About Guitar Pickups
Guitar pickups are pretty much the most important aspect of guitar electronics. Besides using capacitors to effect the tone of a guitar, guitar pickups have the most influence on the sound in guitar electronics. They can be wired to interact with each other to create a variety of different sounds. Guitar pickups work by transducing the vibration of the metal strings into electrical signals (an AC voltage usually between 100 millivolts and 1 volt rms). There are several different pickups but the most popular by far are passive humbuckers and single coils. Single coil pickups are made with a magnet at the base and a coil of wire which rests on top. The coil is one wire that has been wrapped around the six metal lugs or screws that extend to the magnet, thousands of times. the magnet in combination with the coil of wire, resting under a metal guitar string, will act as a transducer. The vibrating string of the guitar will upset the magnetic field generated by the magnet and induce an electrical signal in the coil of wire. The electrical signal can then be manipulated by volume and tone controls in order to shape the tone. The pickups can also be wired to each other to produce different tones. This happens do to the orientation of the magnets and the direction of the coil windings.
The Magnetic Orientation Of Single Coils and Humbuckers
Here is the difference of direction of different pickups polarities. As you can see, the single coil pickup has a different polarity direction than a humbucker. The single coils polarity has its poles facing up towards the strings or away from them. The humbucker has its poles facing towards each of its coils. The reason for this is because the coils need to be one polarity, either North or South. The single coil pickup has either the North side of the magnet facing the coil or the South side of the magnet. The Humbuckers coils were cleverly designed to have one coil resting over the entire South end of the magnet and the other coil over the North end of the magnet. Humbuckers are design that way because having the coils with opposite polarity as well as one coil being wound in the opposite direction cancels out electromagnetic interference but doubles the amplitude of the guitar signal. The combination of the magnets being out of phase and the coils being out of phase actually causes the pickups signal to be in phase. This can be confusing but almost everyone, when talking about phase in the context of guitar electronics, is referring to the guitars signal rather than the magnets or coils. So when you hear someone talking about thin sounding pickups wired out of phase, they are reffering to the guitars signal from each coil being out of phase when they combine due to the coils actually being in phase (wound in the same direction) but with magnets still out of phase (opposite polarity).
Single coil pickups in the standard Stratocaster set up have the middle coil with opposing polarity as well as reversed windings. This is for the same reason as with the humbuckers. The reversed coil wiring and the opposite polarity cause the guitars signal from both pickups to be in phase with each other. Quite often you will see guitar pickups that are reverse wired and reverse polarity abbreviated to RPRW around the internet. That stands for: Reverse Wound Reverse Polarity, as you might have guessed. In some old Stratocasters, the middle pickup was not RWRP. This meant that the guitar signal was actually out of phase. The farther apart the pickups are, the more difference there is in the signal that they receive which means that pickups out of phase with each other that are far apart will be louder than if the pickups were next each other because less of the signal is cancelled out.