RE: Magnetic Field in a Flat Spiral

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

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

Hi Paul,

You can get the magnets in many shapes and sizes from Forcefield
http://www.wondermagnet-dot-com/dev/magnets.html pretty cheap.  I used magnet
number 11.

Do you have a MathCAD sheet for magnetic plots?  I'll check out the Biot and
Savart's law anyway, but if time can be saved, that's always a good thing.
It will be interesting to plot the magnetic field for a flat spiral
secondary.

I've been measuring the characteristics of my coils.  The data will be
forthcoming after double checking.  BTW, what is the formula for calculating
the Q of a coil by taking the two voltages on either side of the resonant
frequency?  I'd like to check the efficiency function on an LCR meter.

My latest coil, that is named Chiron, is nearly finished.

Outer Diameter = 25.25"
Inner Diameter = .25"
Wire gage = 12
Turns = 145 (counted, but will double check after resin dries)
Measured inductance = 4.78mH on first day before polyurethane, 4.17mH on
third day after 3 coats of polyurethane.
Measured efficiency = .20 before polyurethane, .32 after polyurethane

The windings on this coil are flat and near perfectly spiral.  The geometry
is a near perfect circle.  I've made the outer lead feed directly through
the 3/4" base at the outer winding to reduce interference with the primary.

Perhaps the bigger diameter wire and lower inductance will give better
results.  This coil will be fed with up to 3600 watts through a 14.4KV
potential transformer after testing with the 15KV NST.

Dave

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, March 05, 2002 1:46 PM
To: tesla-at-pupman-dot-com
Subject: Re: Magnetic Field in a Flat Spiral


Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<paul-at-abelian.demon.co.uk>

David Thomson wrote:
> NIB magnet in various positions around the coil.

That's a really neat experiment - what a nice demonstration of the
shape of the field around a coil, every school physics lab should
have some of those NIB magnets, not to mention a TC or two!

Your description of the shape of the field as determined by the
magnet's vibration is very clear and spot on.

Beats the heck out of iron filings any day!  Nice one.

FYI,
It's quite easy to plot the magnetic field around an air cored coil,
just apply Biot and Savart's law to lots of points in a plane
containing the axis of the coil.  Work in cylindrical coordinates so
that you only have to sum components in the r and z directions, then
plot a little arrow at each point.  An easy and worthwhile
programming exercise - an hour or two, tops.  A little more of a
challenge if you want to plot field lines.

BTW, what's an NIB disk magnet, and where do I get one from?
--
Paul Nicholson
--