Ferrite Magnets

[Clonus]

Hi there, I was hoping someone, anyone can help me with this question.

I have a few small fridge magnets from the dollar store, very cheap coin style magnets.

I found that one of them is conductive? I was surprised since I read that ferrite magnets are non conductive?

Can someone shed some light on this for me please?

thanks a mill!
Clonus

 

[electricpete]

I tried it myself. I used a 1 x 2 x 0.25" (approx) ferrite/cermic magnet.

I place my voltage probes flat on the biggest surface, approx 1" apart. The reading was 4 megaohms. Not what I'd call conductive.

As you probably know, that's why these magnets are used in high frequency application - very little eddy current developes in the highly resistive ferrite magnet.

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[electricpete]

"high resistivy" would have been a better term than "highly resistive"

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[Clonus]

yes, i understand.. BUT, i have one that is?

It's not a major conductor but I can power an LED through it...

any comments please?

 

[electricpete]

I would say either it's not ferrite (maybe AlNiCo?) or else it has some conducting surface treatment.

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[Clonus]

Well, I purchased a bunch of them. They are about the size of a nickel, they are black and are definetly ferrite.

I tested all of them, about 12-20 and only about 3 of them are able to conduct. This is why I am wondering if there are impurities in the magent.

One of the three is stronger at conduction than the other two. It's pretty interesting. Can anyone recommenda some I can send the magnet to for analysis?

 

[Clonus]

sorry I meant...


Can anyone recommend someone I can send the magnet to for analysis?

 

[electricpete]

The variability peaked my interest, so I did some more tests.

I tested 15 more ferrite magnets (total 16) in the same fashion as above. Results were 3 megaohms to 25 megaohms and fairly repeatable on a given magnet.

I tested an aluminum bar in same manner.... probes flat and parallel 1" apart (also about 1" overlap of probe onto test specimen). Result was 0.2 ohms.

I tested steel bar in same manner. Result was 0.4 ohms.

I tested Nickel coated Neodymium magnet in same manner. Result was 0.4 ohms.

There may be some error in those latter three resistance measureents since direct check probe to probe jumps around 0 - 0.1 - 0.2 ohms.

But a huge difference ~ 10^7 between the conducting and non-conducting stuff. It places my ferrite magnets squarely in the non-conducting category.

As for your situation, I'm curious why you would invest any money in testing on a magnet that is worth much less than a buck. Why not buy them, test them, and throw out the ones that test bad?

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[electricpete]

Now you've got me wondering about Neodymium magnets. The coating I know is conducting. Does anyone know whether the magnets themselves are good conductors or bad conductors?

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[MagMike]

electricpete:
I don't have the electrical conductivity numbers handy, but NdFeB magnets can conduct electricity. They'll never replace copper as a conductor though.

BTW: I don't know why the ferrite magnets that Clonus tested are conductive.

 

[Clonus]

Here's the real reason.

I've been experimenting with hydrogen cells and decided to use a few ferrite magnets as even and quick spacers between the plates.

To make a long story short, in my observations I noticed that one cell in particular was producing so much more hydrogen than the rest. Even in distilled water, it did't seem to make much difference.

So I narrowed it down to one magnet I had, this one works great. Although not a huge amount, I can create hydrogen using 2 - 4 AA batteries.

This is why I am asking, I'm trying to figure out why this ferrite magnet is doing this and the others are not?

 

[Clonus]

Ok, I finally made the videos

Part 1

http://www.youtube.com/watch?v=rIZb0Gs2U1g

Part 2

http://www.youtube.com/watch?v=_IGoI7IoqcQ

Remember, this is a ferrite magent, the other magnets do not display the same property this particular one does.

 

[electricpete]

So the purpose of your setup is to produce hydrogen (correct?)

At one location (the magnet in question) there is a lot more hydrogen produced than the rest.

I don't know anything about hydrogen production, but I'll take a guess.

You yourself I think answered the mystery when you said this particular magnet is conducting.

Now instead of having a 1/4" gap with 1.5vdc accross it, you may have created much smaller gap (the one between the magnet and the plate). Smaller gap means higher E field (E = V/d). Ok, it's alittle more complicated than that because we don't know exactly where the voltage drop occurs... but if we connect several good conductors together with semi-loose connections, we may reasonably expect the voltage drops to occur primarily at the connections.

If you were to throw your ferrite magnets away and move your main electrodes closer together, I suspect you would see the same thing. Perhaps separate the main electrodes with a very thin nonconducting screen.

Again, just some thoughts from a guy who doesn't know much about separating hydrogen or electrochemistry.

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[RMWS]

Electrical resistivity of sintered ceramic ferrite magnets is typically around 10^+6 ohm-cm. Sintered Neodymium-iron-boron ("Neo" or "NdFeB") magnets have around 160x10^-6 ohm-cm resistivity. That's nearly 12 orders of magnitude difference! By comparison, the value for copper is about 1.7x10^-6 ohm-cm.

I suspect that your "very cheap" refrigerator magnets may be manufactured under conditions where the chemistry is not closely controlled. If all the magnet has to do is hold itself and a sheet of paper on the refrigerator, you can get away with some really sloppy chemistry and still accomplish the mission. There is probably some electrically conductive metal, or maybe a die release compound that is slightly conductive, present in the magnet or at its surface. If you really care to find out more, break one of the "conductive" magnets and check the resistance on the freshly broken surface.

Magnetic Instrumentation, Inc.

http://www.maginst.com