Re: Dimensions of my flat spiral coil

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:
> 
> Original poster: "David Thomson by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>

Several comments about measuring techniques...

> >Interestingly, if you hook the windings in parallel, since they are very
> tightly coupled, I'd ballpark the resulting inductance at 9 times that of
> the single winding, or around 50-60 mH, just on the N^2 principle.
> 
> Well, let's try that and see what we get.  I hadn't thought of that.

This would be for three coils -in series-. In parallel the inductance 
of the association should be about the same of each individual coil
(you have a single coil made with three wires in parallel).

> Just out of curiosity I made the copper ring a complete circle.  Starting in
> the front of the meter and measuring the midpoint between outer leads and
> then the adjacent outer lead, next midpoint, next outer lead, etc., I
> received the inductances of 4.92mH, 4.90mH, 4.92mH, 4.90mH, 4.92mH, and
> 4.90mH successively.  The 4.92mH readings were on the midpoints.  After I
> finished the readings and went back to double check (about 1 minute,) all of
> the readings were 2mH lower than the first reading.

You added an extra coil with single turn, short-circuited, and coupled
to the others. This reduces the effective inductance.

> When I measured for internal capacitance by using the same configuration as
> the open ring above, I got the same capacitance measurements on where each
> lead connected to the ring.  But the capacitance reading continually
> fluxuated between 126.6pF and 128.0pF.  The same degree of fluxuation
> occurred at each point.

If you measured it by connecting a capacitance meter across the coils,
what you measured is a parasitic effect of the meter, that depends on
how it is built. The reading may be on the right magnitude order, but
it is surely not the "self-capacitance" of the coil.

You can have an estimate value of the self-capacitance of a coil by
measuring the capacitance between the coil and the ground. Connect
one of the terminals of the coil to one terminal of the capacitance
meter, leaving all other connections open, and connect the other
terminal of the capacitance meter to ground. 
The "self-capacitance" of the coil is then about one half of this value.
This is correct for a solenoidal coil. For a flat coil would be close 
too. 

> These are unique properties that may be able to be exploited.  Essentially
> this is implying that the resonant frequency of the coil can be changed on
> the fly.  Using the inductance range of 4.90mH to 6.25mH and the constant
> internal capacity of 128pF, the coil can be oscillated within a range of
> 200.96KHz to 177.94KHz.  Since you asked the question, I'll share this
> discovery with you, Jim.

You can really lower the inductance with a shorted ring near the
coil. For the "self-capacitance", revise you measuring method.

Antonio Carlos M. de Queiroz