An adjustable inductor used in radio circuits. About 4.2 cm (1 5/8 inches) tall. It consists of a wire wound around a plastic form, with a threaded ferrite slug down the center (visible at top). The slug can be turned with a hex wrench to move it into or out of the coil. Moving it further into the coil increases the permeability, increasing the magnetic field and thus the inductance. Another part of the ferrite core (dark cylinder), called a "pot core", surrounds the wire winding. This serves to confine the magnetic field lines, so they don't induce currents in other nearby components, causing electromagnetic interference. It is probably used in bandpass filters in the intermediate frequency (IF) section of radio receivers to filter out interference from radio stations near in frequency to the desired station.