Polished vs Un-polished Magnets

[Mike Sulzer]

If I lay it on the desk and try to get a piece of 1/8" thick steel to stick right in the middle, between the two poles, it wont.

I said to try it with the head of a small flat head screw. It samples the field on a smaller scale than a flat piece of steel. You know there is a field; you showed me a picture. Why would a small piece of steel not be attracted?

[David Schwab]

I said to try it with the head of a small flat head screw. It samples the field on a smaller scale than a flat piece of steel. You know there is a field; you showed me a picture. Why would a small piece of steel not be attracted?

It will not stick to the very middle between the north and south poles. It will always move more towards the ends. That's what I said in the first place. In the very middle is a null point.

So at the very middle there is no magnetic force attracting the steel, and it's pulled by the stronger pull of either pole. You can easily feel it is you swing the steel plate over it.

If you measure and draw a line up the center with a sharpie, you will see that you can't stick any thing right on that line.

[Mike Sulzer]

It will not stick to the very middle between the north and south poles. It will always move more towards the ends. That's what I said in the first place. In the very middle is a null point.

So at the very middle there is no magnetic force attracting the steel, and it's pulled by the stronger pull of either pole. You can easily feel it is you swing the steel plate over it.

If you measure and draw a line up the center with a sharpie, you will see that you can't stick any thing right on that line.

But that does not mean that there is no field there. Right at the the line, and out from it, the field points along the surface of the magnet; that is, it has no component perpendicular to the surface.

As you move towards one of the poles, the field gains a component perpendicular to the surface (see your picture) and thus can induce magnetization in the steel of the screw that results in an attraction. The pole itself is just a convenient reference point and plays no special role in the process.

[David Schwab]

But that does not mean that there is no field there.

Right, the field travels through the magnet. I understand that. Cut the magnet shorter and it's still there. What I'm saying is on a physical bar magnet, when you are measuring on the outside between the poles, you are going to hit that spot when the domains switch from north to south. Obviously they don't switch in the magnet. But we are dealing with a physical magnet. So if you hold an object over the magnet, it is going to be attracted to one pole or the other.

So take measurement a gauss meter and what do you see?

Here we see one pole on a ceramic magnet.




Now move the meter to the center and when you hit the right spot it will read 0. It was very hard to hold it still with one hand, so it was fluctuating between 0 and 1. I had to press my thumb down on it to keep it still while holding the camera. But here you go, 0 Gauss and no indication of S or N.



So that was my point. On a bar magnet you will hit a spot between the domains and get no reading. Move a slight distance in either direction and you start to get a reading of that pole.

I think you were just saying the same thing in your last post.

[Mike Sulzer]

Right. You have shown that there is a null in the component of the field into/out of the side face of the magnet. Now turn the probe so that the field lines going from one pole to the other pass through it perpendicular to its surface. For this component (lines running parallel to the side face of the magnet) there is no null.

[Joe Gwinn]

Well, that is a pretty big parting of the ways. I hear you saying that if there is a field from the magnet under the pickup, then there is an effect that simply does not happen if there is no field; that is, the vibrating string somehow disturbs that field, resulting in secondary disturbances through the coil. That does not happen. Linearity applies here; the field under the pickup simply adds with the field from the vibrating string that exists there also. This means that you can conceptually subtract off the permanent field, that is, set it to zero, without affecting the results. (Note that the magnet can otherwise affect the circuit through its permeability, a property that it has even if the magnet is not "charged", although it might be somewhat different in value.)

It is often said that the pickup explanation using reluctance justifies what I hear you saying. It does not, and this misunderstanding shows the danger of trying to derive intuitive understanding from a secondary analogy rather dealing directly with the basic physics (Maxwell's equations, in particular the one that has the law of magnetic induction within it).

What we are discussing is well understood physics. There is no room for opinion or differing interpretations.

Umm. This has been discussed at length in multiple prior threads. There are a number of mathematically equivalent ways to formulate the problem, and one chooses one's favorite formulation. All equivalent formulations will get the same answer, where "equivalent" means that the same simplifying assumptions were chosen.

[David Schwab]

Umm. This has been discussed at length in multiple prior threads. There are a number of mathematically equivalent ways to formulate the problem, and one chooses one's favorite formulation. All equivalent formulations will get the same answer, where "equivalent" means that the same simplifying assumptions were chosen.

Right, such as variable reluctance.

[Joe Gwinn]

Well you can see that then you look at the magnetic field with something, but obviously in the middle of the magnet is a null point. Orienting a magnet's poles the "wrong way" in a pickup, i.e., not facing the pickup's pole pieces, gives you a pickup that doesn't work very well. So there is not much useable flux on the sides. Same is true with ground or cast magnets.

Yes, there will be null points, But they are just that, points. Getting a whole region to be null is impossible, although there are some special arrangements that do pretty well.

Like a Halbach array: Halbach array - Wikipedia, the free encyclopedia.

So how would surface texture affect this?

I wouldn't think it would have much effect, for two reasons:

1. Iron body: Because the field quickly smooths out after leaving the rough surface. The reason it smooths out is that the magnetic field repels itself, which quickly smears out any irregularity in the surface of the iron pole.

2. Alnico body: Because the permeability of alnico is about three times that of air, so a little ripple in the surface has little effect even right at the surface. And the field repels itself, even right at the surface.

[Mike Sulzer]

Yes, and it is time to stop with the misinterpretation of just that equivalence. The law of magnetic induction is considered fundamental and the variable reluctance method is an analogy between J =(sigma)E and B = (mu)H. If one thinks that the variable reluctance method implies something different from what the law of induction says, then that is a misinterpretation. You also know that fields in a vacuum are linear: they add. Non linearity occurs only when two high level fields interact in a ferromagnetic material, for example. In a pickup, the field from the vibrating string is small and it adds with the permanent field. There is no interaction of the kind that was suggested above, suggested with no evidence of any kind, either theoretical or experimental. The variable reluctance method does not imply what he was saying. It just does not, and it is time to give up these ideas.

I have no idea why so many want to understand this in this completely wrong way. It does not help with learning how pickups work, but merely distracts from any real understanding.

By the way, the usual use of variable reluctance is in a situation where one has a voltage source equivalent, low reluctance known elements, and an unknown higher reluctance. This is easily analyzed as voltage source, wire, and resistor. In a pickup we are dominated by the high reluctance of the large air regions, and so the voltage source equivalent plus air regions is best thought of as a current source in parallel with a large resistor. I do not see any reason why one cannot an analyze a pickup in this way, nor do I see anyone doing it so, nor do I see any particular advantage over an analysis using the law of induction directly. The latter provides a direct intuitive interpretation in which the vibrating string is magnetized, creating a variable field which adds with the permanent field, the latter being ignored because of linearity. What could be simpler?

Umm. This has been discussed at length in multiple prior threads. There are a number of mathematically equivalent ways to formulate the problem, and one chooses one's favorite formulation. All equivalent formulations will get the same answer, where "equivalent" means that the same simplifying assumptions were chosen.

[David Schwab]

There is no interaction of the kind that was suggested above, suggested with no evidence of any kind, either theoretical or experimental.

And where exactly has that been shown to be true?

The latter provides a direct intuitive interpretation in which the vibrating string is magnetized, creating a variable field which adds with the permanent field, the latter being ignored because of linearity.

Now this is different from what you have been saying, in that you are now adding to the permanent magnet's field. You used to say it was just the (temporarily) magnetized strings, which clearly it isn't, since even with the strings magnetized the pickup doesn't function well without the magnet.

So again we can see that the string is disturbing the static field of the magnet. Kind of works out the same.

[Mike Sulzer]

And where exactly has that been shown to be true?

The linearity of E&M fields in a vacuum is fundamental in physics. As the sun rising in the morning is to day to day life. It has been measured to a very high degree of accuracy. It it fails, so goes much of physics.

For example, to get an idea how much linearity is a part of physics you could look at how significant it is that the equations of general relativity are not:

Nonlinearity

The nonlinearity of the EFE distinguishes general relativity from many other fundamental physical theories. For example, Maxwell's equations of electromagnetism are linear in the electric and magnetic fields, and charge and current distributions (i.e. the sum of two solutions is also a solution); another example is Schrödinger's equation of quantum mechanics which is linear in the wavefunction.

---http://en.wikipedia.org/wiki/Einstein_field_equations

Now this is different from what you have been saying, in that you are now adding to the permanent magnet's field. You used to say it was just the (temporarily) magnetized strings, which clearly it isn't, since even with the strings magnetized the pickup doesn't function well without the magnet.

So again we can see that the string is disturbing the static field of the magnet. Kind of works out the same.

I am saying the same thing I have been saying. Perhaps your understanding is changing. But let's try it one more time:

1. The permanent magnetizes the strings. That is, it causes some magnetic domains to line up. A magnetic field results from this magnetization, which goes away if the permanent magnet is removed.

2. The string vibrates, resulting in a varying magnetic field.

3. This field induces a voltage around the coil. The field must pass through the coil for this to happen.

The string is not disturbing the field of the permanent magnet, it is adding to it, as I explained in more detail to Frank earlier. These disturbances that Frank hypothesizes supposedly cause secondary disturbances through the coil when the field of the string disturbs the permanent field. No such thing happens. The time varying field and the permanent field do not interact. They merely add (in a vector sense) because of the linearity of E&M fields.

[David Schwab]

The linearity of E&M fields in a vacuum is fundamental in physics. As the sun rising in the morning is to day to day life. It has been measured to a very high degree of accuracy. It it fails, so goes much of physics.

Who's talking about the linearity of E&M fields in a vacuum?

I meant this:

There is no interaction of the kind that was suggested above, suggested with no evidence of any kind, either theoretical or experimental.

So you need evidence to back that up, which you are implying there is.

[Mike Sulzer]

Who's talking about the linearity of E&M fields in a vacuum?

I meant this:



So you need evidence to back that up, which you are implying there is.

If the relationship between the fields is linear, they do not interact. For example, two radio waves in a vacuum "pass right through each other". Neither affects the other. The kind of interaction that Frank was talking about requires a non-linearity. Such non-linear behavior can occur with E&M fields in matter at high level. For example, a speaker with an Alnico magnet might have high enough current to move significantly along the hysteresis curve over the audio cycle. However, in a pickup, the field from the vibrating string is too weak. (Speakers deal with watts, pickups with fractions of a nano watt or less, typically.)

[Joe Gwinn]

If the relationship between the fields is linear, they do not interact. For example, two radio waves in a vacuum "pass right through each other". Neither affects the other. The kind of interaction that Frank was talking about requires a non-linearity. Such non-linear behavior can occur with E&M fields in matter at high level. For example, a speaker with an Alnico magnet might have high enough current to move significantly along the hysteresis curve over the audio cycle. However, in a pickup, the field from the vibrating string is too weak. (Speakers deal with watts, pickups with fractions of a nano watt or less, typically.)

It's certainly true that the magnetic response of non-magnetic materials (air, vacuum, wood, plastic, et al) is strictly linear at all field strengths we are likely to achieve, and so superposition applies: Superposition principle - Wikipedia, the free encyclopedia

In iron at flux levels typical in pickups, the linearity is good enough that we can usually act as if superposition applied, because the resulting error is small enough.

So, one can analyze a pickup by the variable-reluctance formulation, or by the induced-field (string is magnetized by the field) formulation, and get equivalent if not precisely equal results.

[Plectrum]

OT but important IMHO.

An amazing phenomenon of internet forums is the way a crowd can initiate and self-confirm complete technical garbage. To those of us with actual experience and knowledge, it's somewhere between amusing and scary. Just like scandalous rumors, bad technical information spreads so much faster than accurate technical information. I guess we Engineers are eventually going to have to hire thug squads to defend rational knowledge.

I'm not an EE, but have a certain degree of knowledge and "higher than average" education in physics.
I totally agree. The Internet has changed things or accelerated them, in that some patently potty ideas are bounded around by the wide-eyed - and it seems the more nuts the idea, the more people get excited.

I know many are trying to establish and affirm their positions as experts in their little fields, and feel that weird arcane beliefs will help. I just don't know why they don't want to read a physics book.

One that gets-to me recently is the incredible "Tracking speed" idea that even gets aired on pickup manufacturing sites.