II. Give the English equivalents for the words below. Find in the text the sentences with these words and translate them

1) трение; 2) электродвижущая сила; 3) элемент; 4) параллельное соединение; 5) сопротивление; 6) электромагнитная индукция; 7) переменный ток; 8) постоянное напряжение; 9) фотоэлемент.

III. Guess the meaning of the following international words and translate them:

reaction, electrostatic, electrometer, electroscope, volt, metallic.

IV. Say whether these sentences are true or false:

1. Alternating force always exerts its effort in one direction.

2. Alternating force is produced by electromagnetic induction.

3. The electromotive force is induced by motion of a conductor.

4. Resistance is an important factor that greatly influences electrical flow.

5. The type of the material employed doesn’t influence the resistance.

V. Answer the questions:

1) What factors cause the motion of electrons from one atom to another? 2) When is the electromotive force developed? 3) When does an electrostatic field appear? 4) How is the electromotive force induced? 5) What unit is used for measuring the electromotive force? 6) What are the sources of electromotive force? 7) What is called “resistance”? 8) How do the types of circuit and material influence the resistance? 9) Name the factors that influence the resistance.

7

THE ALTERNATOR

The alternator is an electric machine for generating an alternating current by a relative motion of conductors and a magnetic field. The machine usually has a rotating field and a stationary armature. In a synchronous alternator the magnetic field is excited with a direct current. The direction of an induced e. m. f. is reversed each time when a conductor passes from a pole of one polarity to a pole at another polarity. Most machines of this type are used for lighting and power, but there are alternators with a revoking armature and a stationary field. They are used in small sizes mostly for special purposes.

Any electrical machine is reversible. When a machine is driven by a source of mechanical power, it works as a generator and delivers electrical power. If it is connected to a source of electrical power, it produces mechanical energy, and operates as a motor. The alternator may also be operated as a motor.

The a-c .generator, or alternator, does not differ in principle from the d. c. generator. The alternator consists of a field structure and an armature. The field structure is magnetized by a field winding carrying a .direct current. An electromotive force is generated in tine winding of the armature. In alternators the field is usually the rotating element and the armature is stationary. This construction has a number of advantages. Only two rings are needed with a rotating field. These rings carry only a relatively light field current, at a voltage generally of 125, and seldom exceeding 250. The insulation of such rings is not difficult. A stationary armature requires no slip rings. The leads from the armature can be continuously insulated from the armature winding to the switchboard, leaving no bare conductor. The alternator with a rotating field may be further divided into the vertical and the horizontal types.

The vertical type is usually applied for large water-wheel generators where it is desirable to mount the water turbine below the generator. The more common horizontal type is used with diesel and steam engine drive. A low speed alternator of this type is suitable for a diesel engine drive, a high speed alternator is suitable for a steam turbine drive.

II. Form nouns, denoting devices with the help of the suffix – or. Translate them

To alternate, to commute, to conduct, to generate

III. Read the text and write out the key words, characterizing the alternator

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