Pot coresThere is another way to confine the magnetic flux in a coil so that unwanted mutual in-ductance does not occur. This is to extend a solenoidal core completely around the out-side of the coil, making the core into a shell (Fig. 10-9). This is known as a pot core.Whereas in most inductors the coil is wound around the form, in a pot core the form iswrapped around the coil.192 Inductance10-9A pot core shell. Coil winding is not shown.The core comes in two halves, inside one of which the coil is wound. Then the partsare assembled and held together by a bolt and nut. The entire assembly looks like aminiature 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 mag-netic flux from extending outside the physical assembly. Inductance is greatly in-creased 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 largeinductance within a small volume of space.The main disadvantage of a pot core is that tuning, or adjustment of the induc-tance, is all but impossible. The only way to do it is by switching in different numbers ofturns, using taps at various points on the coil.Filter chokesThe largest values of inductance that can be obtained in practice are on the order ofseveral henrys. The primary use of a coil this large is to smooth out the pulsations in di-rect current that result when ac is rectified in a power supply. This type of coil is knownas a filter choke. You’ll learn more about power supplies later in this book.