Paper capacitors
In the early days of radio, capacitors were commonly made by placing paper, soaked
with mineral oil, between two strips of foil, rolling the assembly up, attaching wire leads
to the two pieces of foil, and enclosing the rolled-up foil and paper in a cylindrical case.
These capacitors can still sometimes be found in electronic equipment. They have
values ranging from about 0.001 μF to 0.1 μF, and can handle low to moderate voltages,
usually up to about 1000 V.
Mica capacitors
When you were a child, you might have seen mica, a naturally occurring, transparent
substance that flakes off in thin sheets. This material makes an excellent dielectric for
capacitors.
Mica capacitors can be made by alternately stacking metal sheets and layers of
mica, or by applying silver ink to the sheets of mica. The metal sheets are wired
together into two meshed sets, forming the two terminals of the capacitor. This scheme
is shown in Fig. 11-6.
Mica capacitors have low loss; that is, they waste very little power as heat, provided
their voltage rating is not exceeded. Voltage ratings can be up to several thousand volts
if thick sheets of mica are used. But mica capacitors tend to be large physically in proportion
to their capacitance. The main application for mica capacitors is in radio receivers
and transmitters. Their capacitances are a little lower than those of paper
capacitors, ranging from a few tens of picofarads up to about 0.05 μF.
