Basic Magnet Questions

[automan]

I think I may keep asking these basic questions. I think they might be a good resource for new people entering this craft.

So this question has to do with the theory of magnets. In my mind, I see magnetism as a flow of magnetic energy from one pole to another. Is this premise correct?

If it is, does it matter how long a cylindrical magnet can be and still be magnetized? Rephrased, could I have an Alnico magnet that is 3 feet long and it still be a magnet? Would the flow move that far? I would think the length of a magnet would be limited by the strength. It would have to be strong enough the throw a field from one pole to the other.

A working example of what makes me ask this question: I was wondering how far apart the 2 coils of a humbucker could be and still have the magnetism move from one set of pole pieces to the other. Could you have a blade in a coil by the neck and have a magnet go from the bottom of it to another blade by the bridge and have the magnetic field encompass the whole playing area between those 2 coils? The Charlie Christian pickup is what got me thinking about this.

I look forward to hearing what people much more educated on the subject have to say about it.

[uvacom]

I think I may keep asking these basic questions. I think they might be a good resource for new people entering this craft.

So this question has to do with the theory of magnets. In my mind, I see magnetism as a flow of magnetic energy from one pole to another. Is this premise correct?

Somebody with a background in physics might be able to provide a more comprehensive response, but here's my shot at it:

There is a magnetic field which flows in loops through the magnet from south to north, exits the north pole, and loops back around in the area surrounding the magnet to the south pole. In most cases, it is more useful to view the magnetic field as a force than as a flow of energy, for the practical reason that in electromagnetism energy is usually a result of electrons moving or being stored, and AFAIK there is no electron flow in a static magnetic field, and permanent magnets are not typically used as energy storage devices or energy sources. Of course there is energy involved, magnetism is due to a photon field, but I don't actually know anything more than that, because magnetism is never discussed that way in classical electromagnetic theory.

If it is, does it matter how long a cylindrical magnet can be and still be magnetized? Rephrased, could I have an Alnico magnet that is 3 feet long and it still be a magnet? Would the flow move that far? I would think the length of a magnet would be limited by the strength. It would have to be strong enough the throw a field from one pole to the other.

Actually, and I'm sure somebody will correct me if I'm off, but I believe all other things equal, a magnet with a longer magnetic length (the length between poles) can produce a greater magnetic field.

A working example of what makes me ask this question: I was wondering how far apart the 2 coils of a humbucker could be and still have the magnetism move from one set of pole pieces to the other. Could you have a blade in a coil by the neck and have a magnet go from the bottom of it to another blade by the bridge and have the magnetic field encompass the whole playing area between those 2 coils? The Charlie Christian pickup is what got me thinking about this.

Well, I think you could build a pickup like that, but if I understand what you're saying there would be one pickup with one blade perpendicular to the strings at the neck position, and another blade perpendicular to the strings at the bridge position, magnetically coupled by one long magnet? It would work but the magnetic field would be much stronger at the blades, so you would not have a continuous magnetic field across the playable area of the strings. That's more or less a good thing, as the greater the sensing area the more cancellation would occur. Ideally, you'd like an infinitely narrow magnetic field, assuming you'd like to perfectly capture the strings vibrations. Generally, we don't actually want that (optical pickups would probably have taken off if we did), but still I think the sound would be very dull and rather attenuated if the entire area from the base of the neck to the bridge were being sensed.

[belwar]

Some good question indeed. I would think that you would make the magnet stronger by making it longer in the case of a cylindrical magnet.. However, I see what you're saying as would the flux lines from the north pole reach the south pole? ... interesting. Time to dig out some magnets and stack them to try!

The Charlie Christian pickup seems like a wierd animal. I dont have experience in them, but as I "understand" it, the magnets are large because of the type of magnet.. The CC pickups where made before Alnico was available, and I believe were made from one of the Magnet Steels. Most magnet steel is quite weak in comparison to modern alnico. The two two bars are probably magnetized through thier thickness, not the width like a humbucker magnet.

I just started researching magnet steels and i've got a long way go. So much information on them just is not readily searchable. A lot of the information is in books that predate ISBN's. You have to get lucky and find a book, then look at the references, track those books down and look at thier references. I just got a book from 1930 with BH curves of 8 different magnet steels. sweet find.

[defaced]

Magnets are really well described by vector fields. They're a bit of a pain to visualize, especially in three dimensions, but they're good tools, Vector field

As for the length of a magnet/distance thing, magnetic fields are governed by the inverse square law, Inverse-square law, so length is a non issue because magnetism is conserved, but redistributed. Meaning, total magnetism doesn't change, it's just redistributed to occupy more volume, so each vector gets smaller as you get further away from the source, but there are more of them. This is where you start getting into flux density. Gauss levels (flux density) are highest at the ends of the poles, and smallest in the center where the flux density hits zero (think of a bipolar power supply) because that is where a magnet changes from a source to a sink.

I hope I didn't screw this up too bad. It's been a while since 3D vector Calc and EM Physics.

[Mike Sulzer]

So this question has to do with the theory of magnets.

The flow of electrical charge (current) is the source of the magnetic fields we deal with in audio. These currents can be macroscopic (for example, current in a wire) or microscopic (tiny loops of current associated with each atom or molecule).

There is a force between two charges moving relative to each other; this is the magnetic force. A moving charge can be affected by the motion of many other charges; these forces simply add up, and one calls such a process linear.

A magnet consists of a huge number of tiny current loops, one for each atom. In a material with permanent magnetism, these currents tend to be lined up in the same direction. It is not practical to deal with the forces resulting from each atom. However, it is possible to deal with their summed overall effect. This effect is described by the magnetic field. A magnetic field allows one to forget the currents and just deal with their effect.

A small loop of current has a so-called dipole field. It decreases with the cube of the distance from the dipole. The field produced by a material containing many current loops is just the sum of all the individual fields.

In an unmagnetized material, the loops point in random directions. The magnetic field is zero because all the individual fields from each current loop cancel out.

In a material like steel, the currents initially aligned randomly tend to line up when placed in a magnetic field. Such a material is said to have a high permeability.

In a permanent magnet, many of the loops of current tend to remain aligned by themselves.

It is often convenient to think of magnetic field lines as a flow. This is an analogy, useful, but one must remember how it really works.

In a material such as Neodymium, the currents in one magnet are hardly altered at all by other magnets. So one can think of building up a long magnet by adding on shorter magnets. Let's start with a small cylindrical magnet with the field aligned along its length and pointing horizontal. Consider the strength of the field a small distance to the right of it. Now stick another such magnet on the left end. The field at our measurement point gets stronger, but it does not double because the second magnet is farther away from the measurement point than the first one. As we continue to add more magnets, the field at the measurement point gets stronger, but by a smaller amount each time.

If you look at the field lines resulting from summing the fields of the individual magnets, you will see that they do not all flow down the magnets. Some leak out the sides and flow back around without going the entire length of the stack of magnets.

[salvarsan]

...does it matter how long a cylindrical magnet can be and still be magnetized? Rephrased, could I have an Alnico magnet that is 3 feet long and it still be a magnet? Would the flow move that far? I would think the length of a magnet would be limited by the strength.

Magnets manifest their maximum field strength beyond a certain length and width, any longer and the field doesn't increase.

Short magnets may tend to self-demagnetize, particularly Alnico and Cunife types. Ceramic and rare earth magnets are usually immune to self-demagnetization.


-drh

[David King]

I think I read somewhere that the ideal length to dia ratio for an alnico is between 3:1 and 5:1. I might have read it on a magnet maker's website but I can't remember where.

[salvarsan]

I think I read somewhere that the ideal length to dia ratio for an alnico is between 3:1 and 5:1. I might have read it on a magnet maker's website but I can't remember where.

"Ideal" is the smallest you can use to get the results you like. *I'm looking at charts in the Moskowitz bible on permanent magnets and seeing 7:1, 8:1 length:width ratios for maximum field strength.


Given that a generic Strat pole is 3/16" dia. X 5/8" long = 3.3:1 ratio, it's clear that we don't want max magnetic strength; we only need to hold a usable strength without self-demagnetizing.

The self-demagnetizing is more likely with low number Alnicos 1,2,3, and 4 since their coercivities (Hc spec) are lower than the rest.

-drh

[automan]

Very informative thread, guys. I really appreciate everyone's input.

[Mike Sulzer]

" *I'm looking at charts in the Moskowitz bible on permanent magnets and seeing 7:1, 8:1 length:width ratios for maximum field strength.

It will keep getting a bit stronger as you make it longer, but 1/r^3 is a pretty fast fall off. You get very little more.

For a pickup the issue is not really the permanent magnet; ever since Alnico was invented it has been possible to get too much field strength. You can always find a way to get the field strength that you need. The length of the magnets is pretty much the depth of the coil. How deep it should be is more related to detecting the field from the vibrating string.

[Amit]

From experience, how short can I make an alnico rod and alnico bar without them demagnetizing them selves?
Also why two of them pointed with same polarity to each other, don't demagnetize?
Third magnet ralated question. in Bill Lawrence's SCN patent he uses 6 slugs as core and neo magnets in the middle of the slugs. this makes them (the slugs) into sort of two magnets with same polarity in head to head form.
Can this be done with alnico rods and neos on their side?
Kind of to get a single rod behave like two virtual rods without phisicly breaking it to two pieces

[automan]

do you have a link to that patent or the patent number?

[Amit]

do you have a link to that patent or the patent number?

7227076

[David Schwab]

in Bill Lawrence's SCN patent he uses 6 slugs as core and neo magnets in the middle of the slugs. this makes them (the slugs) into sort of two magnets with same polarity in head to head form.
Can this be done with alnico rods and neos on their side?
Kind of to get a single rod behave like two virtual rods without phisicly breaking it to two pieces

Bill Lawrences SCN does not use neos.. it uses SmCo magnets, hence the name, Samarium Cobalt Noiseless.

The magnets are actually attached to a keeper, which he calls a moderator bar.

If you are going to use alnico rods, you don't need the neos. A lot of the early stacks were made that way.

[J M Fahey]

Hi automan. Some short answers:
1)

I see magnetism as a flow of magnetic energy from one pole to another.

Not exactly, to begin with there's no "flow" at all, there's nothing that flows, it's a magnetic field.
Field describes a portion of space where we can measure something, in this case magnetism.
Fields are in principle static, meaning that if for example you place a magnet somewhere, on certain point close to it you will measure a certain value, that will not vary, the same happening in different parts of space.
That space is the magnetic field of that magnet and a graphic representation of those values is its "map", where you will see "lines of force" joining equal magnetic intensity points.
Of course, if you move magnets (as in generators or some record pickups), or vary the path along which those "lines of force" move (guitar pickups, some disc pickups), the values will change in sync, which can be used to get an electrical signal.
2) a magnet can be *any length*.
Any? Indeed?
We live on a very long one: Earth itself is a huge magnet, 12.000 KM last time I checked.
The magnetic field goes from one Pole to the other (that's where the word pole came from) and you can easily check that anywhere with a compass.
Yes, the magnetic field extends from one pole to the other, or it would not be a magnet.
3) Yes, That, say, 8 inch separation pickup would work.
The electrical signal would depend heavily on how much the string movement would modulate it and the pole separation would influence what harmonics would be picked.
Such a pickup would be relatively dull, for lack of a better word.