Another measurement Involving String Magnetization Along Its Xxis

[Mike Sulzer]

Aquin43 wrote this on Guitarnutz2ickups: pickup testing and modeling: "I am pretty sure that it is the longitudinal magnetization of the string that produces the output in a pickup with poles or pole magnets. If you take the magnets out of a strat type pickup and then orient just one magnet along or at right angles to the coil axis with the pole facing the string, the output remains the same. Only the longitudinal magnetization would be unaffected by this manoeuvre." (https://guitarnuts2.proboards.com/post/88751/thread)

It follows from this and symmetry that the magnetization along the string points in opposition directions on either side of the magnet, and we expect that it decreases away from the magnet with some distance scale. The purpose of this discussion is to show a simple way to measure this distance scale approximately. (This discussion is related to some of the discussion in Mark Hammar's recent discussion.)

Let's review how a cylinder magnetized along its axis produces a magnetic field. Magnetic dipoles can be thought of as a closely pair of + and minus magnetic charges. (These charges do not actually exist, but they give a valid model of how it works.) Inside the cylinder the effects of all + and - charges cancel out at some observation point, and so most of the dipoles contribute nothing to the measured field. But at the ends, the dipoles are exposed, and thus create an observable field. Also if the magnetization is non uniform, then charges are partly exposed and a corresponding field is produced. In this measurement we are creating magnetization that decreases as the distance for the magnet increases, and so a magnetic field is produced.

The field maximizes in the direction of the axis of the string. But remember that magnetic field lines are curved, and this curvature contributes to the component that passes through the pickup coil along its axis, causing the sensed voltage when the string vibrates.


This measurement requires a guitar with amp or oscilloscope or both, and one or several magnets, at least moderately strong. The idea is to place the correct pole of the magnet close to the E1 string, some distance from the pickup magnet, and then move the magnet closer to the pickup magnet until the output from the string (plucked with the other hand) cancels out, or nearly so. Of course, you only can get cancelation if the correct pole is the one close to the string. The idea is to saturate the string where the magnet is close to it so that small changes in distance from the string do not affect the magnetization very much, making the measurement easier.

The distance I got was 6-7 mm from the edge of the magnet closest to the pickup pole. I tried it with several magnets:
neo disk (dia. 3/8", 3/16" thick, 3200 G on a face): works but inconveniently strong
neo rod (dia. 1/8", 1/4" long, 3200 G on a face): works but too strong
neo rod (dia. 3/32", 1/4" long, 2000 G on a face): still kind of strong, and needs to be mounted in a handle
some radon Alnico pole piece, 1000G on a face, works, but I prefer a smaller diameter for more precision.
some random Alnico Humbucker bar (600G on edge) works, convenient because is has its own handle, but should be a bit stronger so it does not have to be inconveniently close to the string
ceramic bar from a mini humbucker. (800+G) My favorite, thin and precise with a convenient handle.

The measurement yields the distance over which the field decreases from saturation down to the level induced by the pickup magnet. Probably one could nail this down better numerically by testing pickups with different strength magnets, but for now this simple measurement gives me some useful approximate information.

[Antigua]

That's interesting, and I guess that makes sense, since the bulk of the guitar string is longitudinal. I'm trying to paint a picture in my head of your test setup, but I'm not very good at doing, even when I read is slowly and several times over, but what of it I did pick up on gives me ideas of my own to try out.

[Mike Sulzer]

Antigua, I spent some time replying, but the software responded with a "reply too short" message. I will start over again later when I have some time.

That's interesting, and I guess that makes sense, since the bulk of the guitar string is longitudinal. I'm trying to paint a picture in my head of your test setup, but I'm not very good at doing, even when I read is slowly and several times over, but what of it I did pick up on gives me ideas of my own to try out.

[Mike Sulzer]

Here is some more detail:

Connect the guitar to scope.
Put guitar flat in lap (or on bench, I suppose.)
Hold the ceramic (or other magnet) vertical against the pick guard with the right hand, edge towards the E1 string. (only one edge will cancel, of course.)
Make sure the edge is close the the string, closer than normal spacing between the pole and string. The idea is to saturate the string so that the degree of magnetization is not very dependent on small changes in the distance to the string.
Start a couple of centimeters from the pickup pole piece. This is too far away to have a significant effect.
Excite the string with the left hand and note that the pickup puts out a normal level.
Now move the magnet towards the pickup pole along the string.

You get to a point where the pickup output drops a lot. This is about 6 or 7 mm from the left edge of the magnet to the pickup pole. Apparently the magnetization induced along the string for that distance produces a field that is just about exactly as stoing (and oppositely directed) as the field from the pole piece. You can play around and convince yourself that the string does it; that is, the magnet alone has no significant effect at that distance.

That's interesting, and I guess that makes sense, since the bulk of the guitar string is longitudinal. I'm trying to paint a picture in my head of your test setup, but I'm not very good at doing, even when I read is slowly and several times over, but what of it I did pick up on gives me ideas of my own to try out.

[Antigua]

Here is some more detail:

Connect the guitar to scope.
Put guitar flat in lap (or on bench, I suppose.)
Hold the ceramic (or other magnet) vertical against the pick guard with the right hand, edge towards the E1 string. (only one edge will cancel, of course.)
Make sure the edge is close the the string, closer than normal spacing between the pole and string. The idea is to saturate the string so that the degree of magnetization is not very dependent on small changes in the distance to the string.
Start a couple of centimeters from the pickup pole piece. This is too far away to have a significant effect.
Excite the string with the left hand and note that the pickup puts out a normal level.
Now move the magnet towards the pickup pole along the string.

You get to a point where the pickup output drops a lot. This is about 6 or 7 mm from the left edge of the magnet to the pickup pole. Apparently the magnetization induced along the string for that distance produces a field that is just about exactly as stoing (and oppositely directed) as the field from the pole piece. You can play around and convince yourself that the string does it; that is, the magnet alone has no significant effect at that distance.

So is the reason this works because the steel is magnetically saturated, or is it because it's magnetically nullified/cancelled out by having an opposing magnetic field near the pickup?

[Mike Sulzer]

The saturation is just for convenience. Of course, you cannot get cancelation unless the magnetization is strong, and it is a lot easier to saturate it.

So is the reason this works because the steel is magnetically saturated, or is it because it's magnetically nullified/cancelled out by having an opposing magnetic field near the pickup?