In this case, without even doing any calculations, you can say that the total is 100µF for practical purposes. The 100-pF unit is only a millionth of the capacitance of the100-µF component; therefore, the smaller capacitor contributes essentially nothing tothe composite total.Dielectric materialsJust as certain solids can be placed within a coil to increase the inductance, materialsexist 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 verymuch capacitance using air as the dielectric. Some solid material is usually employed asthe dielectric for most fixed capacitors, that is, for types manufactured to have a con-stant, unchangeable value of capacitance.Dielectric materials conduct electric fields well, but they are not good conductorsof electric currents. In fact, the materials are known as good insulators.Solid dielectrics increase the capacitance for a given surface area and spacing ofthe plates. Solid dielectrics also allow the plates to be rolled up, squashed, and placedvery close together (Fig. 11-5). Both of these act to increase the capacitance per unitvolume, allowing reasonable capacitances to exist in a small volume.204 Capacitance11-5Foil sheets can be rolled up with dielectric material sandwiched inbetween.Paper capacitorsIn the early days of radio, capacitors were commonly made by placing paper, soakedwith mineral oil, between two strips of foil, rolling the assembly up, attaching wire leadsto the two pieces of foil, and enclosing the rolled-up foil and paper in a cylindrical case.