Radio parts are essential components of a radio. A DIODE is a two-terminal electronic component that allows current to flow in one direction. A DIAL is a KNOB or lever used to adjust the frequency of a radio. A KNOB is a circular handle used to control the volume or tuning of a radio. A TUNER is a device used to select and amplify a desired frequency from a range of frequencies. An AM TUNER is a device used to select and amplify an amplitude modulated frequency from a range of frequencies.