Dielectric materials

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 works quite well as a dielectric. It has almost no loss. But it is difficult to get very much capacitance using air as the dielectric. Some solid material is usually employed as the dielectric for most fixed capacitors, that is, for types manufactured to have a constant, unchangeable value of capacitance.
Dielectric materials conduct electric fields well, but they are not good conductors of electric currents. In fact, the 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 (Fig. 1). Both of these act to increase the capacitance per unit volume, allowing reasonable capacitances to exist in a small volume.

Fig. 1: Foil sheets can be rolled up with dielectric material sandwiched in between.