Inductors, chokes, and coils are passive devices.  Inductors store energy in the form of a magnetic field. In their simplest form, inductors consist of a wire loop or coil. The inductance is directly proportional to the number of turns in the coil. Inductance also depends on the radius of the coil and on the type of material around which the coil is wound.  For a given coil radius and number of turns, air cores result in the least inductance. Dielectric materials such as wood, glass, and plastic are essentially the same as air for the purposes of inductor winding. Ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance obtainable with a coil having a given number of turns. In some cases, this increase is on the order of thousands of times. The shape of the core is also significant. Toroidal (donut-shaped) cores provide more inductance, for a given core material and number of turns, than solenoidal (rod-shaped) cores. Toroids look exactly like doughnuts and come in various diameters, thickness, permeability and types depending upon the frequency range of interest. They have a high inductance for the physical space occupied.