I have experimented with many ways to direct the magnetic field to the strings and have found that using different pole piece shapes can change the way a magnetic field leaves the pickup. Seth Lover who invented Humbucking Pickup for Gibson in the 50 s also worked for Fender during the 60 s and spent time developing the shape of the polepiece used on the Fender Rhodes Piano. He showed me several designs and felt a tapered polepiece worked best. Different shapes will direct the magnetic field to the string and many shapes have been used by many different pickup manufactuers and many models have been developed throughout the years. Types of common polepieces and basic examples are: Steel blades (Charlie Christians) Magnetic blades (Gibson Melody Maker & Firebirds) Magnetic rods (Fender single coils) Magnetic rods- threaded Cunife magnet (Fender Humbuckers) Ferrous rods- (non-adjustable or stud side of Gibson humbuckers) Ferrous screws- (adjustable side of humbuckers, Gretsch Filtertrons