Capacitors playa very important part today in various electrical circuits, especially radio circuits. For practical purposes, capacitance is measured in microfarads (IlF). A microfarad is a millionth part of a farad (page 388). Three common types of capacitor are illustrated in Fig. 33.12. Fig. 33.12 (a) shows a variable capacitor of the kind used for tuning radio sets. It
consists of two sets of semicircular aluminium or brass plates separated by air. One set of plates is fixed and the other is rotated by a knob to alter the effective area of the plates.
Fig. 33.12 (h) illustrates the Mansbridge capacitor. It contains 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 the entry of any moisture which would spoil the insulation.
Fig. 33.12 (e) shows the construction of the “dry” electrolytic capacitor. This takes the form of two sheets of aluminium foil separated by muslin soaked in a special solution of ammonium borate. These are rolled up and sealed in an insulating container. Wires attached to the foil strips are then connected to an electric battery. Electrolysis (page 428) takes place and a thin film of aluminium oxide forms on the positive foil. This film is highly insulating, and so the combination now forms a capacitor in which the oxide film acts as the dielectric. Owing to the extreme thinness of the film, very large capacitances which take up very little space may be obtained by this method. Obviously it is important to use this type of capacitor under conditions in which the oxide-covered foil never becomes negative with respect to the other foil.