[Doug]

from Jay's find...
"The polarity of a signal generated in a conductor depends on the magnetic polarity of the flux lines and the direction of travel. In a humbucking pickup, the two coils are wound with opposing electrical polarity, but the magnetic polarity for each coil is also reversed. Therefore, in a humbucking pickup we have two coils of wire, each of which is carrying two electrical signals. Each coil has the intentional signal generated by string vibration within the magnetic field and the unwanted noise signal picked up from ambient electromagnetic radiation. The polarity of the desired signal is dependent on both the polarity of the magnet and the direction of the coil winding so this signal is "in-phase" across the two coils. The polarity of the noise signal is independent of the magnets -- it depends only on the direction of the coil winding -- so the noise signal across the two coils is cancelling, or out-of-phase. "

thanks, JayJay, fish!