Capacitor, a device whose principal electric property is capacitance, the ability to store an electric charge. They are important components in many kinds of electrical equipment, including radio and television transmitters and receivers, some automobile ignition systems, and some types of motors. An early form of capacitor, the Leyden jar, was used by 18th-century scientists in studying the nature of electricity and is used today in physics laboratory demonstrations.
The ability of a capacitor to store an electric charge is useful in controlling the flow of an electric current. In some automobile ignition systems, for example, a capacitor (called a condenser) temporarily stores a charge when the breaker points of the distributor open. If there were no condenser, the charge would jump the gap and damage the points.
Another use of capacitors is in circuits that filter electrical signals. A capacitor whose capacitance can be varied is used in the tuning circuit of radio and television receivers. Varying the capacitance changes the resonant frequency of the tuner circuit so that it matches the frequency of the desired station or channel, filtering out signals of all other frequencies.
The typical commercial capacitor consists of two plates (conductors such as metal plates or foils) separated from one another by an insulator, or dielectric, with each plate connected to a terminal. There are two principal types of capacitors, those with continuously variable capacitance and those with a fixed capacitance.
When voltage is placed across the terminals of an uncharged capacitor, charge flows up to the plates but not across the insulator; one plate receives a positive charge, the other a negative charge. As the plates become charged, a voltage is produced across them that opposes the externally applied voltage. When these two voltages have the same magnitude, the current ceases and the capacitor is said to be charged. A charged capacitor is discharged by reducing the external voltage; when this occurs, charge flows off the plates, producing a current and decreasing the voltage across the plates until the external voltage and the plate voltage are equal.
See the next page to learn about common commercial capacitors.