XVI. Complete the sentences using the correct variant.

A transformer is applied A transformer consists of The function of the pri-mary winding is The function of the se-condary winding is A transformer with an air core transfers. Iron-core transformers are used Impedance-matching transformers are applied Isolation transformers are employed

to store charge; to prevent the change of energy; to transfer energy; to change the voltage and the current value in a circuit. cores only; the primary and the secondary windings; an iron core and the primary and the secondary windings. to prevent the change of voltage; to supply energy; to receive energy. to receive energy; to decrease the value of charge; to supply energy; to transfer energy. high-frequency currents and low frequency currents; high-frequency currents only. for high-frequency currents; for low-frequency currents. to match the impedance of a source and that of its load; to step down the secondary voltage. for stepping up the primary voltage; for safety in order to isolate a part of machinery from the power source.

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.