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 general practice in the radio communications. The frequency of a radio circuit can be adjusted in this manner.

Because moving the core in and out changes the effective permeability within a coil of wire, this method of tuning is known as permeability tuning. The in and out motion can be controlled precisely by attaching the core to a screw shaft, and anchoring a nut at one end of the coil. As the screw shaft is rotated in the clockwise direction, the core enters the coil, such that the inductance increases. As the screw shaft can be rotated counterclockwise, the core moves out of coil and the inductance decreases.

Air-core coils:

The simplest inductors are coils. A coil can be wound on the plastic, wooden or other nonferromagnetic material, and it will work quite well, although no air-core inductor can have much inductance. Practically the maximum attainable inductance for such coils is around 1 mH.

Air-core coils are used mostly at radio frequencies, in receivers, transmitters, and antenna networks. Generally the higher the frequency of an alternating current, the less inductance is required to produce significant effects. Air-core coils can be made to have almost unlimited current carrying capacity, by using heavy-gauge wire and making the radius of the coil large. Air does not dissipate much energy in form of heat; it is almost lossless. For these reasons, air core coils are made very efficient.