An inductor is generally constructed as a coil of conducting material, characteristically copper wire, wrapped around a core either of air or a ferromagnetic material. Core materials have higher permeability than air; confine the magnetic field closely to the inductor, thereby increasing the inductance. Inductors come in variable shapes. Most are developed as enamel coated wire wrapped round a ferrite bobbin with wire uncovered on the outside whereas some enclose the wire totally in ferrite and are known as "Shielded". Some inductors have adjustable core, which enables changing of the inductance. Small inductors can be imprinted directly onto a printed circuit board by set out the trace in a spiral pattern. Small value inductors can be built on integrated circuit using the same process that are use to make transistors. Inductors use to block very high frequencies are some time made with a wire passing through a ferrite cylinder.