The magnetic field lines flow through the coil(s) and a short section of the strings. Some pickups have a metal cover for shielding and protection of the coils, others have a plastic cover that does not shield against electromagnetic interference, and still others have only isolating tape for protecting the wire. In many cases these cores are screws, so level differences between strings can be evened out by screwing the core further in or out. Some types have rod or bar magnets inserted directly in the coils, while others have magnets below the coils, and cores of soft iron in the coils. The arrangement of the magnets is different for different pickups. signals such as hum that radiate into both coils with equal amplitude) cancel each other. Because these coils are electrically out of phase, common-mode signals (i.e. Dual coil or "humbucking" pickups use two specially configured coils to minimize this interference. Singlecoil pickups are sensitive to magnetic fields generated by transformers, fluorescent lamps, and other sources of interference, and are prone to pick up hum and noise from these sources. Magnetic pickups work only with steel strings, and consist of magnets and coils. The latter type work with all kinds of strings (steel, nylon, or gut). There are two basic pickup types, magnetic pickups and piezoelectric pickups.
I am not affiliated with any manufacturer. So some corrections of facts will be necessary. I am sorry to say that most pickup manufacturers spread misleading information on their products, in order to make more money and to agitate their competitors. Electrically, though, pickups are fairly easy to understand - so this article will examine the connection between electrical characteristics and sound.
There are lengthy discussions between musicians about the advantages and disadvantages of different models, and for someone who has no knowledge of electronics the subject may seem to be very complicated. An electric bass or guitar's sound depends greatly on its pickups.