XVII. Make a short summary of the text (See appendix, lang. Learn. Focus, p.117-118).

XVIII. Read text 3 c.Describe the process of electric current flow in a vacuum in your own words. Text 3 c. Electric Current In a Vacuum

If a free electron were in a vacuum within the electric field set up between positive and negative electrodes, the negatively charged electron would be attracted to the positive electrode. The movement of the electron would constitute a flow of electric current. It is upon this principle that the electron tubes used in radio and television receivers operate.

We can construct an electron tube by sealing a pair of metal electrodes into opposite ends of a glass bulb and by evacuating the air from within the bulb, leaving a vacuum. Connecting the electrodes to a source of electromotive force makes them positive and negative, respectively. A question now arises: how can we get the free electron into the tube?

There is always a disorderly movement of free electrons within all substances, especially metals. If the difference of potential between the two sealed-in electrodes is made great enough, some of free electrons of the negative electrode will be attracted so strongly to the positive electrode that they will leave the former fly through the vacuum to the latter.

If a substance is heated, the movement of free electrons within that substance is increased. If the temperature is raised high enough, the movement of free electrons is increased to the point where some of the actually fly off from the substance. We call this process thermionic electron emission.

In most electron tubes, the negative electrode is heated to the point where it emits electrons. These electrons are attracted to the positive electrode and constitute a one-way flow of electric current through a vacuum from the negative to the positive electrode.

Lesson 4. Power transmission

TEXT 4 A. Power Transmission;

Grammar Exercises (The Gerund. The Gerundial Complex. The Functions of the Gerund);

TEXT 4 B. Electric Power Consumers and Power Systems;

TEXT 4 C. Electric Power Interruptions.

Pre-Text Exercises

Active Vocabulary

I. Practise reading the words written below. You’ll come across them in text 4 a.

to expand …………………… to convert …………………... interconnection …………….. owing to ………….………… wire ………………………… branch …………….………… to subdivide ………………… distribution ……….………… to deliver …………………… overhead ……………………. conductor …………………… insulator ……………………. support ……………………… to exceed …………………… load ………………………… loss …………………………. to protect …………………… fuse ………………………… relay ………………………… overcurrent …………………. short-circuit ………………… cord ………………………… bus …………………………. consumer …………………… feeder ………………………. isolator ……………………… switchgear ………………….. circuit breaker ……………… step up (step down) substation to supply ……………………. storage battery ……………… auxiliary structure ………….. flexible ……………………... fault …………………………

расширять, увеличивать превращать объединение (энергосистем) благодаря провод ветвь, ответвление подразделять распределение доставлять воздушный, надземный проводник изолятор опора, опорная стойка превышать нагрузка потеря защищать плавкий предохранитель, пробка переключатель, реле перенапряжение короткое замыкание шнур электрическая шина потребитель питатель, подающий механизм изолятор переключатель автоматический переключатель подстанция с растущим (падающим) напряжением доставлять аккумуляторная батарея вспомогательное устройство, вспомо-гательный механизм гибкий авария, повреждение, неисправность