The 'role' of a pickup pole piece?

[Dave Kerr]

So, the upshot of Mike's observations is that Bartolini had it ass-backwards? He's more likely to pick up more side-to-side string motion by having his pole pieces's flux have more of a horizontal bias?

[bbsailor]

Field lines show the direction that the magnetic field points. The density of field lines shows the strength of the field. So if you follow a field line out from the pole of a magnet, you move from a region of high density of lines to a region of lower density. This agrees with the observation that the magnetic field gets weaker as you move away from the pole of the magnet.

When the string becomes magnetized by the permanent field you can indeed think of this as a modification of the total field. But it is equally correct to think of the total field as made up of the sum of two parts (by linearity), that is, two fields. The field we are interested in is the one resulting from the magnetization of the string. The reason we are interested in that field is that if we look at it inside the coil, this field changes when the string vibrates, inducing a voltage around each turn of the coil.

This field changes most when the string moves towards or away from the coil because the field of the string weakens with distance from the string. There is a second effect, too. The magnetization of the string changes with its distance from the pole piece. This is because the strength of the permanent field decreases with distance. These two effects multiply together, and it means, as Joseph has pointed out, that the pickup generates some harmonics. How big this effect is depends on the excursion of the string.

Mike, Steve, and all others interested

The neck pickup has a higher asymmetrical output because the string movement is greater. The string output is higher when the string is in the lowest portion of its vibration nearer the magnet and is the lowest when it in the highest portion of its vibration farther away from the magnet. As the pickup is placed closer to the bridge the string vibration is less and the asymmetry is also less but still there to a lesser degree. This can be quickly and easily verified by observing the output of the guitar directly on an oscilloscope.

North and South poles have their fields connect, hopefully through the string and the coil under the string wound around the magnet(s). I have found that if I place a ferrous metal plate under the magnet and have that plate extend to under the full coil width that the output increases by about 20 percent (as observes on an oscilloscope with and without the plate). This is because the metal plate helps redirect the non-string-facing pole end and helps to reflect that magnetic field back up through the coil in a more concentrated manner. This ferrous matal plate then affects the coil inductance and since the guitar pickup acts like a tuned resonant circuit, this plate tends to alter the perceived tonal quality and loudness.

The two most popular instances of this is the metal plate under the bridge Telecaster pickup and the metal "U-shaped" plate on the Fender Jaguar. Grounding this piece of metal also helps to tame the noise pickup.

Joseph Rogowski