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SECTION THREE

I.

Pronunciation Drill 1

Practise the sound [ou]:

both, diode, envelope, low, most, close, below, only, show, know, known, though, coat, control, whole, process, hollow", flow, follow, cathode, motion, cold, glow, photo­tube

Pronunciation Drill 2

White reading these words pay attention to the stress mark­ings:

certain constitute external eventual

circuit actual electron phenomenon

follow difference determine evacuate

either envelope collector replenishing

TEXT 4

DIODES

1. The simplest combination of elements constituting an
electron tube is the diode. It consists of a cathode, which
serves for emitting the electrons, and a plate or anode sur­
rounding the cathode, which acts as a collector of electrons.
Both electrodes are enclosed in a highly evacuated envelope
of glass or metal. If the cathode is indirectly heated, there
must be a heating spiral or a heater. The size of diode tubes
varies from tiny metal tubes to large-sized rectifiers. The
plate is generally a hollow metallic cylinder made of nickel,
molybdenum graphite, tantalum or iron.

2. A basic law of electricity states that like charges repel
each other and unlike charges attract each other. Electrons
emitted from the cathode of an electron tube are negative
electric charges. These charges may be either attracted to or
repelled from the plate of a diode tube, depending on wheth­
er the plate is positively or negatively charged.

3. Actually, by applying a potential difference (voltage)
from a battery or other source between the plate and cathode
of a diode, an electric field is established within the tube.

The lines of force of this field always extend from the negatively charged element to the positively charged element. Electrons, being negative electric charges, follow the direction of the lines of force in an electric field.

4. By establishing an electric field of the correct polari­
ty between cathode and plate and "shaping" the lines of for­
ce of this field in certain paths,¹ the motion of the electrons
can be controlled as desired. A battery is connected between
plate and cathode of a diode, so as to make the plate positive
with respect to the cathode, the lines of force of the electric field extending in a direction from the cathode to the plate.

5. Again, applying a heater voltage results in emission of electrons from the cathode. The electrons follow the lines of force to the positive plate and strike it at high speed. Since moving charges comprise an electric current, the stream of electrons to the plate is an electric current, called the

plate current.

6. Upon reaching the plate the electron current continues
to flow through the external circuit made up of the connect­
ing wires and the battery. The arriving electrons are absorbed
into the positive terminal of the battery and an equal
number of electrons flow out from the negative battery ter­
minal and return to the cathode, thus replenishing the sup­
ply of electrons lost by emission.

7. As long as the cathode of the tube is maintained at
emitting temperatures and the plate remains positive, plate
current will continue to flow from the cathode to the plate
within the tube and from the plate back to the cathode
through the external circuit.

8. Now a battery connection has been reversed so as to
make the plate negative with respect to the cathode. When
voltage is applied to the heater the cathode will emit a flow
of electrons. However, these electrons are strongly repelled
from the negatively charged plate and tend to fill the inter-
electrode space between cathode and plate. Since no electrons
actually reach the plate, the tube acts like an open circuit.

9. The total number of electrons emitted by the cathode
of a diode is always the same at a given operating tempera­
ture. The plate voltage (voltage between plate and cathode)
has no effect, therefore, on the amount of electrons emitted
from the cathode. Whether or not these electrons actually
reach the plate, however, is determined by the plate-to-cath­
ode voltage,2 as well as by a phenomenon known as space
charge.

10. The term space charge is applied to the cloud of elec­
trons that is formed in the interelectrode space between cath­
ode and plate. Since it is made up of electrons, this cloud
constitutes a negative charge in the interelectrode space that
has a repelling effect on the electrons being emitted from the
cathode. The effect of this negative space charge alone, there­
fore, is to force a considerable portion of the emitted elec­
trons back into the cathode and prevent others from reaching
the plate.

11. The space charge, however, does not act alone. It is
counteracted by the electric field from the positive plate,
which reaches through the space charge to attract electrons
and thus partially overcomes its effects. At low positive plate
voltages only electrons nearest to the plate are attracted
to it and constitute a small plate current. The space charge
then has a strong effect on limiting the number of electrons
reaching the plate.

12. As the plate voltage is increased, a greater number
of electrons are attracted to the plate through the negative
space charge and correspondingly fewer are repelled back to
the cathode. If the plate voltage is made sufficiently high,
a point is reached eventually, where all the electrons emitted
from the cathode are attracted to the plate and the effect of
the space charge is completely overcome. Further increases
in the plate voltage cannot increase the plate current through
the tube, and the emission from the cathode limits the maxi­
mum current flow.

4000 Commentary

1. "shaping" the lines of force of this field in certain
paths — придавая силовым линиям определенные направле­
ния

2. whether or not these electrons actually reach the plate,
however, is determined by the plate-to-cathode voltage —
однако попадание или непопадание электронов на анод
определяется анодным напряжением

EXERCISES I. Review questions:

1. What elements does a diode consist of? 2. What metal is the plate made of? 3. What does a basic law of electricity state? 4. What charges are emitted from the cathode of an electron tube? 5. When is an electric field established within

the tube? 6. For what purpose is a battery connected between the plate and the cathode of a diode? 7. What is the plate current? 8. When does the tube act like an open circuit? 9. What is space charge? 10. What is the effect (action) of the space charge?

II. Make up an abstract of the text basing on the answers to the above questions.

III. Translate the international words without a dictionary: to vary, to control, to absorb, to indicate, to tend, to limit, to discuss, to form

IV. Define what parts of speech these words are and translate
them:

direction, directive, directly, indirectly, director; par­tial, partially, partly, party; operator, operating, operation, operative, operatively; converter, convertible; dependable, dependent, dependence, dependency

V. Translate these antonyms and memorize them:

1. remain (v), leave

2. correct (adj), wrong

3. simple (adj), complicated

4. continue (v), interrupt

5. lose (v), gain

6. attract (v), repel

7. stop (v), start

8. strongly (adv), weakly

9. externally (adv), internally
10. open (adj), closed

VI. Translate these words and word combinations and learn

both, whether... or, each other, with respect to, as long as, generally, eventually, therefore, correspondingly, as well as

Pronunciation Drill 1

Practise the sound [au]:

how, without, now, however, ground, power, outside, output, around, about, found, encounter, amount, surround, bound, profound

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