UNIT 15 .VACUUM TUBES

1. Read the following words:

emitter, electrodes, collector, transistor, filament, cause, classify, include, create, separate, strengthen, essential.

2. Read the following groups of words:

consume power, glass container, flow of electrons, sourse of direct current, wire mesh, microwave tubes, gas – filled tubes, electronic equipment, outer part, common vacuum tubes.

3. Memorize the following words and word-combinations.

1. envelop – оболонка

2. light bulb – електрична лампочка

3. enclose [inklouz] – оточувати

4. emitter – випромінювач

5. collector – колектор

6. coating – покриття

7. filament – нитка розжарювання

8. low-pressure space – простір низького тиску

9. internal gas-pressure – газозаповненний з внутрішнім тиском

10. common vacuum tube – звичайна електронна лампа

Text 15(a)

Vacuum tubes

Vacuum tube is a device widely used in such electronic equipment as radios, television sets, and computers. Vacuum tubes control the electric currents, called electronic signals, that are necessary to the operation of such equipment. The tubes help to create these signals, strengthen them, combine them, or separate them from one another.

The outer part of a vacuum tube consists of a tubelike glass or metal shell. Inside the shell are specially designed wires and small metal plates that control the electronic signals. The vacuum tube gets its name from the fact that almost all the air must be removed from the tube for it to work. A partial vacuum is created inside the tube by pumping out as much air as possible.

How a vacuum tube works. The outer part of 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 corresponds to the electronic signal being controlled by the tubes. The electrodes usually are connected to electric circuits outside the tube by wires that pass through the base of the envelope.

Two basic electrodes in a vacuum tube are the emitter, or cathode, and the collector, or anode. The emitter gives off electrons. These electrons flow to the collector, which in most tubes surrounds the emitter. A coating on the emitter gives off electrons when heated. Close to the emitter is a filament (fine wire) much like that of a light bulb. Electric current from outside the tube flows through the filament and heats it. The filament heats the emitter, causing it to give off electrons.

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’s negative charge helps to push away the electrons it produces. This happens because electrons have a negative charge, and two negative charges – or two positive charges - always push away from each other. But a negative and a positive charge always attract each other. Thus, the positive collector attracts the negative electrons. In this way, a current of electrons flows from the emitter to the collector.

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. A strong negative charge on the grid prevents many of the electrons from reaching the collector. If the negative charge becomes weaker, more electrons get past the grid and reach the collector. The strength of the charge on the grid corresponds to the strength of the electronic signal entering the vacuum tube.

A vacuum tube may have several other parts between the emitter and collector. It may also have charged metal plates that can “bend” a stream of electrons created in the tube. Magnets outside the tube can also bend the stream of electrons.

Kinds of vacuum tubes. 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 widely used in radio and television receiving sets, are classified by the number of electrodes they have. Receiving tubes include (1) diodes (two-electrode tubes), (2) triodes (three-electrode tubes), and (3) multielectrode tubes. Other types of tubes include (1) cathode-ray tubes, (2) microwave tubes, and (3) gas-filled tubes.