# How do you magnetize a circular magnet?

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1. May 26, 2016

### Part Toon

< Mentor Note -- thread moved from New Member Introduction Forum >

Hello, this is actually my first post on Physics Forums (I just joined)

The question above is the one that I thought that would grab the most attention.

I actually have 3 questions concerning magnets...

I require a special kind of magnet to complete my project, a magnet that the flux field lines continuously go around clockwise or counter-clockwise. Very similar to the magnets that were used in the REALLY old computers to store memory (I learned that in my set of old encyclopedias :D). I have a "donut" magnet that is the correct size and shape, except the flux lines act like a regular magnet (like a disc magnet with a hole in the center :P), in my project, this magnet is practically un-usable because of this (inconvenient) fact. I do not have such a magnet that the flux lines continuously go around, so I either need to make one or buy one :P

Question 1: What do you call such a magnet? I want to make sure that if I buy one, it is not like the one that I already have :P

Question 2: Is it possible to convert the magnet that I have now into the magnet that I need (by changing the direction of the magnetic domains)? By doing some sort of electro-magnet thingy to switch the direction of the field???

Question 3: If I can't make one, where would I be able to purchase such a magnet with the clockwise or counter-clockwise field, and not simply a regular field? I'd prefer it'd be some sort of store that I can examine the magnet in person before purchase (i.e. not on the internet). Maybe Menards or Staples, but I don't have a way to go from place to place, so I'd prefer to know exactly where I can get it before I drive my Mom crazy going from place to place :P

Remember, the magnet I need MUST have a clockwise or counter-clockwise field.

Thanks :)

Last edited by a moderator: May 26, 2016
2. May 26, 2016

### Staff: Mentor

What size donut are you talking about? Do you need it as a permanent magnet?

I haven't tried this, but I imagine you could magnetise two semicircular pieces of suitable material then tape or glue them together to form the complete circle. Picture semicircular horse-shoe magnets, maybe they can be purchased in that very shape?

If you can get hold of a toroid (ring-shaped) piece of suitable unmagnetised material you'll probably find it can be neatly sawn in half with a fine hacksaw.

The computer memory used solenoids around the core to magnetise it; tiny ferrite cores with fine wires.

How are you going to make use of the magnet?

3. May 26, 2016

### Tom.G

Search for Core Memory on ebay. Prices range from $3 for 500 bare cores to$2000 for a large rackmount assembly.

http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=core+memory&rt=nc

Or if you want a larger one, get a large nut from the local hardware store. To magnetize it you can wrap small, well insulated, wire thru the hole (as many turns as will fit) then momentarily connect the coils ends to a "D" cell flashlight battery. The wire will get hot so don't leave it connected very for long!

4. Jun 1, 2016

### Part Toon

That might work, the magnet that I need must have a "donut" hole that is at least 1.5" across. It *must* be a permanent magnet, an electro magnet wouldn't work :P

I don't think what NascentOxygen said about the semi-circle thing would work for me, the issue I have with this magnet to begin with is that it has to be a single piece. If it is multiple, it has a higher chance of having exposed North and South poles, my entire goal (as for this magnet) in my project is to obtain a circular magnet, that is a single piece so that it will not have poles. I think I may have forgotten a few details in my first post :P I might be able to make a miniature version of my device using those "Core Memory" thingies that you were mentioning :)

To conclude (for now), I require a permanent magnet, that is large enough to do stuff with (like put another setup of magnets inside), and has a circular magnetic field (clockwise or counter-clockwise)

5. Jun 1, 2016

### Staff: Mentor

Can you start with a bar magnet, drill a hole in it, then machine it to the donut shape?

6. Jun 1, 2016

### Part Toon

So...

(20 seconds after other post :P)

I looked up the stuff about core memory stuff, and that stuff looks pretty cool :) I think I know what to call the magnet now: "Big ferrite core". But yes, I still need it to be a few inches in diameter.

Sorry anorlunda :P I cannot do that, I need the magnet to have a circular field along with a circular shape :P

EDIT: I did an Ebay search for "Big ferrite core" and I got this...

http://www.ebay.com/itm/Green-Iron-...sh=item485f73a6e8:g:bOcAAOSwDNdVlYhN#rpdCntId

You guys may want to take a look at this, and maybe give me your opinions on whether it has a circular magnetic field or not :D

7. Jun 1, 2016

### Staff: Mentor

Ferrite is the core for an electromagnet, so is not what you seek.

I doubt that you will get a permanent magnet donut, because with no exterior field it would have no uses. I think 2 semicircular horse-shoe magnets would produce what you are asking for. Whether it will achieve the feats you're planning is quite another matter.

8. Jun 1, 2016

### Part Toon

Well, ferrite is like a ceramic magnet material, nearly every permanent magnet that I have is made of the stuff, very handy. I do in fact need a donut permanent magnet, it has, special uses...

The Ebay post has a 30 day back thingy, so if it is not what I'm looking for, I can simply return it.

Please don't stop giving me free-info about the stuff, I never know when I might need some good-old knowledge to use for my secret project :)

9. Jun 1, 2016

### Staff: Mentor

That's true, fridge magnets being a good example. But if it's sold as a ferrite core then it's almost certainly a soft ferrite for a transformer or electromagnet. If it was a ferrite toroid for a permanent magnet then it would be sold already a magnet. There would be no market for the item you seek because it sounds like something of almost no practical value---at least, not until it's broken.

Does it need to be both circular and of round cross-section? Otherwise, you could readily get hold of a pair of squarish horseshoe magnets and join them into a neat rectangle with rounded corners.

10. Jun 1, 2016

### davenn

again, it isn't what you seek, don't waste your time and money

the only donut shaped magnets, that I know of, and of the size you require, are in loud speakers
every modern speaker has one, I have even dismantled a few blown speakers over the years and removed the magnets

Dave

11. Jun 1, 2016

### Part Toon

Well, let me redefine the magnet I need, it has to be a toroid circular donut-shaped magnet, that is a single piece, that does not contain any creases or weld/glue points :P But most importantly, it must have either a clockwise or counter-clockwise magnetic feild, and it must be a permanent magnet, an electromagnet simply couldn't work for my uses :P

I could check out the ones found in speakers, there's probably an old-junky speaker lying around somewhere on the farm that I live on, I could dismantle it, and see if the magnet will work for my project :D

But the magnet I require is in no way useless, I have done some experiments with several 1 7/8" x 7/8" x 3/8" ceramic ferrite magnets, 24 of them to be exact. I assembled them into a ring (North pole touching South pole of the next magnet) to see if it would work for my project. Unfortunately, some of the magnets were slightly more powerful than the others, and they weren't all perfectly aligned, so that created set North and South poles, so I need it to be as a single piece. And with a circular magnetic field (I tend to call circular magnetic fields "Flux Rings").

12. Jun 1, 2016

### Staff: Mentor

13. Jun 1, 2016

### OmCheeto

So you want a permanent magnet like this:

only without the wire?

I don't think anyone makes such a thing. As NascentOxygen says:

There was no market for such a thing, until you showed up.
Testing to see whether or not it had a permanent magnetic field might make a good homework problem.
I guess you could set it up like the magnetic-core memory, from your original post, and have a single bit place to store all your information.

ps. I think a large flat washer will work. Appropriate material of course.

14. Jun 1, 2016

### davenn

see my post
they are plentiful

Dave

15. Jun 1, 2016

### OmCheeto

But the magnetic flux field is not as requested.

Unless of course, you have really weird speakers down under.

Here is a picture of one of my old speaker magnets, with the poles annotated.

The sewing needle is suspended in air, and is pointing to the strongest part of the south pole.
The north pole is strongest in the center of the top.
The poles may have shifted, from what was originally designed, as the space around the central magnet is now filled with iron filings.

(I used this magnet to look for my keys one day, 10 years ago, underneath 20 feet of water. I found my chair. [ref] I eventually found my keys, 3 months later. One of these days, I'll finish that story. )

16. Jun 2, 2016

### Part Toon

It appears that there are a lot of ideas for what I need, I decided to make a .png image to demonstrate...

(it may look a little cheap, I assembled it in a couple of minutes :P)

EDIT: I just realized how small the image is :P You may need to download the image by dragging it to your desktop to see the details :P

This is the kind of magnet that I need, it must be a permanent magnet, that is all a single piece. I simply cannot use multiple magnets that were attached together :P

So... would any of you know what this sort of thing is called? And how I could either make or purchase one? Maybe a specific Ebay post, or a store that carries a wide variety of magnets? Or I might be able to create one if I knew how...

P.S. The magnet must also be a few inches in diameter.

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17. Jun 2, 2016

### davenn

going on from Om's comment about my speaker magnet idea

if that doesn't fit your criteria, I'm starting to pick that you are not likely to find one with the magnetic properties you state

18. Jun 2, 2016

### Part Toon

I may be able to try the speaker thing some time soon, but so far, I haven't had enough time to do that :P I have a question, the magnets used in old computers (core memory as I heard it is called), aren't they simply miniature versions of what I need?

19. Jun 2, 2016

### davenn

they were not magnets, they were ferrite cores that would be magnetised with the data signal passing through them on a wire
and in general they were tiny ... I used to have a chunk of core memory, the cores were a couple of mm in diameter

https://en.wikipedia.org/wiki/Magnetic-core_memory

Dave

20. Jun 2, 2016

### Staff: Mentor

@OmCheeto mentioned a flat washer. Could you permanently magnetized one with a coil, then remove the coil?