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Using Femm To Model Pickups


Mike Sulzer

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"Why haven’t some one tried to wind “half a strat pickup” (only upper part that is) and discovered that they can cut manufacturing costs with 50% for the wire and 50% for the time winding the pickup(rhetoric question)?

I have not done the integral, but the bottom half probably contributes about a third of the signal. But if you left it out you would reduce the inductance by more than half. This would change the sound a lot; the resonance would be too high.

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Ah...it seems I'm good at missing the point. :D BASEplate not BRIDGEplate....thanks Peter for sticking with me.

The small steel plate on the bottom of the pickup increases the inductance of the coil some.

I was going to suggest the same. I've gathered that a lot of pickup winders recognize that coil shape also has an effect on inductance, and changes in inductance are known to change the peak resonant frequency.

______________

Yes Mike, I usually get to Fall AGU, more often since they've moved it further from Xmas. I'll be there this year, Union session U6.

Are you an AGU type as well?

Edited by erikbojerik
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Mike:

I have a sound card Frequency analyser/oscilloscope/signal generation software now, but I have still not learned to use it very well. I have also built a jig to hold pickups in front of “half a headphone”. I use the signal generator software and see the response on the frequency analyser. With that I could of cause see if the resonance peak changes. No absolute numbers, but an indication about what’s happening. But I have not used an oscilloscope for like 20 years now and I’m not sure how I would go on using one for those measurements.

What would be extremely interesting to do is to alter the inductance without altering the pickup itself. That would tell us if the change in sound is due to the altered inductance or the change in magnetic field in the lower part of the coil. But please help me (I haven’t used this knowledge for 15-20 years now as mentioned before). How can I simulate the change in inductance without altering the pickup? And how would I use the software to measure the change in inductance when adding the steel plate (I know there have to be some equation to this, but I have none of my book left…) to match it to the simulated change?

I would really like to test all of this in the real world. As I don’t care for learning simulation software that can be my contribution to this. But then you guys have to help me a bit here…

About Leo Fender: Take it from someone that has researched the Tele thoroughly. I have read about every book that is available on the subject and searched the web for all available info. The only reason for the steel bridge cover and the steel pickup base plate was shielding. Nothing else. Why didn’t he make anything to reduce hum via shielding on the Strat? Simply because he saw that the players threw away the bridge cover and some even took off the base plate (to reduce the most extreme treble). He was a production man with economical interest in cutting costs. No way was he going to add production costs for something people threw away.

Place a transformer (or anything similar) up front from a Tele and you will notice that a pickup surrounded by a steel plate is not well shielded. Not at all.

About the history of Fender and the transition to more mellow sounding guitars:

Leo went from the trebly Tele to the more mellow jaguars and jazzmasters that were his biggest successes at the time. Actually it was the Jazzmaster that was the high end model at the time, not the Strat. The Strat was a logical transition to those other model (OK, my memory might fail me here and fooling me to mix up the timeline here but I’m 90% sure) going from a twangy Tele to the less ear piercing Strat to the followers. And you have to remember that the Strat was a commercial failure in the beginning. The Tele remained popular but it was the jaguars, mustangs and Jazzmasters that were Fenders high sellers at the time. Now enter a left handed guitar player. It wasn’t until the smashing success of Jimi Hendrix the Strat took off commercial.

Erik: no problemo.

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Place a transformer (or anything similar) up front from a Tele and you will notice that a pickup surrounded by a steel plate is not well shielded. Not at all.

OK let's talk about shielding a bit. Hum can come from two kinds of fields:

1. electric

2. magnetic

Electric fields can be shielded out by metal, copper, steel, whatever, the conductivity does not need to be very high. What happens is that the charge in the metal re-aaranges itself so as to produce a field which tries to cancel the interfering field. The best way to shield something is to fully enclose it in metal. But even an open arrangement has a significant effect.

Magnetic fields are not affected by conductors. You cannot shield them with a thin layer of metal like an electric field. You can shield them out pretty well with a significant amount of ferromagnetic material. But you cannot effectively shield a pickup from magnetic fields or it could not sense the field from the strings.

A transformer produces magnetic fields. The metal in the tele does not shield this out. It is pretty effective against electric fields, however.

Hum from magnetic fields must be canceled, as in a humbucker. The law of induction can be manipulated to get oposing signals from the hum and addition from the fields from the strings. Electric fields cannot be canceled with coils because the law of induction is for magneitc fields. Electric fields can be canceld with a differential amplifier or a transformer used as a subtractor. But electric fields can be shielded out, and that is easier.

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This is how we measure the inductance of magnet coils in mass spectrometer electromagnets.

Measuring inductance

This uses a test signal of a single frequency; Mike do you have a method that uses a frequency sweep?

That method compares the resistance of a resistor to the magnititude of the impedance of the inductor at a fixed frequency, chosen so that there is a significant volage across both the R and the L.

I prefer to make a frequency measurement, rather than voltage. If you feed a current into a parallel LC circuit (by means of a voltage into a large series resistor), you can then measure the frequency response, and from the peak, get the inductance if you know the capacitance. The simple formula 2*pi*f = 1/sqrt(L*C) is approximate for low Q inductors like pickups, but often good enough if you just want something close. I like to feed random noise into the circuit and then measure the spectrum using the 'scope program, adding enough estimates to get accurate results. You can do this at the end of a cable connected to a guitar (or just to a pickup) and get the response of the whole circuit including the cable capacitance. It takes some fooling around to get it right, but so does everything.

Edited by Mike Sulzer
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Peter, I forgot to respond about changing the inductance without changing anything else. One way to get close to this is to put another coil in series with the pickup. This coild could just be another pickup. People do this experiment when they use a dummy coil to cancel hum. Doing this usually results in a noticeable loss of treble, since it raises the inductance and so lowers the resonant frequency. But if the initial resonance is very high it could actually increase the apparent treble by putting the peak of the response wherre the guitar has more response. This is why it is important to be able to measure the resonance; then you know for sure where it is.

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If you feed a current into a parallel LC circuit (by means of a voltage into a large series resistor), you can then measure the frequency response, and from the peak, get the inductance if you know the capacitance. The simple formula 2*pi*f = 1/sqrt(L*C) is approximate for low Q inductors like pickups, but often good enough if you just want something close.

I like that, I'll have to try that out. Might have to borrow the digital scope from the lab for a weekend. :D

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Hey there...I missed this thread, good on you guys for discussing it further in a thread of it's own. I will have to read up on the discussion so far and may well have some questions later.

Keep in mind that it is a stimulating discussion and debate with people who are truly exploring the issues on all sides to further our understanding. I of course have my own point of view and hypotheses, but I welcome them being challenged and corrected.

Learning how to do some comparative tests alone is worth following along with the thread...

keep it up... pete :D

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Guys, I have been offline for 24h and so much to read during that time. Unfortunately not now. I’m on my way right now (travelling on the weekend really sucks) on a week long business trip with limited possibilities to get on line. I’ll do my best to check in from time to time.

Just a few quick comments:

Erik and Mike:

I will have to look deeper into inductance measuring. Hopefully I will come up with something that allows us to compare the inductance for a tele pickup with ond w/o the steel plate, and then simulate the same change without using the steel plate. If so I can make comparing measurements and see how the frequency response differs in the two cases.

Mike: Does a fluorescent light radiate electric or magnetic fields? Then substitute the transformer with that, or what ever you feel like the appropriate thing. A tele will not be shielded enough from the front only because it has the steel bridge plate as you suggested.

All of you:

If you put another pickup in series to change the inductance, you will double the resistance. That will also affect the frequency response, wouldn’t it? A resistor in series with an inductor is some type of filter if I’m remembering things right (but I’m probably not).

Next problem: The “feed voltage into the pickup“ is doable but what if I/we don’t know the capacitance of the pickup? I think that there are some expensive multimeters that can do this, but my budget doesn’t allow any big expenses for things like that at the moment.

But an update on the “upper part of the coil contribute more” issue.

I made a test pickup. One magnet (A5 and symmetrically charged with 15 Gauss at each end)). Wire wound like “half a strat” on that magnet (or half a stacked strat HB but with one magnet). 4000 turns (equal to half of a beefy strat, AWG42) I then put it in my test rig I have for measuring frequency response of the pickups I wind., first with the coil “at the bottom”, then at the top. This way I have eliminated all factors I can think of. It is the same magnet, the same windings the same lead wires the same measurement amplifier (my sound card) and what more…nope cant think of anything.

I then measured the output of the coil when “flipped” and “unflipped” keeping a close eye on the positioning so I hit the same spot in the rig (this rig isn’t made for this type of “pickup”). The result (drum roll please): The ration between the upper and lower part of the coil isn’t 1:2 as an earlier educated guess. It is 1:1! Exactly! There was a difference in less than 2%! So maybe we can put the hole “upper versus lower part of the coil debate behind us as this proves in real life that the hol pickup contributes equally to the output.

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I then measured the output of the coil when “flipped” and “unflipped” keeping a close eye on the positioning so I hit the same spot in the rig (this rig isn’t made for this type of “pickup”). The result (drum roll please): The ration between the upper and lower part of the coil isn’t 1:2 as an earlier educated guess. It is 1:1! Exactly! There was a difference in less than 2%! So maybe we can put the hole “upper versus lower part of the coil debate behind us as this proves in real life that the hol pickup contributes equally to the output.

Well Peter, I think you just proved that a stacked humbucker of the old-fashioned kind does not work. Or are we missing something about stacked humbuckers? I think it is just two coils connected out of phase. But I do not understand your test setup. The signal is made by a string at the normal distance from the pole piece?

Flourescent lights make all kinds of interference.

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Yep, by adding a dummy coil in series, you'll change the resistance and probably the response. Maybe measure the inductance with the two coils in series, then again on the main coil in series with a surface-mount resistor (SMD resistor) of the same resistance as the dummy coil (which I would think would not do as much to the inductance as the dummy coil itself).

I made a test pickup. One magnet (A5 and symmetrically charged with 15 Gauss at each end)). Wire wound like “half a strat” on that magnet (or half a stacked strat HB but with one magnet). 4000 turns (equal to half of a beefy strat, AWG42) I then put it in my test rig I have for measuring frequency response of the pickups I wind., first with the coil “at the bottom”, then at the top. This way I have eliminated all factors I can think of. It is the same magnet, the same windings the same lead wires the same measurement amplifier (my sound card) and what more…nope cant think of anything.

I then measured the output of the coil when “flipped” and “unflipped” keeping a close eye on the positioning so I hit the same spot in the rig (this rig isn’t made for this type of “pickup”). The result (drum roll please): The ration between the upper and lower part of the coil isn’t 1:2 as an earlier educated guess. It is 1:1! Exactly! There was a difference in less than 2%! So maybe we can put the hole “upper versus lower part of the coil debate behind us as this proves in real life that the hol pickup contributes equally to the output.

OK this is not what I would have expected! Now (naturally) I need to figure out why...got a photo? I'm having trouble visualizing the test pickup, and I'd be interested to see the test rig as well.

(I can't get to Mike's conclusion about stacked humbuckers from this particular test)

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"(I can't get to Mike's conclusion about stacked humbuckers from this particular test)"

My understanding of the stacked humbucker (original old kind) is that it is just two coils, one wound on the top half of the magnets, the other on the bottom half. They are connected in opposite polarity. A source of magnetic field that is far away changes little in magnitude or direction over the coil separation and so the opposite polarity causes cancelation. A source that is close by and small (such as the string) falls of with (1/r)^3 in air. Does it fall off with the pickup present? I am saying yes, although not as fast, and so the string signal is reduced, but does not completely cancel.

If Peter's test is valid we have some explaining to do. The other possibility is that the source of the signal that Peter used is large, and so its signal does not fall off much over the length of the magnet. I think a remember from another thread that he uses a small dynamic speaker or headphone driver? That would be too big. The first thing to do is just to hold the pickup over the strings in both orientations and see if there is a big difference.

"Yep, by adding a dummy coil in series, you'll change the resistance and probably the response. Maybe measure the inductance with the two coils in series, then again on the main coil in series with a surface-mount resistor (SMD resistor) of the same resistance as the dummy coil (which I would think would not do as much to the inductance as the dummy coil itself)."

Any old resistor will work. A pickup coil is about a Henry. No resistor in the 5K to 15K range gets anywhere close to that. The extra resistance of the second coil does flatten the responses a bit. But even without using the resistor, you still should hear enough difference to convince you that the circuit properties of the pickup are important.

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"(I can't get to Mike's conclusion about stacked humbuckers from this particular test)"

My understanding of the stacked humbucker (original old kind) is that it is just two coils, one wound on the top half of the magnets, the other on the bottom half. They are connected in opposite polarity. A source of magnetic field that is far away changes little in magnitude or direction over the coil separation and so the opposite polarity causes cancelation. A source that is close by and small (such as the string) falls of with (1/r)^3 in air. Does it fall off with the pickup present? I am saying yes, although not as fast, and so the string signal is reduced, but does not completely cancel.

I'm missing something...what about the stacked single doesn't work according to Peter's test? Are you saying that (for a strong uniform perturbation of the pickup's field) the signal itself should also cancel if the two coils are in opposite polarity and have equal output?

Maybe there's something here:

I then measured the output of the coil when “flipped” and “unflipped” keeping a close eye on the positioning so I hit the same spot in the rig

Peter, did you want the coil itself to be located in the same spot in the rig? Or the ends of the A5 rod(s)?

Need photos!

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"I'm missing something...what about the stacked single doesn't work according to Peter's test? Are you saying that (for a strong uniform perturbation of the pickup's field) the signal itself should also cancel if the two coils are in opposite polarity and have equal output?"

The result of Peter's test implies that the two coils of the SHB would sense the same signal. Then the the out-of-phase connection would give nothing across the two coils in series. This is indeed what does happen for signal far from the coil, one that changes litte over the length of the pole piece. The SHB can only produce output from a signal that changes in amplitude or phase over the length of the pole piece. (Or it works in some other way that I do not understand.)

So the conclusion is: if Peter's test did use a source that simulates the string and given that the result is that the signal is the same whether or not the coil is closer to the source, then the SHB does not work as described above.

I think it is more likely that Peter's source does not properly simulate the string, but we do not know until we hear from Peter, and that might be a few days.

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You guys keep going in the same circle because your basic premise is in error. That premise (seems to) state that the string must become a magnet, and having done so becomes the primary means of generating a voltage in the coil. This explanation also requires the string to have the magical ability to instantly de-magnetize when removed, and to somehow form "multiple magnets" of apparently different poles along it's length (otherwise how would guitars with multiple pickups ever work).

Going back to basic basics, all that's necessary for a magnetic field to generate a voltage in a conductor is a relative motion between a magnetic field and a conductor.

Relative motion does not have to be physical motion, and I'm thinking that a unconscious need for some kind of physical motion is what's really behind this "string is the magnet" theory. After all, the string being in motion is pretty obvious - and therefore it "must" be the magnet.

One thing Peter proved with his experiment is that the magnetic field of the magnet (i.e. pole piece) extends from one end of the magnet to the other. You should know that already, from the elementary school experiment where you get to sprinkle iron filings on a sheet of paper covering a bar magnet.

Magnetic conductive material (material that will be attracted by a magnet, and not necessarily material that "becomes" a magnet) will deflect the magnetic field of the magnet. In other words, the magnetic field - the lines of force - the "flux" will move into a new location. Each line will still extend from one pole to the other - a line of flux is not "firmly attached" at one end, and only capable of being deflected at the other.

In a pickup, the varying deflection of the magnetic field of the pole piece by the vibrating string provides the relative motion necessary for the magnetic field to "cut" the conductor of the pickup coil to generate the signal voltage. Peter's experiment also proved that this action of relative motion of the magnetic field is consistent along the length of the polepiece/coil.

Instead of counting on the "fall off" of the strength of the "string as magnet" field to explain a stacked coil pickup, wouldn't it be much simpler to not have the same pole piece extending through the second coil? - also justified by Peter's experiment.

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Joe, Didn't you like Peter's example of how a magnetized nail picks up other nails? Can you offer any physical explanation of how this happens unless the nail becomes magnetized? (Magnetization means that the current domains tend to line up. What other physical change in the nail could cause it to pick up other nails?)

Of course the string is not magnetized the same along its whole length. Why would it be?

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Joe, Didn't you like Peter's example of how a magnetized nail picks up other nails? Can you offer any physical explanation of how this happens unless the nail becomes magnetized? (Magnetization means that the current domains tend to line up. What other physical change in the nail could cause it to pick up other nails?)

Of course the string is not magnetized the same along its whole length. Why would it be?

Mike, Your model depends on the string becoming a magnet itself and producing it's own unique magnetic field instead of merely influencing the field of the pole piece. Your model ignores the fact the the pole piece is a magnet. Therefore, your model is faulty which is why it doesn't explain anything.

Can you take a nail and magnetize it in some way so that both ends are north poles? If not, what mechanism allows a string to have multiple poles along it's length - or width?

Can you show me any magnet where one side has multiple poles which are opposite to the other side of the magnet? Like this:

N S

+---------------+

S N

That has to be possible before your string as magnet theory has any chance. Am I wrong about that, or are you admitting that the string doesn't actually become a magnet?

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"...producing it's own unique magnetic field instead of merely influencing the field of the pole piece."

Those two things are the same thing because the influence results from magnetization. A ferromagnetic material material that is not really "hard" (such as neo) can be magnetized in quite complicated ways with an external field. Making those pole patterns is not a problem.

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Going back to basic basics, all that's necessary for a magnetic field to generate a voltage in a conductor is a relative motion between a magnetic field and a conductor.

Not *quite* true. You probably know that the second "conductor" you mention (your conductor in bold...which is the string) has to be made of a ferromagnetic material. If you used a guitar string of a different metal (say aluminum) it would not disturb the pickup's magnetic field, and would generate no current in the pickup coil. This is the reason why some acoustic strings won't work with magnetic pickups.

The reason ferromagnetic strings disturb a pickup's field is because they (A) obtain an induced magnetism from the nearby polepiece, and (.:D may have a small inherent permanent magnetism retained during manufacture (usually negligible).

This is why Mike is modelling the string as a small permanent magnet in FEMM.

One thing Peter proved with his experiment is that the magnetic field of the magnet (i.e. pole piece) extends from one end of the magnet to the other. You should know that already, from the elementary school experiment where you get to sprinkle iron filings on a sheet of paper covering a bar magnet.

Yes, rest assured we do already know this. :D

Magnetic conductive material (material that will be attracted by a magnet, and not necessarily material that "becomes" a magnet) will deflect the magnetic field of the magnet. In other words, the magnetic field - the lines of force - the "flux" will move into a new location. Each line will still extend from one pole to the other - a line of flux is not "firmly attached" at one end, and only capable of being deflected at the other.

Yes, and the reason this is true for ferromagnetic material (and not other metals) is because of the field induced in the string by the nearby permanent magnet (pole piece), even though they are not touching. Ergo...string as magnet.

In a pickup, the varying deflection of the magnetic field of the pole piece by the vibrating string provides the relative motion necessary for the magnetic field to "cut" the conductor of the pickup coil to generate the signal voltage.

Yep. The vibrating string "cuts" the field lines and (if ferromagnetic) moves them, inducing current to flow in the coil.

Peter's experiment also proved that this action of relative motion of the magnetic field is consistent along the length of the polepiece.

No, that's not correct...because the test did not involve an actual vibrating string (there was no motion of anything). It involves (I think) perturbing the pickup's field with the time-varying magnetic field of an external electromagnet. Peter's test rig may be perfectly valid for measuring the freq response or output, but I'm not sure if it is set up right for testing this particular phenomenon.

If (for some reason) Peter's test rig produces a test field whose magnitude does not vary over the length of the pole piece, then he'd get exactly the result he got when flipping the test pickup.

Instead of counting on the "fall off" of the strength of the "string as magnet" field to explain a stacked coil pickup, wouldn't it be much simpler to not have the same pole piece extending through the second coil? - also justified by Peter's experiment.

I don't quite see how you get this from Peter's experiment.

__________

OK, taking Peter's experiment to its logical conclusion (thinking out loud here...)

The output of a pickup will depend on how closely spaced the field lines are in the area where the string is vibrating...that's why pickups get more quiet when you lower them further away from the strings, because the field lines diverge from each other as the distance from the pole increases.

Fact is, the magnetic field set up by the pole piece will not be affected very much by the position of the coil along the pole (top, middle or bottom) so long as it contains no ferromagnetic material. By this reasoning, because the string is vibrating at the same position relative to the pole, the pickup's output should be insensitive to coil position....you could have a 12" long pole with the coil at the very bottom, and the output would be the same as if the coil were at the top. I think this is Peter's premise.

But...here's the problem...current is established in the coil only when the turns of the coil intersect the changing magnetic field lines. So the coil at the bottom of the 12" long pole should produce no current if the string is altering the field only at the top of the pole. I think this is Mike's premise.

Edited by erikbojerik
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Joe, at first I misunderstood what Mike was trying to model in FEMM. He is not ignoring the fact that the pole is a magnet, what he's done is to assume that he can subtract out the permanent field of the magnet, and all that's left is the (horizontally) vibrating field of the string...which he is estimating as a thin disc with poles oriented like this:

N

(thin disc)

S

air gap here

S

(pole piece)

N

So the field you see in the FEMM plots is supposed to represent the variable part of the field; the static field has been subtracted. I've thought hard about this, and I am fine with that representation.

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Joe,

Erik's responses reminded me of something else. Are you thinking that the conductivity of the string is an essential part of how the pickup works? It is not. In principle the string could be made of non-conducting ferrite. As Erik pointed out, it is the magnetizability of the string that counts

Mike,

No. Read again - Erik is the one that claimed the string was the conductor. In a pickup, the coil would be the conductor. The string just disturbs the magnetic field of the pole piece.

By subtracting out the magnetic field of the pole piece there would be no magnetic field to cut the wires of the coil and generate a voltage. Sure, you can do it with a model, but it's ignoring reality.

Look at it this way - you are eliminating the cause and claiming that the effect still exists independently without it.

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OK, taking Peter's experiment to its logical conclusion (thinking out loud here...)

The output of a pickup will depend on how closely spaced the field lines are in the area where the string is vibrating...that's why pickups get more quiet when you lower them further away from the strings, because the field lines diverge from each other as the distance from the pole increases.

Fact is, the magnetic field set up by the pole piece will not be affected very much by the position of the coil along the pole (top, middle or bottom) so long as it contains no ferromagnetic material. By this reasoning, because the string is vibrating at the same position relative to the pole, the pickup's output should be insensitive to coil position....you could have a 12" long pole with the coil at the very bottom, and the output would be the same as if the coil were at the top. I think this is Peter's premise.

But...here's the problem...current is established in the coil only when the turns of the coil intersect the changing magnetic field lines. So the coil at the bottom of the 12" long pole should produce no current if the string is altering the field only at the top of the pole. I think this is Mike's premise.

It's a case of backward premises as I tried to point out. Mike's needs the magnet - as the string being a magnet, to be moving and therefore inducing the voltage. If that were correct, then yes, Mike would be right assuming that the field would fall off with distance. That is the only way that the field would be altering the field at the top of the pole - if the field were not "in" the pole to begin with.

But the magnetic field is in the pole and not the string.

Instead of pickups, let's flip it around. Wind a small coil around your 12" pole - something shorter than 12". Put a current through it and create an electromagnet. Will the strength of the electromagnet be any difference depending on the position of the coil along the pole - as long as all of it remains on the 12" pole? No - because the primary poles of the electromagnet will be the ends of the pole.

Flip it back, and you have it - the magnetic lines of the pole must cut the coil regardless of the position of the coil on the pole. As long as the coil remains on the pole.

That's why you can raise individual pole pieces on a pickup, without moving the coil, and still get a stronger signal. The lines of flux are moved closer to the string and are more easily disturbed by it. The lines of flux from the pole still cut the coil as long as it remains on the pole.

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