Comment by jacquesm | Hacker News Reader

Comment by jacquesm | Hacker News Reader

I've played around with some pretty bad ass magnets during the time that I was building wind turbines and one of the more interesting effects was that if you dropped one near anything made of steel you were actually in danger of getting shrapnel embedded in your body.

They move so fast it is scary, sometimes they explode on impact. This makes you pretty nervous about dropping them.

Then, one day one got dropped over a chunk of solid aluminum. It floated gently to the metal landing with a soft 'click'. Besides the initial surprise (I realized the eddy currents induced a magnet field of opposing polarity in the aluminum) what struck me most was the force of that opposing magnet. If you tried to force the magnet close to the aluminum at speed it would resist so strongly that you never managed to smash it into it with any kind of effectiveness. Always just that soft 'click'.

I still have a bunch of 3"x2"x1" neos waiting for some project, and whenever someone visits that's interested in technology I show them what those things can do, if you have tried to pry one of those from a chunk of solid steel (or if you're unlucky, another magnet) you know what I mean when I say I have a lot of respect for those little golden blocks.

Video of a large magnet being dropped onto aluminum: http://www.youtube.com/watch?v=iABmUEH5s0k

I wonder if anything happened to the guy's watch? It looks like a movement piece rather than digital.

I think you can wear a steel watch around a strong magnet, there will be little magnetic filed penetrating the outer body. AFAIK, gold watches are not recommended.

I had no idea the magnetic screening was so effective.

There's a story floating around my school of a student a few years ago who wanted to make a sort of hovercraft by fixing in position two opposing large supermagnets (I go to an art school). He couldn't build it when his supermagnets arrived in the mail because it was deemed too dangerous, supposedly they could have snapped or crushed someone's arm/leg easily had they come undone, and that was assuming he even survived the construction himself.

Let me give you one example of just how dangerous these things are from my own experience.

I was assembling the rotor/stator assembly of my windmill, to be safe I'd bolted the stator to the welding table and was lowering the the rotor around it using a chainfall.

Suddenly and to my complete surprise the welding table leapt up to the rotor just to hang there suspended by the magnetic field.

The table surface was a 3/8" thick 8x4 sheet with a fairly sturdy steel frame under it. You'd be hard pressed to move it, let alone lift it.

Fortunately I didn't have any fingers in the airgap or I would probably be typing this a whole lot slower.

Wow, I have experience welding and am trying to imagine this happening to the table I used. Sounds like that could have been a lot worse.

I don't really know much about this sort of thing short of seeing a huge generator on a field trip to a dam in sixth grade. The stator spins around the rotor which contains magnets, and these magnets have to be constant, permanent ones. Is that correct? I assume it's not viable to use electromagnets for this purpose since they use electricity. I bet it would be nice to be able to turn them off.

Yes, they're permanent magnets.

The reason why you don't use electromagnets on a windmill this size is that the additional complexity isn't worth it. Though it would make assembly and dis-assembly a lot easier / safer ;)

It definitely could have been a lot worse, I recall having a tea break after it happened, once I stopped shaking enough to hold a glass without spilling.

Here is a picture of what the thing looked like in that stage of the final assembly:

http://pics.ww.com/v/jacques/renewables/windmill/dscf0312.jp...

And is this the table? http://pics.ww.com/v/jacques/renewables/windmill/dscf0220.jp...

Hehe, yes. And on the table is my Texan friend Ron B. who is pretending someone stabbed him with a very large chisel.

Ron is a God when it comes to woodworking and helped out with the making of the blades when I got stuck, he traveled all the way from Texas to Northern Ontario for that. Amazing guy, really.

Actually large generators normally do use electromagnets today. They need an external source of power when starting, but drain off a very small part of the generated power to keep running. As jacques wrote, that isn't done with small generators because the added complexity usually isn't worth it.

Did you take a picture?

I took plenty of pictures (see link above and other pics in that album) but not of that particular mishap, it was decidedly unsafe to leave it all hanging suspended like that so we lowered the table/windmill combo to the ground using the chainfall.

I'd hate to have had the whole thing give and land on someone's foot.

The rotor was only press-fit into two bearings, no point in trying to find out if it would hold or not, it was meant to be pushed on, not pulled on by a welding table.

See also: The gallery of flying objects sent in by people who work with MRI machines:

http://www.simplyphysics.com/flying_objects.html

I worked with an NMR machine in college and would always have to empty my pockets before entering the room. I'm not sure I really understood the danger. I heard stories about people having piercings ripped out though.

A child was killed in 2001 when an oxygen tank got pulled into an MRI machine he was in: http://www.nytimes.com/2001/07/31/nyregion/boy-6-dies-of-sku...

Some system or another was running low on oxygen, someone panicked and grabbed a tank carelessly, next thing you know a tank is free where it shouldn't be.

Projectile injuries are one issue. Heating, induced current, hearing loss are others. FDA is investigating setting up a registry for safety problems to help reduce the frequency of accidents. http://www.auntminnie.com/index.aspx?sec=sup_n&sub=bai&#.... There are some shockers out there. Metal detectors on doors etc. Stopping the guy with a pacemaker is important too.

I've heard of a technician getting killed by flying keys of somebody else when standing next to a NMR machine

And those, i think, are just from the fixed magnets (which may be a type of superconducmting electromagnet thats just normally on.). During a scan, dont mangetic fields go way way up? The fixed ones are just there for stability and whatnot.

They are superconducting magnets kept that way through cryogenic helium. They can lose their magnetism through a process called quenching. If you search on youtube for "MRI quench" there are several videos of people messing around with MRI's before they get decommissioned. Not exactly safe.

http://www.youtube.com/watch?v=5z33ZcDgavY

http://www.youtube.com/watch?v=6BBx8BwLhqg

There is a lot of helium in there and it can explode:

http://www.youtube.com/watch?v=1R7KsfosV-o

Musthave been big. Theyd have to be anchored somehow though..... There is no stable equilibrium for ferromagnetic materials.

I once played with aluminum bottle inside a 3T MRI. It was weird - you could rotate it, but the faster you did it, the harder it was.

I believe it's related to this. Try leaning into the bore fast then pulling your head out, fast. Many people lose their balance temporarily. Something to do with inducing currents in the fluid in the inner ear I believe. A smaller version of the effect causes the sensation that you are moving when you lie still in the bore.

Edit: can't get the link easily as am on iPad, but google "MRI eddy currents" and watch a slab of aluminium fall

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