I got a package in the mail today. A demagnetizer from china. (I have no idea who sent it, someone here?)
Anyway I cracked it open and looked inside.
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The bottom piece seemed to be a simple dipole. Magnetic moment pointing vertically. The top piece was different. (more of a quadrapole) With the center of the top one pole (north say) and the outside south, and opposite that on the bottom, center south, rim north.
I'll still have to think more about how it works.
George H. (If someone here sent it, then thanks for the gift.)
ter of the top one pole (north say) and the outside south,
A few months ago a demagnetizer from china also tempted me. Until someone in the group more-or-less said that my old Weller 8200 N makes a great demagnetizer. Indeed. It quickly and thoroughly demagnetizes tools passed between the twin leads of the heating element. It's perfectly demagnetized tools at least a dozen times for me so far.
There's always a bunch of old hard drives in need of destruction in my bone pile. (One of them recently got destroyed to make voice coil wire measurements for another thread.)
The neodymium magnets in hard drives *love* to instantaneously magnetize any tool that comes close. In the past, that meant keeping a special set of hard drive destruction tools that were unwantedly magnetized.
Nowadays, any tool within arm's reach can be used with impunity and easily demagnetized afterward. Page 122 of the Epcos Data Book [1] explains why this technique works.
The demagnetized state can be restored at any time by:
a) traversing the hysteresis loop at a high frequency and simultaneously reducing the field strength H to H = 0.
Grin yeah, I bought the orchid two winters ago from home depot for ~$10. I repotted it last year and it's blooming for the second time. (It gets totally pampered for a plant, fed with orchid food once a week.)
For mine, the bottom magnet is a simple dipole, the other is like two dipoles one a ring around the other... North on top in the middle of the disk, south pole on the bottom middle, reversed on the edges... does that make sense? I can draw a picture.
I can imagine that as you move away from the center and up that the total field points one way and then the other.
I could try scanning it with a hall probe... but that seems like a lot of work.
It's a laminated sheet of plastic with green and black particles in it. It shows the flugs from magnets placed near it. I'd have to see what company made it, they used to hand them out out at trade shows. It's a handy little thing to have.
I think the magnets are identical. And are held so the field between them is the strongest. And the demagnitising bit is a less strong field and y ou just keep running it thru a less and less strong field by using the mold ed plastic steps.
To me one clue is that the magnets have no marking on them. If they were not the same, one would probably be marked so you could tell which magnet t o put where. Occam's razor would suggest that the simplest way is what it is. How tricky can it be for $2,
em is the strongest. And the demagnitising bit is a less strong field and you just keep running it thru a less and less strong field by using the mo lded plastic steps.
Well you can think all you like... try a measurement! (I've got these little magnet toys,,, magents on the end of plastic rods, I used them to probe the direction of the field.
Here's something you could try.. (I'll give it a whirl when I get home.) Mark your magnets, take the top one out and flip it around. My prediction is it won't de-magnetize as well. (hmmm with identical magnets you might make the same prediction.... Oh well.)
George H.
e not the same, one would probably be marked so you could tell which magnet to put where. Occam's razor would suggest that the simplest way is what i t is. How tricky can it be for $2,
For mine, the bottom magnet is a simple dipole, the other is like two dipoles one a ring around the other... North on top in the middle of the disk, south pole on the bottom middle, reversed on the edges... does that make sense? I can draw a picture.
I can imagine that as you move away from the center and up that the total field points one way and then the other.
I could try scanning it with a hall probe... but that seems like a lot of work.
George H. ================================================================
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is my favorite source for small neodymium magnets and magnetic pushpins. They also sell a viewing film but I've never used it. See
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for their most recent monthly newsletter with some examples.
To demagnetize a tool, first you get the smallest remanence when the field is aligned with the smallest dimension, and second, to demagnetize you need to go around and around the B-H hysteresis loop while slowly reducing the field. That's why an ac coil demagnetizes as you slowly withdraw the tool from the coil. If you insert a screwdriver into the "demagnetization" hole, pressed against the surface of the magnet, any given spot along the shaft will experience alternating fields oriented perpendicular to the shaft (so along the smallest dimension). Then keep inserting it and removing it as you move upwards in the hole, away from the magnet, and it will experience steadily weakening alternating fields, and will hopefully be completely demagnetized on the final removal. Those steps along one side may be a guide for how much to raise the shaft on each pass, but that's my speculation, I've never read any real instructions for one of those. Many, many fewer field reversals than an ac coil, but enough to crudely get the job done. The "magnetizing" hole applies a constant field as you go in and out so will result in a final field perpendicular to the shaft which is not ideal (would be stronger along the shaft) but is easy to do.
As far as I can tell the magnets are identical and are magnetised thru the thickness. And one can put the bottem magnet in the top and the top magnet in the bottom and it works the same as before.
( using cheap compass to tell the field orientation and strength.)
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