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Roger Mosley, age 13, of Lake Charles, La., for his question:

WHAT IS A VACUUM TUBE?

A vacuum tube is a device once widely used in such electronic equipment as radios, television sets and computers. the tubes control the electric currents, called electronic signals, that are necessary to the operation of such equipment.

Vacuum tubes played an important part in the development of the science of electronics. From the 1920s through the 1950s, all electronic equipment used vacuum tubes.

Since that time, the vacuum tube has been replaced in most kinds of electronic equipment by a newer device called a transistor. Transistors do the same jobs as vacuum tubes but are smaller and also more reliable.

The outer part of the most common vacuum tubes is a glass or metal container called an envelope or bulb. The envelope encloses two or more metal parts called electrodes.

The electrodes create and control a flow of electrons within the tube. This flow of electrons corresponds to the electronic signal being controlled by the tube.

The electrodes are connected to wires that pass through the base or bottom of the envelope. Electronic signals enter and leave the vacuum tube by means of these wires.

Two basic electrodes in a vacuum tube are the emitter or cathode, and the collector or anode. The emitter gives off electrons which flow to the collector, which surrounds the emitter.

The emitter usually has a negative electric charge and the collector usually has a positive charge. The electrodes get their charges from a battery or other source of direct current. The emitter is connected to the source's negative terminal and the collector to the positive terminal.

Another basic vacuum tube electrode is the grid. It consists of a wire mesh located between the emitter and the collector. The grid controls the amount of electrons flowing through the tube.

There are many hundreds of vacuum tubes having various sizes and functions. But electrical engineers classify all tubes into a few basic types.

Receiving tubes, the kind once used widely in radio and television receiving sets, are classified by the number of electrodes they have. Receiving tubes include diodes (two electrode tubes), triodes (three electrode tubes) and multielectrode tubes.

Other types of tubes include cathode ray tubes, microwave tubes and gas filled tubes.

Diodes have only an emitter and a collector. These tubes are used chiefly as rectifiers. A rectifier changes alternating current into direct current. An alternating current is one that keeps reversing its direction of flow.

An electrode connected to a source of alternating current gets a charge that constantly changes from positive to negative and back again.

If an alternating current is sent to a diode, the tube will pass the current only when the emitter has a negative charge. This happens only when the current is flowing in one direction. Thus, the current leaving the tube is direct current.

Diodes were used in receiving sets as rectifiers and also as detectors.

 

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