Re: Dimensions of my flat spiral coil

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Dave- 

Thanks for posting this info. comments intersperced. 

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

This one bugs me. Is it possible human error snuck in and it was 3.5mH? If
there is a good deal of spacing between turns (that which accompanies the
difficulties of winding such a coil), the inductance will be less than a calc
if it's not averaged and put into the equation. 

I know I'll have to wind a coil now. I'm sure someone else is also winding one.
I've been needing to put my new LCR through a good test anyway. I will also
measure the resonant frequency and whatever other intricacy's I can find. First
things first, the coil. 
>
> 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!

Now this sounds correct. Each wire is one width apart and have different radius
(slight, but differnt). This tiny variable will stack up for the number of
turns. I would expect to see 3 seperate inductances slightly apart. 

Take care, 
Bart