Capacitor
A capacitor is an electrical device that can store electrical energy. In this way it is like a battery; however, unlike a battery, a capacitor has the property of capacitance, which is the ability to store electric charge and let it out all at once, and it can resist changes of voltage across its terminals. Capicatance is measures in units of farads, though typical capacitors have values of picofarads, one trillionth of a farad.
A capacitor consists of two conductors that are separated by a dielectric (insulator). The larger the surface area of the conductors and the smaller the separation between them, the larger the capacitance will be. There are two main types of capacitors--variable (which are tunable) and fixed (which have a single, fixed capacitance). Fixed capacitors, which comprise the lion's share of the market, come in two types: electrolytic and non-electrolytic.
The origin of the capacitor goes back more than 200 years. In October 1745, Ewald von Kleist, Dean of the Cathedral of Camin (in Germany), made the discovery of capacitance. Three months later, in January 1746, Professor Peter von Muschenbrock at the University of Leyden, made the same discovery and built the first working model of a capacitor. The device consisted of a glass jar partially filled with water, with an electrode dipping into the water, and is called a Leyden jar. A student named Cuneus, experimenting in Leyden, was using a Hawkesbee machine to generate static electricity which was conducted by a chain into water in the jar--a simple type of Leyden jar. When Cuneus touched the chain he received such an intense electric shock that it nearly killed him. The news of this ability to store a large electrical current spread rapidly, and demonstrations in which the public willingly offered themselves as guinea pigs to be shocked took place.
It was one such demonstration that caught the attention of Benjamin Franklin. The electricity he conducted out of the sky with his famous kite experiment was stored in a Leyden jar. Franklin ultimately discovered that the electrical charge was stored within the glass itself, not the water, and determined that the electrostatic action was due to the material that insulated the conductor. He placed two sheets of lead on his jar (one on the inside and one on the outside), electrified them, then removed them and found that the glass insulator held the charge. Franklin's invention became known as the electrical "condenser," a name which is often still applied to capacitors.
The Leyden jar became obsolete in 1775 when Alessandro Volta (1745-1827) invented a device he called the electrophorous. It consisted of one metal plate covered with ebonite and a second plate that had an insulated handle. When the ebonite plate was rubbed, it built up a negative charge. When the second plate was placed above the ebonite, a positive charge was attracted to the lower surface, and a negative charge was repelled to the upper surface. This negative charge was drawn off with a grounding wire, leaving the positive charge behind. Repeating the process caused a strong positive charge to be built up. This "charge-accumulating" device became the basis for today's electrical capacitors.
The use of more efficient dielectric (insulating) material improved the performance of capacitors. In the 1850s mica (a naturally occurring dielectric) was made into sheets for use as insulators, although they didn't come into commercial use until World War I. They could withstand shock better than glass insulators, and their efficiency permitted the size of the capacitor to be reduced. Rolled paper and ceramic insulators came into use in 1876 and 1900 respectively; today plastic film, electrolytes, and even air are used as insulators. Ceramic capacitors are similar to the paper type but they have several advantages; they have larger capacitances, are smaller in size, and operate at voltages up to 1,000 volts. Mica capacitors can be used with voltages as high as 35,000 volts. Plastic film capacitors, using polystyrene as an insulator, are more expensive than the paper capacitor, but they are smaller and have a very high insulation resistance.
The ability to store and control electric charges and currents is essential to modern electronics. The dielectric inhibits the flow of direct current (DC), but the constant charging and discharging allows alternating current (AC) to pass. This type of capacitor is used to couple amplifiers and is known as a blocking, or coupling, capacitor. Electrolytic capacitors have a very thin dielectric and a high capacitance. Because of their small size they are very well-suited for modern electronics, but they do have a disadvantage in that they have a high leakage of current. They are used as electronic filters to rectify AC (convert AC to DC). Electrolytic capacitors are also used in electronic photographic strobe units. There is a great variety of uses for capacitors. They are used in ignition systems to reduce arcing. Trimmer capacitors are used where just a small change in value is needed. Variable (tunable) capacitors are found in old-style radio tuners; two sets of interleaved metal plates, one set fixed and one movable, are used to tune in specific frequencies. The leaves are insulated by the air between them, which serves as the dielectric.
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