SECTION TWO
Pronunciation Drill 1
Practise the sound [o:]:
also, always, already, small, almost, although, thought,
all, important, four, ball, orbit, force, call, brought,
portion, saw, ordinary, orderly, alternating, audio, source, cause, fall
Pronunciation Drill 2
While reading these words pay attention to the stress
process carrier emitter sufficiently
method barrier electrode inadequate
metal energy enable available
unit quantity important majority
TEXT 3
ELECTRON EMISSION
1. The electron tube depends for its action on a stream of electrons that act as current carriers. To produce this stream of electrons a special metal electrode (cathode) is present in every tube. But at ordinary room temperatures the free elec trons in the cathode cannot leave its surface because of cer tain restraining forces that act as a barrier. These attractive surface forces tend to keep the electrons within the cathode substance, except for a small portion that happens to have sufficient kinetic energy (energy of motion) to break through the barrier. The majority of electrons move too slowly for this to happen.
2. To escape from the surface of the material the electrons must perform a certain amount of work to overcome the re straining surface forces. To do this work the electrons must have sufficient energy imparted to them from some external source of energy, since their own kinetic energy is inadequate. There are four principal methods of obtaining electron emis sion from the surface of the material: thermionic emission, photoelectric emission, field emission and secondary emis-
. sion.
3. Thermionic emission. It is the most important and one \ most commonly used in electron tubes. In this method the
metal is heated, resulting in increased thermal or kinetic energy of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons released per unit area of an emitting surface is related to the absolute temperature of the cathode and a quantity of the
work an electron must perform when escaping from the emitting surface.
4. The thermionic emission is obtained by heating the cathode electrically. This may be produced in two ways: 1. by using the electrons emitted from the heating spiral for the conduction of current (direct heating) or 2. by arranging the heating spiral in a nickel cylinder coated with barium oxide which emits the electrons (indirect heating). Normally, the method of indirect heating is used.
5. Photoelectric emission. In this process the energy of the light radiation falling upon the metal surface is transferred to the free electrons within the metal and speeds them up sufficiently to enable them to leave the surface.
6. Field or cold-cathode emission. The application of a strong electric field (i.e. a high positive voltage outside the cathode surface) will literally pull the electrons out of the material surface, because of the attraction of the positive field. The stronger the field, the greater the field emission from the cold emitter surface.
7. Secondary emission. When high-speed electrons sud denly strike a metallic surface they give up their kinetic energy to the electrons and atoms which they strike. Some of the bombarding electrons collide directly with free elec trons on the metal surface and may knock them out from the surface. The electrons freed in this way are known as second ary emission electrons, since the primary electrons from some other source must be available to bombard the second ary electron-emitting surface.
2600 EXERCISES
1. Review questions:
1. What does the action of the electron tube depend on?
2. What is present in every tube to produce the stream of electrons? 3. At what temperatures free electrons cannot leave their surface of the cathode? 4. What forces tend to keep the electrons within the cathode substance? 5. What must the electrons do to escape? 6. What must the electrons have to overcome the restraining surface forces? 7. How many methods are there for obtaining electron emission? 8. What are they? 9. What imparts the external energy to the elec trons in thermionic emission? 10. What energy is used for producing free electrons in photoelectric emission? 11. What is field emission? 12. How is secondary emission obtained? 13. What emission is the most commonly used in electronics?
II. Make up an abstract of the text basing on the answers to the above questions.
III. Translate the international words without a dictionary. cathode, emitter, material, cylinder, portion, energy,
radiation, temperature, thermal, adequate, absolute, special, emission, electron, normally
IV. Define to what parts of speech these words belong and translate them:
realize, equalize, electrify, classify, originate, strength-гп, widen, increasingly, widely, likewise, otherwise, for-ivard, towards, upward, outward, downward
V. Translate these antonyms and memorize them:
1. be present (v), be absent
2. primary (adj), secondary
3. relative (adj), absolute
4. outside (adj), inside
5. majority (n) minority
6. common (adj), special
7. external (adj), internal
8. slow (adj), quick, rapid
9. free (adj), bound 10. strong (adj), weak
VI. Translate these words and word combinations and learn them:
because of, since, except for, a number of, the same, within, in this way, suddenly, sufficiently, literally
Test I
Find the correct answer out of the three given to each question:
1. Which of the following devices depends for its action on a stream of electrons in vacuum:
a gas tube, an electron tube, a transistor
2. Which of the following forces keep the electrons within the emitter substance:
internal forces, external forces, attractive surface forces
3. Which of the following kinds of emission depends on increased thermal energy of electrons: thermionic emission, secondary emission, field emission
4. Which of the following kinds of emission depends on astrong field:
thermionic emission, field emission, photoelectric emission
5. Which of the methods of emission is the most important and widely used:
field emission, thermionic emission, photoelectric emission
Test 2
Find Russian equivalents for the English words and word
combinations (see p. 147):
1. a number of (а) буквально
2. except for (b) тот же самый
3. literally (с) так как; с тех пор как; с
4. suddenly (d) из-за
5. within (e) достаточно
6. in this way (f) внезапно
7. since (g) за исключением
8. the same (h) ряд
9. because of (i) таким образом
10. sufficiently (j) внутри; в; в пределах
Test 3
Find an antonym (a), (b), (c) or (d) to the word in bold type:
1. The primary question — (a) principal; (b) difficult; (c) secondary; (d) new
2. A slow motion of electrons — (a) similar; (b) rapid;
(c) steady; (d) continuous
3. The internal forces — (a) attractive; (b) strong; (c) restraining; (d) external
4. A small portion — (a) great; (b) similar; (c) external;
(d) common
5. A high speed — (a) decreased; (b) low; (c) adequate; (d) sufficient
6. The strong attractive forces — (a) slight; (b) equal;
(c) unequal; (d) weak
7. Outside the tube — (a) near; (b) in front of; (c) in side; (d) around
8. Cold water - - (a) clean; (b) hot; (c) boiled;
(d) mineral
9. To heat the liquid — (a) boil; (b) change; (c) evapo rate; (d) cool
10. The majority of the electrons — (a) a great number of; (b) weight; (c) minority; (d) amount
11. Common measuring devices — (a) various; (b) neces- sary; (c) special; (d) new
12. The flow is rapidly determined — (a) often; (b) com monly; (c) frequently; (d) slowly
13. Many particles — (a) light; (b) few; (c) free; (d) heavy
14. To be present in this tube — (a) be included; (b) be inclosed; (c) be changed; (d) be absent
15. Free particles — (a) small; (b) bound; (c) loose; (d) tiny
16. Absolute temperature — (a) high; (b) low; (c) relative; (d) the same
17. More interested in — (a) usually; (b) always; (c) nev er; (d) less
18. Negative charges — (a) the same; (b) unlike; (c) dif ferent; (d) positive
19. Large particles — (a) a lot of; (b) various; (c) tiny; (d) great
20. In a downward direction — (a) upward; (b) outward; (c) straight; (d) right
Test 4
Finish each sentence choosing one of the three variants (a),
(b) or (c) based on the text from Section II:
1. The electron tube depends for its action on...
(a) restraining forces; (b) a stream of electrons; (с) а magnetic field.
2. A special metal electrode is present in every tube to pro duce...
(a) a magnetic field; (b) a stream of positive charges:
(c) a stream of electrons.
3. At ordinary room temperatures the "free" electrons in the metallic cathode cannot leave its surface because of...
(a) attractive forces acting as a barrier; (b) forces of the external magnetic field; (c) thermal energy of the atoms.
4. A small portion of electrons has sufficient kinetic energy to break through...
(a) the surface of the tube; (b) the plate substance; (c) the surface barrier.
5. To escape from the surface of the cathode the electrons must have...
(a) attractive internal force; (b) sufficient energy from some external energy source; (c) low speed.
6. The energy for electron emission comes from...
(a) internal sources; (b) external sources; (c) external and internal sources.
7. There are...
(a) two principal methods of obtaining electron emission; (b) three principal methods of obtaining electron emission; (c) four principal methods of obtaining electron emission.
8. The cathode metal is heated in...
(a) photoelectric emission; (b) cold-cathode emission; (c) thermionic emission.
9. In thermionic emission the cathode is made of...
(a) metal; (b) semiconductor; (c) an insulator.
10. In thermionic emission the number of released electrons depends on...
(a) resistance; (b) cooling; (c) temperature.
11. In photoelectric emission the energy of the light falls...
(a) upon the surface of the non-conducting material;
(b) into the glass envelope filled with the gas; (c) upon the surface of the metal.
12. In photoelectric emission the energy of the light radia tion is transferred to...
(a) free electrons; (b) bound particles; (c) positive charges.
13. In photoelectric emission electrons to which the energy of the light radiation is transferred are...
(a) on the surface of the metal; (b) in the space about the cathode; (c) within the metal.
14. Field emission is...
(a) hot-cathode emission; (b) photoelectric emission;
(c) cold-cathode emission.
15. Electrons escape from the cathode surface because of...
(a) the attraction of the positive field; (b) the attraction of the negative field; (c) the cooling of the cathode's metal.
16. When high-speed electrons suddenly strike a metallic surface they give up their kinetic energy to...
(a) electrons; (b) positive particles; (c) positive charges.
17. Some of the bombarding electrons collide directly with...
(a) positively charged particles; (b) uncharged particles; (c) free electrons.
18. From the surface the bombarding electrons may knock out...
(a) uncharged particles; (b) free electrons; (c) positive
charges.
19. The electrons freed by bombarding are known as...
(a) secondary emission electrons; (b) thermionic emission electrons; (c) photoelectric emission electrons.
20. The most important and the most commonly used method of emission is...
(a) secondary emission; (b) field emission; (c) thermionic emission.
(See keys to the tests on p. 147.)
After reading the following text be ready to answer these questions:
1. How many methods are there for heating cathodes? 2. Where to is the electric current applied in the direct method? 3. Where to is the electric current applied in the indirect method? 4. What materials are used for cathodes? 5. What current can be used for heating them?
TEXT
1. Cathodes are heated electrically either directly or in directly. In the direct method, the electric current is applied directly to a wire, called filament, that also serves as an elec tron emitter.
2. In the indirect method the electric current is applied to a separate heater element, located inside a cylinder that is coated with the emitting material. The cathode is thus heated indirectly through heat transfer (трансформатор на кала) from the heater element. Alternating or direct current can be used for both methods of heating.
3. The most commonly used materials for cathodes are tungsten (вольфрам), heated to temperatures between 2000° and 2800° C, thoriated tungsten (торированный вольфрам) operated at about 1700° С and oxide-coated cathodes opera ted at about 800° to 1000° C. Most tubes are indirectly heated.
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