Guitar Pickup Analysis Web Link

[David Schwab]

2. The change in magnetic flux through the coil when the string vibrates is greater because the magnetized string is closer to the pickup.

You know Mike, the whole magnetized string thing is greatly exaggerated in my opinion. Yes, the strings become magnetized, but they are very poor magnets on their own, and a brand new set you put on is not as magnetized as set that's been on there a while, yet they sound just fine.

Many years ago I tried removing the magnet from the pickup after the strings were magnetized, and you get very little output without the main magnet. That would lead one to believe the string's own magnetic field plays very little in the overall output of the pickup.

A pickup is a variable reluctance sensor. As the strings move over the pickup, the reluctance of the flux path through the coil changes, as well as the flux linkage through the coil. The coil always has a magnetic field around it because of the permanent magnets. The motion of the string varies the magnetic reluctance in the circuit created by the permanent magnet.

The fact that the strings might be magnetized is of less interest than the main magnet in the circuit. This is why different magnet types will sound different in the same pickup with the same strings. When the pickup is closer to the strings, so is the magnet and coils. You have greater lines of flux to disturb as well, especially in a humbucker where the field is pretty compact over the pickup.

[Mike Sulzer]

You know Mike, the whole magnetized string thing is greatly exaggerated in my opinion. Yes, the strings become magnetized, but they are very poor magnets on their own, and a brand new set you put on is not as magnetized as set that's been on there a while, yet they sound just fine.

Many years ago I tried removing the magnet from the pickup after the strings were magnetized, and you get very little output without the main magnet. That would lead one to believe the string's own magnetic field plays very little in the overall output of the pickup.

A pickup is a variable reluctance sensor. As the strings move over the pickup, the reluctance of the flux path through the coil changes, as well as the flux linkage through the coil. The coil always has a magnetic field around it because of the permanent magnets. The motion of the string varies the magnetic reluctance in the circuit created by the permanent magnet.

The fact that the strings might be magnetized is of less interest than the main magnet in the circuit. This is why different magnet types will sound different in the same pickup with the same strings. When the pickup is closer to the strings, so is the magnet and coils. You have greater lines of flux to disturb as well, especially in a humbucker where the field is pretty compact over the pickup.

Variable reluctance is an analysis method; it is convenient to use for some magnetic problems. However, it adds nothing to the fundamentals of magnetism. Maxwell's equations have that function, with Faraday's law of induction (part of Maxwell's equations) most important for pickups. You do not need the method of variable reluctance to understand how a pickup works. To use the VR method, you must be very familiar with how it works, and how it relates to the fundamentals of magnetism. Understanding the operation of a pickup directly through the law of induction allows one to get a better feel for how observed behavior relates to the basic physics.

When it becomes becomes magnetized, the string does not become a permanent magnet, at least not significantly so. It remains so only while in the field of the pickup's magnet. Please do not think that I am saying that it does.

This temporary magnetization is the only way in which the vibrating string can produce a changing magnetic field resulting in a changing flux through the coil, inducing a voltage. If the string does not become magnetized, there is no induced voltage.

Yet your third paragraph seems to imply that there is some other way that the string could influence the reluctance of the magnetic circuit, other than responding to the applied field by becoming magnetized. Ferromagnetism is the process in which the microscopic magnetic domains in some materials tend to line up when a B field is applied. This lining up is magnetization. If the alignment of the domains does not take place, there is no effect.

The final paragraph of your post confuses two different things. As far as the permanent magnetic field is concerned, it is only the field at the string that matters, because by definition of this field, the magnetization of the string is determined by the field at that location. However, the permanent magnet can have two other effects. It is part of the entire magnetic circuit, and if it has a permeability significantly greater than unity, it can have an effect on the changing flux through the coil due to the vibrating magnetized string. Also, its electrical conductivity can modify the pickup response through the flow of eddy currents. This pretty much sums up what the magnet can do.

[David Schwab]

Mike, pickups work by variable reluctance. If the string isn't moving, you wont get any current. Yes soft magnetic metals become temporarily magnetized around permanent magnets.

But don't take my word for it...

If you look up "variable reluctance sensor" in Wikipedia you get (emphasis added):

A variable reluctance sensor (VRS) is used to measure position and speed of moving metal components. This sensor consists of a permanent magnet, a ferromagnetic pole piece, a pickup coil, and a rotating toothed wheel.
As the wheel rotates, the reluctance of the flux path through the coil changes, and the flux linkage through the coil changes, which results in a change in voltage that is measured by an external circuit.

Wheel... string.. it doesn't matter. Hammond organs worked on the same principal. What matters is you are disturbing the lines of flux.

If you look up "magnetic reluctance" you see:

Applications

...
Variable reluctance (magnetic) pickups

Let see what they have for "Variable reluctance (magnetic) pickup"

The vibration of the nearby soft-magnetic strings modulates the magnetic flux linking the coil, thereby inducing an alternating voltage through the coil of wire.

More generally, the pickup operation can be described using the concept of a magnetic circuit. In this description, the motion of the string varies the magnetic reluctance in the circuit created by the permanent magnet.

Lastly in Bill Bartolini's patent:

The invention relates to a variable reluctance pickup for steel string musical instruments in which the vibrating strings cause variations of reluctance in a magnetic circuit generating electrical signals which, upon electronic amplification, are suitable for driving acoustic speaker systems.

Description of the Prior Art
Generally, variable reluctance pickups for steel string musical instruments comprise an arrangement of magnets and magnetically susceptible materials which establish a magnetic circuit in combination with the playing strings. As the strings vibrate, the changes in their position affect the reluctance and magnetic flux of the magnetic circuit. A sensing coil is inductively linked to the magnetic circuit for converting the variations in magnetic flux into a corresponding electrical signal. The electrical signals from the sensing coils is amplified electronically and fed into an acoustic speaker system producing musical sounds.

Mike, your a smart guy. We all know that. But this is how you used to piss off people at MIMF. All these articles I quoted said the same thing I did in my post.

If you move the pickup too far from the string, the string will have less of an effect because its out of the useful range of the magnet. Thats common sense. Move a microphone too far from a singer and you get the same thing. A stronger magnet will allow the pickups to be farther from the strings, if that's what you wanted to do.

I certainly think Bill Bartolini knows about pickups, just as you know about RADAR. You need to be pedantic were it counts. I've always enjoyed your experiments with pickups, such as at Project Guitar, but have you actually made something that works well?

[Mike Sulzer]

Mike, pickups work by variable reluctance.

I did not say it was wrong to look at it that way. Let's look at reluctance. Suppose you have a magnetic field caused by some currents somewhere. Where do the field lines go? You can say they tend to go where the reluctance is low. What has low reluctance? Ferromagnetic materials.

But now the key question is why do ferromagnetic materials tend to make the field lines go through them? Because the magnetic domains tend to line up in a way that reinforces the field. This process is magnetization.

So when I said two posts ago that the permanent field magnetizes the string, and the vibrating string then results in a varying flux through the coil, this does not differ from the idea that a pickup is a variable reluctance sensor. So when you replied two posts ago that this magnetization was not how it works, but rather it is variable reluctance, I tried to explain to you that the two ways of looking at it are really the same thing. I also tried to explain to you that if you do not understand this, you would have some incorrect ideas about how it works. Apparently I did not succeed in communicating these ideas. I hope that this attempt is more successful.

[David Schwab]

Well that's the way the internet often is... so sorry about the confusion.

However, this was the statement:

Variable reluctance is an analysis method; it is convenient to use for some magnetic problems. However, it adds nothing to the fundamentals of magnetism.

So I said, it's also how pickups work, so it's not just "an analysis method".