Bill Anderson, age 14, of Rutland, Vt., for his question:

WHAT EXACTLY IS A CAPACITOR?

A capacitor, once called a condenser, is a device that stores an electric charge. Capacitors perform many functions in electrical and electronic circuits.

Capacitors can smooth the flow of fluctuating current. They are used in circuits that tune a television or radio to the desired station. They can block a continuous flow of direct current, but allow an alternating current to flow through them.

Almost all electronic devices from small transistor radios to giant television transmitters and electronic computers use capacitors.

One kind of capacitor, often called a plastic film capacitor, consists of a piece of plastic film sandwiched between two sheets of aluminum foil. The sheets are rolled into a tight roll, with the plastic film insulating the two aluminum sheets. A wire is attached to each piece of foil and the device is sealed with plastic. The aluminum foil sheets are the plates of the capacitor and the plastic film is the dielectric.

If the wire from the plates are attached to the terminals of a battery, current will flow into the capacitor for a short time. The current builds up an electrostatic charge in the dielectric. The charge is called "electrostatic" because it does not flow past the capacitor.

After the charge has built up, current no longer flows from the battery. If the wires are disconnected from the battery and touched together, a spark will jump between them. The spark discharges the capacitor.

Capacitors are made from many kinds of materials and in a variety of shapes and sizes. They are classified according to the insulating material used as the dielectric. These materials include ceramics, glass, mica, plastics and paper saturated with oil.

Some capacitors are designed for special purposes. Oil capacitors, as an example, have mineral oil as the dielectric. Most of them are used in devices that require high potential alternating current.

Air capacitors have an air space between the plates.

Most air capacitors have two groups of plates. One group is attached to a shaft that moves the plates between the second group.

Turning the shaft changes the area where the plates overlap. This change, in turn, varies the capacitance of the capacitor. Variable air capacitors are often used to tune in different stations on a radio.

Electrolytic capacitors have their plates in a liquid or moisture retaining material. When voltage is applied to the capacitor, a thin coating of oxide forms on the metal plates and acts as the dielectric.

These capacitors are used in circuits that require a large capacitance in a small space.