IX. Translate tfie following sentences paying attention to the words in bold type:

1. The faster the molecules of a substance move, the higher is the temperature of the substance. 2. The larger the water pipe, the more water passes through it. 3. The more you read, the more you learn. 4. The shorter the wire, the less is its resistance to current flow. 5. The greater the number of free electrons in a substance, the better that sub­stance conducts electricity.

X. Read and retell the following text.

If there were no electricity

At present it is difficult even to imagine the time when * there was no electricity, when people had to do without it.

What would our everyday life be like if there were no electricity?

Can you imagine a situation when all devices producing electricity would stop operating?

If this happened in the evening while you were in the cinema, you would be sitting in the dark without light. Then you would walk along dark streets. You would try to take a trolley-bus or a tram, it would be impossible. As there would be no light at home, you should use either a smoking kerosene lamp or a candle.

You would like to use the telephone or to watch TV but they would not work because they both depend upon the elec­tric current. This example shows the importance of electricity in everyday life.

XI. Speak on the heating effect of an electric current.

LESSON SEVENTEEN

_{THE COMPLEX SENTENCE}

MAGNETIC EFFECT OF AN ELECTRIC CURRENT

The invention of the voltaic cell in 1800 gave electrical experimenters a source of a constant flow of current. Seven years later the Danish scientist and experimenter, Oersted, decided to establish the relation between a flow of current and a magnetic needle. It took him at least 13 years more tp find out that a compass needle is deflected when brought near a wire through which the electric current flows. At last, during a lecture he adjusted, by chance, the wire parallel to the needle. Then, both he and his class saw that when the current was turned on, the needle deflected almost at right angles towards the conductor. As soon as the direc- tion of the current was reversed, the directionjthe needle pointed in was reversed too. :

As seen in Fig. 9 the north end of the needle moves away from us when the current flows from left to right. Oersted

_{Current ^\ N}

^{Fig. 9. Influence of an electric current on a compass needle.}

■ •

also pointed out that provided the wire were adjusted below the needle, the deflection was reversed.

The above-mentioned phenomenon highly interested Am­pere who repeated the experiment and added a number of val­uable observations and statements. He began his research under the influence of Oersted's discovery and carried it on throughout the rest of his life.

Everyone knows the rule thanks to which we can always find the direction of the magnetic effect of the current. It is known as Ampere's rule. Ampere established and proved that magnetic effects could be produced without any magnets by means of electricity alone. He turned his attention to the behaviour of the electric current in a single straight conduc­tor and in a conductor that is formed into a coil;i.e. (that is) a solenoid.

When a wire conducting a current is formed into a coil of several turns, the amount of magnetism is greatly increased.

It is not difficult to understand that the greater the num­ber of turns of wire, the greater is the m.m.f. (that is the magnetomotive force) produced within the coil by any constant amount of current flowing through it. In addition, when doubling the current, we double the magnetism generat­ed in the coil (See Fig. 10).

A solenoid has two poles which attract and repel the poles of other magnets. While suspended, it takes up a north and a south direction exactlv like the compass needle. A

₄

₄

_{MORE MAGNETISM (TWICE AS MUCH) MORE MAGNETISM (TWICE AS MUCH)}

Fig. 10. a) The number of turns of wire in the coil affects the magnetic strength, b) The strength of the current through the coil influences the magnetic strength of the coil.

core of iron becomes strongly magnetized if placed within the solenoid while the current is flowing.

When winding a coil of wire on an iron core, we obtain an electromagnet. (That the electromagnet is a controllable and reliable magnet is perhaps known to everyone, vlt is, so to say, a temporary magnet provided by electricity. Its behaviour is very simple. The device is lifeless unless an electric current flows through the coil. However, the device comes to life provided the current flows. The iron core will act as a magnet as long as the current continues to pass along the winding.

Active Words and Expressions

to add, angle, to adjust, as soon as, coil, constant, core, deflection, electromagnet, to establish, to find out, needle, to repel, rule, straight, turn

Exercises

/. Translate the following sentences:

1. A current-carrying coil of wire which is long in compari­son with its diameter is called a solenoid. 2. The experi­ments Oersted carried on attracted Ampere's attention. 3. The electric circuit can be closed, if necessary. 4. It was Ampere who showed the difference between the current and the static charges. 5. That the unit of current is named after the famous French physicist Ampere is probably known to you. 6. When placing an iron core within a sole­noid, we obtain an electromagnet. 7. The phenomenon Oer­sted pointed at interested Ampere greatly. 8. We know that the direction of the magnetic effect of the current can be found thanks to Ampere's rule. 9. If suspended so that it can rotate freely, the solenoid points north and south when the current flows.

//. Fill in the blanks with suitable words given below: where, which, when, who, that,

1. We know ... Oersted established the relation be­tween the flow of electric current and a magnetic needle. 2. The great scientists Volta, Ampere and Yablochkov may be named among those ... have greatly contributed to elec­trical engineering. 3. The end ... the lines of force leave the coil after passing through its core will act like a north magnet­ic pole. 4. ... there is a certain connection between electric­ity and magnetism was proved by experiments. 5.... he placed the wire parallel to the needle he saw ... the needle deflect­ed. 6. A wire ... is wound in the form of a solenoid acts like a magnet as long as it is carrying a current.

///. Answer the following questions:

1. When was the voltaic cell invented? 2. What did Oerst­ed decide to establish? 3. What did he find out? 4. When did the needle deflect? 5. Who repeated Oersted's experi­ments? 6. Do you know Ampere's rule? 7. What did Ampere establish and prove? 8. When is magnetism greatly increased? 9. Is the magnetic effect produced when the charges are at rest? 10. What is an electromagnet? 11. When does the iron core act as a magnet?