Science of Magnets

[Jerry Bruette]

I was reading in the Lee Valley catalog that setting magnets into cups increases their holding power. What's the science behind this? Is it because you're increasing the surface area or some other magical reason?

[Steven Cooper2]

The little metal bit around the magnet redirects the flux and allows it to reach maybe twice the strength as otherwise.
It's kinda magic.

[Bill Dufour]

Iron cup, not wood or plastic.
Bill D

[Jerry Bruette]

Iron cup, not wood or plastic.
Bill D
Ironwood won't work?

[Bruce Wrenn]

Bought some RE magnets and cups from Lee Valley years ago. They suggested filing a notch in one side of the cup, so you could pry the magnet out if needed.

[Lee Schierer]

The little metal bit around the magnet redirects the flux and allows it to reach maybe twice the strength as otherwise.
It's kinda magic.

The iron cup concentrates the magnetic flux making the holding power increase. No magic is involved, just science.

[Ken Fitzgerald]

The iron cup concentrates the magnetic flux making the holding power increase. No magic is involved, just science.


And then there is mhu metal.

[Larry Frank]

Mu metal..high nickel metal used for magnetic shieldimg. I had to look it up on Wikipedia.

[Kent A Bathurst]

Ironwood won't work?

Drop the mic and walk away, Jerry

I laughed out loud. Well done.

[Jim Dwight]

When a magnet is close enough to a piece of ferromagnetic material, the magnetic field of the magnet causes at least some of the atoms of the ferromagnetic material to align with the field of the magnet. The steel cup becomes magnetized. Normally the steel cup would have atoms arranged randomly. So you then have the original permanent magnet plus a temporarily magnetic cup. The magnetic properties of the cup are not permanent but will be present for awhile even if the magnet is removed.

[Bill Dufour]

If you work around high power magnets all iron stuff will get magnetized. My father always ha a small screwdriver and pocket scale in his shirt pocket. They were both magnetic from his job running a cyclotron. It had over 2,000 horsepower just for the steering magnets. Not including the main drive ones. I believe it had its own 12 KV switch yard to supply incoming power..
Used special bronze wrenches and hammers. He said you could not swing a iron hammer in a straight blow. Welders hated working on it. Even gas welders had issues.
BilL D

[Alan Lightstone]

If you work around high power magnets all iron stuff will get magnetized. My father always ha a small screwdriver and pocket scale in his shirt pocket. They were both magnetic from his job running a cyclotron. It had over 2,000 horsepower just for the steering magnets. Not including the main drive ones. I believe it had its own 12 KV switch yard to supply incoming power..
Used special bronze wrenches and hammers. He said you could not swing a iron hammer in a straight blow. Welders hated working on it. Even gas welders had issues.
BilL D
Did screwdrivers fly towards your father when he walked in your shop?

[Dave Fitzgerald]

Did screwdrivers fly towards your father when he walked in your shop?

Reminds me of a story I heard when high-field NMR was a new thing. (NMR is a lab tool for chemical analysis that uses rf signals in a magnetic field; it's the technology behind MRI scanners.) The machines have liquid nitrogen-cooled electromagnets, typically decorated with signs warning users to keep metal objects away. A maintenance worker came in one night and didn't heed the signage. He was found the next morning, unhurt but unhappy, attached by his tool belt to the casing of the magnet.

[Derek Meyer]

Back in the late 90's I worked at a computer store, and a few of the techs discovered rare earth magnets. I ordered a bunch of them (20 I think) and they came stacked together. They were about 3/4" x 1" x 1/4" or so. We were playing with them, marveling about how strong they were, and someone took the whole stack and walked past my workstation, close to my monitor, which was a 19" tube monitor. The magnets were so strong that they actually grabbed the shadow mask under the monitor's glass and pulled it out of adjustment. From then on, the screen had a moire pattern all the time, looking like it was slightly out of focus. Needless to say, we kept those magnets away from screens from then on.

[mike calabrese]

If you want to experience some very powerful pound for pound magnets tear open an old computer hard drive. in the pivot structure of the disc reader arm you will find some kidney shaped magnets about 3/4 long by 1/2 wide and 1/8 thick. Stick two of them together and try to pull them apart. Even stick one to your table saw cast iron top and try to lift it.
the power in this small form it amazing.
calabrese55

[Alan Lightstone]

Back in the late 90's I worked at a computer store, and a few of the techs discovered rare earth magnets. I ordered a bunch of them (20 I think) and they came stacked together. They were about 3/4" x 1" x 1/4" or so. We were playing with them, marveling about how strong they were, and someone took the whole stack and walked past my workstation, close to my monitor, which was a 19" tube monitor. The magnets were so strong that they actually grabbed the shadow mask under the monitor's glass and pulled it out of adjustment. From then on, the screen had a moire pattern all the time, looking like it was slightly out of focus. Needless to say, we kept those magnets away from screens from then on.
I saw something similar in the OR when someone placed a pacemaker magnet on the side of the anesthesia monitor. Pretty well made it useless (in the middle of an operation) in seconds. Probably $15K screwup.

[Dave Zellers]

If you want to experience some very powerful pound for pound magnets tear open an old computer hard drive. in the pivot structure of the disc reader arm you will find some kidney shaped magnets about 3/4 long by 1/2 wide and 1/8 thick. Stick two of them together and try to pull them apart. Even stick one to your table saw cast iron top and try to lift it.
the power in this small form it amazing.
calabrese55

Whenever I am trashing a hard drive that has failed, I always rescue those magnets and then destroy the discs. They are just fun to have.

[Ken Fitzgerald]

I began roughnecking on oil rigs for my driller father at age 15 full time, going to HS days and working morning tower. Somewhere along the way I was driving a set of worn teeth from a set of tongs when a sliver of the steel flashed through the air, through the skin on my left forearm where it remained for nearly 50 years. When I began servicing 1.5Tesla MR scanners, in and around the magnet I could feel that sliver move but it created no problems. However, shortly before I installed and serviced my first 3Tesla magnet, I decided that sliver needed to be gone. The little routers used in the system computer box were merely held in place with rare earth magnets. I had just changed a router and had a set of those magnets. My personal physician was a surgeon, and I had an appointment. I walked into this office for that appointment, climbed up on the exam table, and laid down as instructed. You could see the blue of the entry hole on my forearm. When he said locating the steel might be a problem, I pulled the rare earth magnets from my pocket, placed them over the blue entry hole and the skin stretched out, leaving no doubt where the steel was. He asked me if I minded he show that to his staff. They were entertained. He prepped the area, numbed it using Novocain and made a small incision. The magnets brought the sliver out rapidly.

The historically accepted magnet strength of the earth was 0.6 gauss. In the last decade or so physicists were reexamining and debating it. 1 Tesla equates to 10,000 gauss. A 1.5 Tesla MR scanner magnet has a field strenght 25,000 times the earth's magnetic field strength. A 3.0 Tesla MR scanner magnet has a field strength 50,000 times the earth's magnetic field strength. The precautions you have to take when working in those conditions are pretty important to be observed. I have seen and retrieved a patient gurney not meant to be used in MR, after an arrogant ER nurse nicknamed Gunny rolled the patient into the exam room. I removed an anesthesia cart from a MR magnet after a nurse-anesthetist accidentally rolled it into the MR exam room. You can't just muscle large pieces of steel off those magnets either. The magnet windings are supercooled with liquid Helium, IIRC the temps are in the -469ºF. The metal is brittle at those temps. Muscling the steel off could potentially physically break the wire, destroying the magnet. The magnets have no applied power. You use giant power supplies to "ramp" it up to field strength, turn off the heated portion of the coil so it goes persistent with the current just circulating within itself and then remove the power supply. It gets shipped back to a regional hub for shipment to the next site who needs it. It takes about 72 hours to ramp a magnet down, remove the steel, ramp it back up to field strength, allow it to stabilize and then shim the magnet for homogeneity.

The first MR scanners required mhu metal walls, about 3" thick to keep the magnetic field in the exam room. Later MR scanner magnets had an outer reverse wound set of magnet coils that used the same current circulating through the field magnet to prevent/or dramatically reduce the magnetic field beyond the magnet itself.

I played with some strong magnets professionally for 23 years until I awoke deaf in 2010.

[Kent A Bathurst]

A 1.5 Tesla MR scanner magnet has a field strength 25,000 times the earth's magnetic field strength. A 3.0 Tesla MR scanner magnet has a field strength 50,000 times the earth's magnetic field strength.

Far out!! You know a lot of really cool stuff, Ken.

I have regular MRI scans as they keep trying, and failing, to find any reasonable facsimile of brains. I'm gonna show off my knowledge to the techs next time. And - yeah - keep wearing the sweat pants with no zippers, eh? Good grooming tip.

[Alan Lightstone]

I used to anesthetize patients in MRI scanners. Pretty difficult to get equipment that both worked in/near magnet plus was safe. Multiple incidents worldwide of catastrophes.

Really not my favorite place to take care of patients.

The most interesting location was the cyclotron secretly hidden under Harvard, built for the Manhattan Project. An hour of setup for a fraction a second of treatment on a tiny baby to cure their retinoblastoma. And running like hell to get into the control room with loud bells going off as the doors automatically closed. Pretty cool place. Plus nice to see daylight as a resident.

[Ken Fitzgerald]

Far out!! You know a lot of really cool stuff, Ken.

I have regular MRI scans as they keep trying, and failing, to find any reasonable facsimile of brains. I'm gonna show off my knowledge to the techs next time. And - yeah - keep wearing the sweat pants with no zippers, eh? Good grooming tip.

By far, the most common MR scanners use 1.5 Tesla magnets. High field magnets as high as 6.0 Tesla I have seen reports of being used.

Believe it or not, there was at least one company who used large rare earth magnets to create MRI scans. Those typically were strictly used on appendages, arms, shoulders, hands, ankles, knees, etc. I know because my employer bought one of those companies resulting in me having to service a couple of those scanners. They were "low" field magnets of about 0.2 Tesla IIRC. It's been a few years and my memory isn't what it may have once been.

Kent, "live" MR scans happened after my awakening deaf in July 2010. I would love to find a source to read about the theory/physics of the RF pulses and magnetic waveforms used to do "live" MR scans!

A story about sweatpants and Gunny. On April 6, 2001, I broke my back. It was pay day and my wife had left early to go shopping. I managed to get myself into the house and laid on the living room floor until my wife arrived home. I rolled over, did a pushup and went to the bedroom. I changed out of my jeans into a pair of sweatpants. Having worked in radiology for 25 years at that time including on x-ray equipment, I was quite aware I didn't want to be wearing any metal as I would need x-rays. My wife drove me to the ER, where I was placed in a wheelchair and given a ride to radiology by "Gunny". She pulled me up the dressing booths and gown area telling me I needed to change into a gown. I said "No I don't! Watch this...." at which point I climbed out of the wheelchair and walked into the x-ray working area with my exam order. At that time I had been working in that radiology department and medical center for 19 years. One of the technologists said Ken what's up. I told her I needed a x-ray, she looked at the order and said "Room B is open. Let's go in there." Gunny was standing in the doorway, mouth hanging open trying to figure out who I was.

[Ken Fitzgerald]

I used to anesthetize patients in MRI scanners. Pretty difficult to get equipment that both worked in/near magnet plus was safe. Multiple incidents worldwide of catastrophes.

Really not my favorite place to take care of patients.

The most interesting location was the cyclotron secretly hidden under Harvard, built for the Manhattan Project. An hour of setup for a fraction a second of treatment on a tiny baby to cure their retinoblastoma. And running like hell to get into the control room with loud bells going off as the doors automatically closed. Pretty cool place. Plus nice to see daylight as a resident.

Alan, I felt sorry for the nurse-anesthetist I wrote about. He had just moved to the area from Southern California. He was, in fact, getting setup to anesthetize a young child. You know how stressful that can be regardless! When I arrived, the cart was literally stuck to the middle of the open bore. We typically use a 2-man rule. If 2 men can remove steel using moderate force, you can remove it without having to ramp the magnet down. However, if they can't, then then call for the power supplies and get prepared for the next 72 hours, minimum. My coworker and I couldn't remove the cart. I thought maybe if we removed each of the individual drawers, one at a time, we could then remove the main body using the two-man rule. Thinking of the possession of the drugs, I had the techs contact the anesthesia department for someone to take possession and bring a key for the cart. The guy shows up standing in the exam room doorway. I asked him "Did you do this?" He sheepishly nodded. I told him the plan, took the key and tried to unlock the cart. I broke the key off. This poor guy wilted! He had tears running down his cheeks. I took my MR safe tools and tried to remove the drawers by breaking them. No luck. Two days later while ramping the magnet back up, I heard he had resigned. The day following the incident, he came into to his locker in surgery and found it covered with refrigerator magnets. I felt so sorry for that poor guy. We are all human, can make mistakes and I can imagine the stress of trying to anesthetize a child in an MR scanner.

[Alan Lightstone]

Alan, I felt sorry for the nurse-anesthetist I wrote about. He had just moved to the area from Southern California. He was, in fact, getting setup to anesthetize a young child. You know how stressful that can be regardless! When I arrived, the cart was literally stuck to the middle of the open bore. We typically use a 2-man rule. If 2 men can remove steel using moderate force, you can remove it without having to ramp the magnet down. However, if they can't, then then call for the power supplies and get prepared for the next 72 hours, minimum. My coworker and I couldn't remove the cart. I thought maybe if we removed each of the individual drawers, one at a time, we could then remove the main body using the two-man rule. Thinking of the possession of the drugs, I had the techs contact the anesthesia department for someone to take possession and bring a key for the cart. The guy shows up standing in the exam room doorway. I asked him "Did you do this?" He sheepishly nodded. I told him the plan, took the key and tried to unlock the cart. I broke the key off. This poor guy wilted! He had tears running down his cheeks. I took my MR safe tools and tried to remove the drawers by breaking them. No luck. Two days later while ramping the magnet back up, I heard he had resigned. The day following the incident, he came into to his locker in surgery and found it covered with refrigerator magnets. I felt so sorry for that poor guy. We are all human, can make mistakes and I can imagine the stress of trying to anesthetize a child in an MR scanner.
Yeah, OR staff are often intensely sarcastic, and sadly cruel to each other. Refrigerator magnets. Sigh.

There have been deaths from flying objects in MRI scanners (oxygen cylinders if memory serves).

And the apocryphal stories of stupidly stood up, dropped oxygen cylinders acting like torpedoes going through walls. Mythbusters showed it really can happen. Impressive episode.

The smart, experienced, gray haired staff seemed more careful than the youngins. Been around the block. Seen and heard about awful accidents.

[Ken Fitzgerald]

Yeah, OR staff are often intensely sarcastic, and sadly cruel to each other. Refrigerator magnets. Sigh.

There have been deaths from flying objects in MRI scanners (oxygen cylinders if memory serves).

And the apocryphal stories of stupidly stood up, dropped oxygen cylinders acting like torpedoes going through walls. Mythbusters showed it really can happen. Impressive episode.

The smart, experienced, gray haired staff seemed more careful than the youngins. Been around the block. Seen and heard about awful accidents.

I actually saw the resultant hole in a concrete block wall in an exam room from an accident where a small empty oxygen bottle, unsecured was knocked over and rolled into the exam room. The bottle rolled into the room, was grabbed by the magnetic field, pulled and accelerated through the bore of the magnet striking the wall behind the magnet. It then returned to the magnet, oscillating back and forth within the magnet for a minute or so until it came to rest in the center of the bore.

In the first instance I wrote about, the tech had come back from getting coffee to find Gunny had rolled the patient into the room and the foot end of the gurney was stuck to the center of the bore. It was a Saturday when the tech called our home asking what to do. I asked if the patient was still on the gurney and she replied he/she was still on the gurney. I told her I was on my way, to carefully get the patient off the gurney and out of the exam room.

[Patty Hann]

Ken (and Alan too), those are amazing stories.:eek:
Thank you for taking the time to tell it all and in detail. :)
Whenever I have an MRI done --(too often as of late *sigh*)--- it's always using the 3T magnet.

[Ken Fitzgerald]

Alan,

I have a cousin, now retired, who was responsible for developing, building, installing and maintaining the magnets used at Chicago's Fermi Lab's particle accelerator. He's the smart one!

[Kent A Bathurst]

Alan,

I have a cousin, now retired, who was responsible for developing, building, installing and maintaining the magnets used at Chicago's Fermi Lab's particle accelerator. He's the smart one!

Sounds like the family bidness. Anyone else?

[Bert McMahan]

Perhaps an interesting anecdote- my company makes actuators for very strange environments, including for use inside of active MRI machines. We once made a small actuator that would sit inside an MRI tube and wiggle whiskers on rats on demand. Some scientists were studying brain activity and were able to watch the rat's brain "do stuff" when wiggled with our actuator. We had to use absolutely no steel and no regular magnets or electromagnets of any kind. Standard motors are right out :)

[Ken Fitzgerald]

Perhaps an interesting anecdote- my company makes actuators for very strange environments, including for use inside of active MRI machines. We once made a small actuator that would sit inside an MRI tube and wiggle whiskers on rats on demand. Some scientists were studying brain activity and were able to watch the rat's brain "do stuff" when wiggled with our actuator. We had to use absolutely no steel and no regular magnets or electromagnets of any kind. Standard motors are right out :)

That's what I was referring to when I mentioned "live" MRI. I would love to find a source of reading material to gain knowledge of the physics behind that type of MR studies. Our MR safe tools were IIRC a bronze/aluminum alloy. Non-ferrous. Some of the tools were brittle, some were soft. "Reefing" on one could ruin it in quick fashion!