Re: magnetic fields

To: tesla-at-pupman-dot-com
Subject: Re: magnetic fields
From: "Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sat, 12 Feb 2000 09:52:47 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme


> >  Just a curious question, or two...If we could actually SEE
> > magnetic fields,
> > what would the field around the average, say 900va coil look
> > like? how far
> > out would it extend, etc...what shape would it take?
> 
> I have been thinking about this a lot lately myself.
> Surely some lab somewhere has equipment to view this.
> Albeit the measuring equipment probably effects the
> fields some what.  We have MRIs after all.  Does anyone
> know of a sight that has movie files or least images
> of these fields.

MRI's don't create images of magnetic fields.  Magnetic Resonance Imaging
actually measures the behavior of the molecules and atoms within the body
in response to a surrounding magnetic field.  On a very simple level, think
of an atom or molecule as a spinning top.  It wants to point a particular
direction. Put a magnetic field on it, and it will tend to move, and you
can detect this with a sensitive receiver. If you put a varying field on
it, at some frequencies the top will wobble more or less, which can also be
detected. These changes are unique to particular  configuration of atoms
(for instance, the CH3 methyl group) Now, sweep the frequency of the field,
and measure the effect over a 3 D area, and you can map which kinds of
things there are in which places.

This takes 1) A huge magnetic field over a large area (hence the
superconducting magnets used in an MRI) ( I restored a pre superconductor
NMR machine and the magnets weighed about 8 tons to produce a 25 kG field
(100Amps at 100V regulated to a few microamps)), 2) Very sensitive
detectors and multiple measurements to average out the noise, and 3) a
whole ton of processsing to reduce the data.