Capacitors are one of the primary electrical components, acting as stores of charge in circuits. Capacitance is measured in Farads, or more usefully, microfarads
Three common types of capacitor are illustrated below.
Variable capacitors are commonly used in radio tuning sets. They consist of semicircular aluminium or brass plates separated by air. One set of plates is fixed and the other is rotated by a knob to change the overlap hence the capacitance.
Mansbridge capacitors consist of two long strips of tinfoil separated by thin waxed paper or polyester film. These are rolled up and sealed inside a metal box to prevent entry of moisture.
Electrolytic capacitors take the form of two sheets of aluminium foil separated by muslin soaked in a type of ammonium borate, rolled up and sealed in an insulating container. Wires attached to the foil strips are then connected to a battery. Electrolysis takes place and a thin film of aluminium oxide forms on the positive foil. This is an insulator and serves as the dielectric. Very large and compact capacitances may be made this way.