Curious phenomenon stacking magnets

[MattM]

Using ferrous magnets stacked didn't seem to amplify the magnet field strength.  It barely got stronger with each successive magnet in a stack.  So I decided to play with steel bars to try different ways to connect them.  When all six were stacked on the same side with polarities all in the same direction it was surprisingly ineffective.  The bar itself did not act magnetic at all.  One way using the bar, however, had an interesting ability to increase the strength.  I used two stacks of 3 ferrous magnets on a 5"x3/4" 16ga bar.  But when I moved one stack of 3 magnets on the opposite side and at opposite end, where the polarities were also mirrored, North touching one side of bar and South on the other, they suddenly got substantially stronger.  Not only that, but the steel bar is suddenly strongly magnetized.  The stacks are completely touching the bars lengthways, no overlap.  And there is about an 1-1/2" gap between the two stacks, where neither stack covers.  If I widen or shrink that gap between then they don't combine field strengths as well.  With a gap at 1-1/2", there is a significant combining effect of the magnetic field.

Does anyone know why the magnets suddenly strengthen along the gap in the bar - and under each stack - far more than all six magnets in a single stack?  And why is it when that gap is only at a certain distance it works better?

[Aamir]

I tried this with 18mm x 5mm N45 neo disc magnets and 2.25inch x 20mm round solid iron bar checking different configurations for my future planned IRON CORE AFPMG. When I stuck 2 magnets on opposite sides of iron bar using north and south poles, magnetic field in the bar was not strong enough but when I used north pole on both sides then the magnetic field in the bar was extremely strong. May be this is due to opposite poles cancelling each other causing weak magnetic strength in iron bar. Correct me if I am wrong.

[MattM]

I didn't see the effect really become significant with one or two magnets  on each side.  On the third per side it became significant.  But I also did not see the effect unless the stacks were on opposite sides, on opposite ends of the bar, and with opposite poles touching the bar.

The gap distance was also key to seeing the effect.

[joestue]

a ferrite magnet maxes out at .4T.

if you have a closed loop of iron surrounding a ferrite magnet, its not going to "feel magnet"

[MattM]

I'm talking about flat bar stock, not loops.  The combing effect of the magnetic field is concentrating in the bar, but between the stacks and not as significant directly underneath a stack.

I think maybe this shapes the magnetic field in relationship to the bar.

I originally was putting one magnet on a two foot long bar.  The whole bar would become magnetized.  Adding additionsl magnets to the bar caused either little difference or negated the magnetism in the bar altogether.  I tried the horseshoe shape I talked about in a different thread and it showed no promise carrying a magnetic pole.  I cut some pieces off the bar to try different combinations when I stumbled upon this stacking phenomenon.  It was the only situation where stacks seemed to get stronger than a single magnet.  But the stronger field occurs in the bar, not necessarily in the magnet stacks themselves.

The ferrous magnets are the $2 pairs found at a local hardware store.  They are approximately 1.5"x3/4"x1/2" blocks.

[Adriaan Kragten]

Without a picture it is difficult to understand what you have done. But may be I can give an explanation how the flux density changes if you pile up different magnets.

Assume that you have one single circular magnet with a thickness t1 in open air. In the magnet itself, all field lines are about in parallel to the magnet side. Outside the magnet, the field lines are about ellipses from the north to the south pole. The length of the ellipse which originates from the side of the magnet is shortest. The length of the ellipse which originates from the heart of the magnet is longest. Assume that the average length of the ellipses outside the magnet is called t2. t2 is much longer than t1 especial for thin magnets with a relatively large diameter. The remanence Br is the flux density in the magnet when the magnet is short-circuited by an iron loop which isn't saturated. Assume Br is 1.2 T for neodymium magnets. The magnetic resistance of air is about the same as that of the magnet itself.
So the flux density is reduced by a factor t1 / (t1 + t2). Assume t2 = 5 * t1. So the flux density is reduced by a factor t1 / (t1 + 5t1) = 1 / (1 + 5) = 1/6.
So the flux density becomes 1/6 * 1.2 = 0.2 T.

Next assume that you pile up two magnets. This means that the magnet thickness doubles but that the length of the average ellipse increases with about the thickness of one magnet. So now Br is reduced by a factor 2 / (2 + 5) = 2/7. So the flux density becomes 2/7 * 1.2 = 0.343 T.
Next assume that you pile up three magnets. So now Br is reduced by a factor 3 / (3 + 5) = 3/8. So the flux density becomes 3/8 * 1.2 = 0.45 T.
Next assume that you pile up four magnets. So now Br is reduced by a factor 4 / (4 + 5) = 4/9. So the flux density becomes 4/9 * 1.2 = 0.533 T.
Next assume that you pile up five magnets. So now Br is reduced by a factor 5 / (5 + 5) = 5/10. So the flux density becomes 5/10 * 1.2 = 0.6 T.
Next assume that you pile up ten magnets. So now Br is reduced by a factor 10 / (10 + 5) = 10/15. So the flux density becomes 5/10 * 1.2 = 0.8 T.

So you see that the flux density is increasing if you pile up more magnets but that the increase is reduced at increasing number of magnets. Increase of the number of magnets by a factor five results in increase of the flux density by only a factor three. Increase of the number of magnets by a factor ten results in increase of the flux density by only a factor four. You need a device with which you can measure the flux density at the surface of the magnet to check if my calculations are correct. An iron bar has only influence of the flux density if it is placed in the path of the field lines.

[MattM]

It looks like four works as good as three.  Without a way to measure the effects, its impossible to measure how much.  But here is a picture.
The pieces of clear tape on the magnets can be ignored.

[electrondady1]

i did lots of experiments with ferrous  mags. early on i found it was easier see results working with repulsion  . you can set things up on a measured surface see how far or close before there is movement .

[MattM]

If anyone has ferrous magnets and a steel bar, I'd appreciate if you can confirm this effect.

Might make for an alternative from rare earth magnets.

[electrondady1]

so you have a stack of 8 magnets that are offset 1.5" by this steel strapping. you suggesting the flux density is enhanced by presence of the steel strapping  over and above what it would be with 8 mags stacked?

[MattM]

"so you have a stack of 8 magnets that are offset 1.5" by this steel strapping. you suggesting the flux density is enhanced by presence of the steel strapping  over and above what it would be with 8 mags stacked?"

Without a doubt.  I was expecting the stacks to do that, but they did not.  The stacking of the ferrous magnets is actually pretty underwhelming as they do not add much if anything with each successive magnet attached.  The steel bar and this alignment seems to concentrate the field.  The stacking did not.  It was so stupid simple when it occurred I did not believe the difference it made.  That is why I'm asking for somebody to confirm this phenomenon.

[MattM]

I cannot find any similar thoughts on what is taking place.  On YouTube I found a lot of thought on stacking magnets and everything was focused on the poles running on the flat long sides.  The fieldlines goes out the sides much further than from the poles, which is consistent with my accidental discovery.  Too far apart there is a complete collapse of the effect.  Too close it gets weaker, too.  So there is a sweetspot and my guess the effect is closely tied to the size of fieldlines.  My guess is that the fieldlines merge in the gap.  Like Adrian said, we are combining fields but our thickness causes the fields to thin out.  Since the effect is obviously coming from the short end in this phenomenon, it probably doesn't have the same drop off you'd have from stacking 6-8 total.  I'll need to get wider bar stock to see if side-by-side has any additional benefit.  Maybe it will be more worthwhile to be 2x2 on each side versus stacks of fours.

[electrondady1]

i just happen to have 60 of those same magnets so i will no doubt be experimenting with this phenomenon as well. thanks

[Adriaan Kragten]

Without a picture it is difficult to understand what you have done. But may be I can give an explanation how the flux density changes if you pile up different magnets.

Assume that you have one single circular magnet with a thickness t1 in open air. In the magnet itself, all field lines are about in parallel to the magnet side. Outside the magnet, the field lines are about ellipses from the north to the south pole. The length of the ellipse which originates from the side of the magnet is shortest. The length of the ellipse which originates from the heart of the magnet is longest. Assume that the average length of the ellipses outside the magnet is called t2. t2 is much longer than t1 especial for thin magnets with a relatively large diameter. The remanence Br is the flux density in the magnet when the magnet is short-circuited by an iron loop which isn't saturated. Assume Br is 1.2 T for neodymium magnets. The magnetic resistance of air is about the same as that of the magnet itself.
So the flux density is reduced by a factor t1 / (t1 + t2). Assume t2 = 5 * t1. So the flux density is reduced by a factor t1 / (t1 + 5t1) = 1 / (1 + 5) = 1/6.
So the flux density becomes 1/6 * 1.2 = 0.2 T.

Next assume that you pile up two magnets. This means that the magnet thickness doubles but that the length of the average ellipse increases with about the thickness of one magnet. So now Br is reduced by a factor 2 / (2 + 5) = 2/7. So the flux density becomes 2/7 * 1.2 = 0.343 T.
Next assume that you pile up three magnets. So now Br is reduced by a factor 3 / (3 + 5) = 3/8. So the flux density becomes 3/8 * 1.2 = 0.45 T.
Next assume that you pile up four magnets. So now Br is reduced by a factor 4 / (4 + 5) = 4/9. So the flux density becomes 4/9 * 1.2 = 0.533 T.
Next assume that you pile up five magnets. So now Br is reduced by a factor 5 / (5 + 5) = 5/10. So the flux density becomes 5/10 * 1.2 = 0.6 T.
Next assume that you pile up ten magnets. So now Br is reduced by a factor 10 / (10 + 5) = 10/15. So the flux density becomes 10/15 * 1.2 = 0.8 T.

So you see that the flux density is increasing if you pile up more magnets but that the increase is reduced at increasing number of magnets. Increase of the number of magnets by a factor five results in increase of the flux density by only a factor three. Increase of the number of magnets by a factor ten results in increase of the flux density by only a factor four. You need a device with which you can measure the flux density at the surface of the magnet to check if my calculations are correct. An iron bar has only influence of the flux density if it is placed in the path of the field lines.

I realize that there is something wrong in my calculation. For an infinite number of magnets, the length of the air gap becomes equal to the total thickness of all magnets. The flux density for an air gap which is the same as the magnet thickness is just half Br. So Br eff can never be larger than 0.6 T if there is no iron in a magnetic loop.

[SparWeb]

Matt,
Have you tried iron filings?  If you have a file, any chunk of steel will do.   Something like that bar you're using in the photo.  It takes a bit of time to accumulate enough, but not much of your stock is consumed, and it is worth it.
If you place a paper over your magnet stack, then sprinkle the filings, the field lines will appear.

[MattM]

My grinder is mounted on a table outside and I've been shooting them on thr ground rather than collecting them /headslap

[Mary B]

Some iron filings, some aluminum filings... you can use it to weld copper wire to ground rods! Bit energetic for that but it works in a pinch(we used it at the casino when it was impossible to find Cadweld One Shots). Basically makes thermite...

[SparWeb]

Mary,
The more casino job stories you tell, the more sympathy I feel for your helpers.  Or "sacrificial victims" if words matter.

[Mary B]

LOL we did what we had to! You should have seen us lifting a 20 foot round sign up... all 3,000 pounds of it... couldn't get a forklift in so we had 2 hand crank pallet lifts rated at 500 pounds each... I was off for the final lift thankfully! Amazed they didn't kill someone. They grabbed 10 beefy guys from other departments to help lift along with the 2 pallet lifts... that sign ended up breaking 2 mounting legs one night when that area was empty. Sign crushed 8 slot machines($20,000 EACH). Sign company got sued, we had a dangerous mess to clean up... finally weakened the other 2 legs and pushed it... sign was at 12 feet over the top of 2 rows of slot machines. Cut it up right there and hauled it off

[MattM]

electrondady1-

Would you mind humoring me by trying this magnet arrangement?  It sounds like you probably have these simple items on hand.  I'm simply looking for independent verification here.  If you see the same effect I'm seeing it would perhaps benefit the comunity here.

I'm ordering some n52 magnets after the holidays to try them in the same way.  My feelings is there are practical limits to this.  I'm thinking that there is a saturation limit to these things at some point, so a stack of neos should have exactly the same maximum as ferrous magnets.  Its just a theory at this point, but my experiences with most things 'science' tends to point in this direction.  Being able to match neos at a much lower price point would not be a bad result.  It could be practical in a lot of situations that would lower entry level costs to projects such as DIY windmills.

[MattM]

I went back to one magnet per side and played around with other magnets around the array.  The strength of one pole gets stronger on one side and weaker on the other.  The same poles are facing the steel bar, which means the back side of the magnets gets considerably weaker, while the same side poles on the bar get stronger.  The bar appears to take the polarity of the poles touching it.

So when I make two identical arrays they repel each other.  I was expecting the attraction to be perhaps weaker on one side and stronger on the other, not repel when putting opposite charges (frontside to the backsides of the arrays) together.  Yet they want to repel in all directions.  This reminds me of the Halbach array phenomenon.

When I make one using south poles and bring it near one with north poles, they attract together very strongly.  Some parts of the arrays are stronger than others.  But that first time I drew them together I was surprised by the violence of them coming together.  I would warn anyone trying this not to pinch your fingers, because they seem to combine all the forces of the magnets together where they connect.

Remember, this phenomenon works with a single magnet on each side, and when there are multiple magnets on each side.  It is really boggling how two magnets and a simple steel bar create something akin to a single pole.  I cannot believe these oddities are not documented somewhere.

All fun and games unless there is a use to these forces.

[MattM]

Maybe people are not comprehending this phenomenon?  They say a picture is worth a 1,000 words so let's see if this helps people visualize this phenomenon I'm describing.


...and...


I would think this could prove useful.