Pot cores

There is another way to confine the magnetic flux in a coil so that unwanted mutual inductance does not occur. This is to extend a solenoidal core completely around the outside of the coil, making the core into a shell (Fig. 1). This is known as a pot core. Whereas in most inductors the coil is wound around the form, in a pot core the form is wrapped around the coil.

Fig. 1: A pot core shell. Coil winding is not shown.

The core comes in two halves, inside one of which the coil is wound. Then the parts are assembled and held together by a bolt and nut. The entire assembly looks like a miniature oil tank. The wires come out of the core through small holes. Pot cores have the same advantages as toroids. The core tends to prevent the magnetic flux from extending outside the physical assembly. Inductance is greatly increased compared to solenoidal windings having a comparable number of turns. In fact, pot cores are even better than toroids if the main objective is to get an extremely large inductance within a small volume of space.
The main disadvantage of a pot core is that tuning, or adjustment of the inductance, is all but impossible. The only way to do it is by switching in different numbers of turns, using taps at various points on the coil.