Capacitor

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Capacitor, or electrical condenser, device for storing an electrical charge. In its simplest form a capacitor consists of two metal plates separated by a nonconducting layer called the dielectric. When one plate is charged with electricity from a direct-current or electrostatic source, the other plate will have induced in it a charge of the opposite sign; that is, positive if the original charge is negative and negative if the charge is positive. The Leyden jar is a simple form of capacitor in which the two conducting plates are metal-foil coatings on the inside and outside of a glass bottle or jar that serves as the dielectric. The electrical size of a capacitor is its capacitance, the amount of electric charge it can hold.

Capacitors are limited in the amount of electric charge they can absorb; they can conduct direct current for only an instant but function well as conductors in alternating-current circuits. This property makes them useful when direct current must be prevented from entering some part of an electric circuit. Fixed-capacity and variable-capacity capacitors are used in conjunction with coils as resonant circuits in radios and other electronic equipment. Large capacitors are also employed in power lines to resonate the load on the line and make it possible for the line to transmit more power.

Capacitors are produced in a wide variety of forms. Air, mica, ceramics, paper, oil, and vacuums are used as dielectrics, depending on the purpose for which the device is intended.