Wire and DC resistance
For musical instrument pickups, the most commonly used wire gauges are 42AWG, 43AWG and 44AWG. AWG = American Wire Gauge.
The diameter of the wire diminishes with increasing gauge number:
42AWG has a diameter of 0.063mm
43AWG has a diameter of 0.056mm
44AWG has a diameter of 0.05mm
The resistance per unit length of the wire is inversely proportional to its cross sectional area, so the smaller the cross sectional area, the higher the resistance per unit length becomes. It is a similar effect to water flowing in a pipe, the water will flow more freely in a larger pipe and similarly electrons will flow more freely through a larger diameter wire.
As an example, a P90 wound with 10,000 turns of 42AWG wire would have a DC resistance of around 8.5k
The same P90 wound with 10,000 turns of 43AWG wire would have a DC resistance of around 10.75k and in 44 AWG wire, around 13.5k. The actual output levels of each pickup would not differ by an immense amount so winding with a thinner gauge of wire to increase the DC resistance does not really increase the output of the pickup.
I usually wind pickups to specific turn counts, occasionally to a specific DC resistance. It is important to note that tolerances on the diameter of wire, batch variations and ambient temperatures affect the resistance for any given turn count. So 10k in my cold workshop may read 10.2k in your warm lounge.
Wire and tone
For a given number of turns of wire, the treble content of the tone will increase with:
Increasing diameter of wire.
Increasing wire insulation thickness.
Lower winding tension.
All three of these factors increase the separation of the individual layers from each other and reduce the capacitance of the coil. The effect of capacitance in the coil works in a similar way a tone control in allowing higher frequencies to pass to earth so reducing the brightness of the pickup.
So reducing the wire tension during winding reduces the compaction of the windings and increased wire diameter and insulation thickness further reduce the interaction between the layers of wire. Capacitance reduces as a result and the higher frequencies are less subdued.
Scatter winding of the coil is believed to further reduce capacitance but the effect is hard to quantify as particularly when hand winding both tension and the winding pattern will vary.
Magnets
Whether Alnico, FeCrCo or ceramic, there are two basic types - Isotropic (Un-oriented) and Anisotropic (Oriented).
An un-oriented magnet can be magnetised in any direction, an oriented magnet can only be magnetised across one dimension.
Ceramic magnets are generally quite powerful and developed a bad reputation in the past as many cheaper imported pickups used the material with little consideration to how the coils were wound. Ceramic magnets can produce very good sounding pickups providing that the coils are wound appropriately. This usually means putting a lot of wire on the bobbins to tame the high end with the result that many ceramic pickups tend to be also higher output types.
Alnico 3 is the weakest of the Alnico grades used for musical instrument pickups and found it’s way into earlier single coil pickups where it produces good top end clarity.
Alnico 2 is stronger that Alnico 3 and is a true AlNiCo alloy whereas Alnico 3 is really “AlNi” as it contains no cobalt. Alnico 2 produces a warm, articulate sound and is particularly suited to humbuckers for the neck position and for semi hollow guitars in both bridge and neck positions.
AlNiCo 4 sits between AlNiCo’s 2 and 5 in terms of magnetic strength. Allegedly used in some PAF humbuckers. It has a nice balanced tone and is best suited to vintage/moderate output pickups.
AlNiCo 5 is perhaps the most common and versatile of the AlNiCo’s, more powerful than AlNiCo 4 and producing a brighter, more dynamic response.
Un-oriented AlNiCo 5 shares the warmth and articulation of AlNiCo 2 with the brightness and punch of AlNiCo 5, it makes for great sounding humbuckers for many genres.
AlNiCo 8 is the most powerful of the more common AlNiCo’s and is a good alternative to ceramic magnets for higher output pickups where it retains more warmth. As with ceramic pickups, the winding needs to be appropriate.
Ferri-Chrome (FeCrCo). Used primarily as a modern replacement for CuNiFe and has very similar tonal characteristics.
Rare earth magnets (Neodymium, Samarium) are extremely strong and I generally only use these materials in bespoke pickups where there space constraints and a smaller, strong magnet is the best option. As with ceramics and AlNiCo 8, rare earth magnets are more generally found in high output pickups.