Permeability tuning

Solenoidal, or cylindrical, coils can be made to have variable inductance by sliding ferromagnetic cores in and out of them. This is a common practice in radio communications.
The frequency of a radio circuit can be adjusted in this way, as you’ll learn later in this book.
Because moving the core in and out changes the effective permeability within a coil of wire, this method of tuning is called permeability tuning. The in/out motion can be precisely controlled by attaching the core to a screw shaft, and anchoring a nut at one end of the coil (Fig. 1). As the screw shaft is rotated clockwise, the core enters the coil, so that the inductance increases. As the screw shaft is rotated counterclockwise, the core moves out of the coil and the inductance decreases.

Fig. 1: Permeability tuning of a solenoidal coil.