On Fri, 27 Jan 2006 12:07:42 -0700, Charles Spitzer wrote:

"Dave Hinz" wrote in message
...

Titaniaum hand tools are what we use when _working on/in_
the magnets. I'm curious about your ring - does a magnet stick to it?
Could the moving around have been, instead, a _reluctance_ to move
in the field? A subtle difference.

well, i'm not sure it's pure Ti, nor the screws used to hold it to the bone.

I can't speak for your surgeons, of course, but the whole point of going
to titanium was to provide MRI-compatible implants. Even stainless
normally thought of as non-magnetic is magnetic at 1.5 or 3T.

on an xray, the screws look like 1" fine thread drywall screws.

Yup.

no, a magnet, even a super magnet doesn't stick to the ring. i thought it
was reluctance too, as it seemed to try to lineup a certain way in the tube.
it almost had enough force to move my finger if i relaxed my hand. do the
force lines move in an mri?

Well, there's the static field, which is very very (to a few parts per
billion) even within the imaging area. Then there's the dynamic
(Gradient) fields, in the x,y, and z directions. Those are at mostly
audio frequencies, which is why the scans are as noisy as they are.

So. Ring on a finger, dynamic magnetic field, inducing current into...

i wondered if they'd generate eddy currents in
the ring at the time.

I can see how that could create _torque_, if the ring was at an angle to
the flux lines. Hm. If I still worked there, I'd do a little
experiment. Ah well.

a neat trick: suspend a large horseshoe magnet on a string centered over an
Al plate that is on a low friction turntable. rotating the magnet will cause
the Al plate to rotate.

Sounds like the same eddy current effect I was describing with the coin,
can, and aluminum plate, yup. If that was more portable, it'd be a
great bar-bet...