Re: Can I move the pole pieces?
Artie,
This is getting a bit complicated, but let me try again. Permanent magnets have nothing to do with the pickup, as I said above, and as you say. The only purpose of the permanent magnetic field in a pup is to induce a magnetic field in the strings. The vibrating string then induces a varying magnetic field in the pup. The coil produces a voltage from the time-varying magnetic field. The cores play an essential role: they amplify the changing magnetic field. This is a large effect; without ferromagnetic cores the pup would not work. The cores also increase the inductance of the coil by the same large amount. It is really the same effect; inductance is a result of the magnetic field produced by current flowing in the coil producing a voltage in the coil that opposes the flow of current. The cores amplify this field and so increase the inductance a lot.
The cores in the standard humbucker play a dual role: 1. they increase the sensitivity as described above; 2. they are magnetized by the magnet and thus they then magnetize the strings.
In a single coil pup using magnets as the cores, these magnets play a dual role:1. they magnetize the strings 2. they amplify the field from the strings.
One of the advantages of using separate cores and magnets is that the two types of materials can be optimized. Permanent magnets are not necessarily the best materials for amplifying fields. Using soft iron for the cores and permanent magnet behind them is more efficient.
Suppose one used a very strong magnetic material, neodymium, as magnets in a single coil pup. There would be two problems: 1. string pulling from the way too strong field; 2. the pup would have low output and the wrong voicing. The problem with using very strong magnets as cores is that nearly all the little magnetic domains are already lined up in the same direction. Amplifying the field requires flipping lots of the domains, and there are not many left to flip, and these few are very difficult to flip.
Interference comes in three types:
1. electromagnetic fields. These are propagating waves (radio waves) that can come from distant sources. This is usually not a probelm with pups, but it could be. A metallic layer shields out electromagnetic waves.
2. electric fields. Electric fields come from electric charge. Charge that changes in time produces electric fields that change in time, like hum from an electrical system. Unbalanced or poorly gounded electric systems produce electric fields as does any electrical equipment. A metallic layer shields out electric fields.
3. magnetic fields. Generally only electrical devices containing ferromagnetic material cause magnetic fields that are strong enough to make problems with pups. A power transformer in an amplifier is an example. The transformer core uses a ferromagnetic material to get a large inductance and a high coupling between the primary and secondary windings. A metallic layer does not shield out magnetic fields.
Thus shielding techniques using a metalic (or otherwise conducting) layer can handle the first two types of interference, but they cannot handle the third. Since a pup must be sensitive to changing magnetic fields in order to sense the strings, it must also be sensitive to changing magnetic field from hum sources unless:
1. You use a magnetic shielding material. This is possible, but not so easy.
2. You cancel the hum but keep the sensitivity to the strings by a clever arrangement of two or more coils.
This is what the humbucker was intended to do. Is it possible that the humbbucker also cancels electric fields? Sure, but apparently the inventor was not so certain that it would do a good job, because he put it inside a metal box to keep out the electrical fields.
