, and making the
> measurement from outside to inside. In any case
> complications seem to rise from the energy storage
> between windings being increased. Whether or not this
> is a myth becomes relevant from a study of Tesla's
> patent indicating this, where I have placed an article
> entitled
> Myth of Tesla Patent 512,340 on 12/19/2000, at message
> board linked below, here a quote extracted;
<snip>

```