All i can say is wow. Was surfing the tube, and stopped on the discovery
channel. They were talking about Leonard Lee developing a new scapel for
plastic surgery, and a new method of holding wounds closed. The one that got
me though was the mechanism for holding a hand open while it is operated on.
My wife commented "Wow. I think those are the rare earth magnets your always
Was an excellent show, i missed the first part though.
Just goes to show, a good idea is a good idea.
I don't remember the circumstances but one day LOML saw me using a
rare earth magnet from LV and was amazed. Fast forward a week or so -
she comes home from shopping and couldn't wait to tell me that she saw
a rare earth magnet used as the clasp mechanism on the front of a bra
(brassiere, Jeff). I wonder if that was Leonard's idea.
On 5 Oct 2004 16:40:09 -0700, half firstname.lastname@example.org (Mike) wrote:
I used to work at HP Labs, where every dilbertcage was issued with a
handful of assorted magnets to stop us sticking pins in the walls.
Naturally some people became collectors - they'd build cantilevered
coathooks out of stacks of 3/4" magnets, all sorts of magnet-art.
Two hazards we discovered pretty soon were that a stack of big magnets
flying together could blacken a fingernail, let alone raise pinch
blisters. The second was that the magnets chipped easily, and peeling
plating was razor sharp.
It's much harder than you'd think. You can't kill magnetic media with
externally applied magnetic fields, for most rational values of
"field" and almost all possible fields from a permanent magnet.
Distance is your enemy - even if you try to bulk demagnetise tape, you
need to place tape _between_ pole pieces to achieve any reliable
One of the best sources for cheap rare earth magnets is scrap disk
drives. Most contains two powerful ones, with large pole faces and
normally mounted only a few mm from the disk itself.
CRTs are a _much_ easier target.
Years ago when floppies were actually floppy I was teaching an intro
computer class, mostly college freshmen, some sophmores. One student
came in complaining that something was wrong with her data on her disk.
I inserted it in one of our Apple IIc's (yes, that long ago) and she
was right, nothing could be read. I asked her how she stored the disk
at home and she told me she kept it on the refrigerator with a magnet so
she wouldn't lose it. Go figure.
An old girlfriend decided to clean my computer room one
day and I came home to an unusable computer. The monitor
was all screwy. I checked the cabling, R&Red the graphics
card and cleaned the contacts, nothing helped. Then I
noticed that she had put my electric clock directly over
the screen, on a shelf 3 inches above the damned thing.
Since _I_ hadn't moved the thing, and everything on my
desk and shelves was all moved, I hadn't noticed it right
Needless to say, she didn't clean my computer stuff again.
Say, do you think we could use the local hospital's $1.3Mil
MRI to check for metal pieces in our boards? It should also
remove the pieces of nail, etc., right?
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Nope. I worked at GE, on MRI scanners, for about 10 years. In that
time there was, er, ample time for experimentation. It will not remove
a metal sliver from a finger, although it does make it move around
(to align with the flux lines) in a very uncomfortable way. It's
disturbing to wear steel-toe shoes near an MRI magnet. You could
scan the board to see the iron (it'd show up as a warpy-black-hole in
the image). A pumpkin looks really cool on a 3D MRI scan, but if you
scan it in "research mode" (no power limitation/monitoring) it turns
soft by the next morning. Liquid helium is an interesting substance
to work with, and superconductors can be very unforgiving.
So, yeah, you could _find_ the metal with the MRI scanner, and do a
spectroscopic analysis of the lumber even, but it's not useful for
removing anything. A hard drive rare earth permanent magnet has a
strength of maybe 5 Tesla, where most MRI scanners are only at 1.5 Tesla
(although with a _much_ larger field).
Dave "ask me what aluminum does in an MRI magnet...you'll be surpised" Hinz
I was hoping someone would ask me that. It's non-magnetic, even at
that field strength, but it's still electrically conductive. The flux
lines in that magnet run lengthwise through the bore, and out and around
the magnet in the shape you'd expect. But inside the bore, things get
interesting. The field is _very_ even in there, or imaging wouldn't
work. "even" down to "measured in parts per billion".
So anyway. Take an aluminum soda can. Not much weight, but a good
diameter. Balance it on your fingertips and let it fall away from you.
Tips over right away & falls down, as expected. Now do this in an MRI
magnet, and it tips veerrrrrrrrrrryyyyyyy sllllllooooowwwwwlllllyyyyy
until it's tipped enough to fall off your fingers, then falls at the
expected speed. Tipping, you see, involves this conductive thing
breaking flux lines. It sets up eddy currents inside the conductive
item, and the electrons going around the soda can, while very low mass,
are moving a lot of them at the same time. Keeps it from tipping, not
sure if that's a gyroscopic effect or something else. Gravity, of course,
works as expected.
A quarter takes maybe 1 second to tip over, a silver dollar about 3
seconds. But turn it 90 degrees (so the long part of the coin is in line
with the flux lines / bore of the magnet) and it tips over as normal.
Fun stuff. Almost makes going back to GE sound attractive some days.
Dave "Almost..." Hinz
On Thu, 07 Oct 2004 19:03:27 +0000, Dave Hinz wrote:
Sounds like fun. The eddy currents induced in the Al come with their own
magnetic fields, of course.
Are MRI machines' fields dipoles, quadrupoles, or something exotic? MRI is
just NMR, right? (Organic chem was a looong time ago.)
Back on topic, had a thought while reading the Toolbox Book and the LV
catalog in close succession: use the re magnets behind a felt layer for an
invisible french fit. (not "fit" -- blanked on the correct term.)
Right, so I suppose it's those two opposing fields that slow down the tipping.
On some of the test magnets, we had 2 foot square aluminum doors on the
ends of them; opening them just involved hanging on, leaning, and waiting.
It could _not_ be hurried. Good isometric exercise, though.
Dipole, north at one end of the bore, south at the other. It's the same
as NMR but the "N" was dropped because the public gets scared by the
I kind of like the hidden latch/cow magnet idea myself. haven't used it
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