A fixed capacitor has a value that cannot be adjusted, and that (ideally) does not vary when environmental or circuit conditions change. Here are some of the characteristics, and common types, of fixed capacitors.
Just as certain solids can be placed within a coil to increase the inductance, materials exist that can be sandwiched in between the plates of a capacitor to increase the capacitance. The substance between the plates is called the dielectric of the capacitor. Air is an efficient dielectric; it has almost no loss. But it is difficult to get very much capacitance using air as the dielectric. Some kind of solid material is usually employed as the dielectric for most fixed capacitors.
Dielectric materials accommodate electric fields well, but they are poor conductors of electric currents. In fact, dielectric materials are known as good insulators. Solid dielectrics increase the capacitance for a given surface area and spacing of the plates. Solid dielectrics also allow the plates to be rolled up, squashed, and placed very close together (following figure). This geometry acts to maximize the capacitance per unit volume.
A cross-sectional drawing of a capacitor consisting of two foil sheets rolled up, and two sheets of dielectric material rolled up between them