Exercises

1. Listen, read and translate the text.

2. Answer the following questions in a written form:

1. What system does the most widely used ammeter or voltmeter have? 2. In what does an ammeter differ from the voltmeter? 3. Where is a coil of fine copper wire mount­ed? 4. How is a voltmeter connected to the circuit? 5. What resistance must an ammeter have?

3. Translate the following sentences and explain the function of the infinitive:

114

1. When an ammeter or voltmeter is made and the scale is fixed it is necessary to check the scale readings, and if they are very far out a correction table must be drawn up. 2. To measure the magnitude of any phenomenon calls for the use of the effect produced by it.

4. Make up and write 10 sentences using the words given in the table:

An ammeter A voltmeter A coil of fine copper wire Coil rotation A long poin­ter

is mounted is restrained has is fastened is connected is

basically a moving-coil sys­tem. between the two poles of a permanent magnet. by a hairspring. to the coil. in series. in parallel. a device to measure electric current. a device to measure the po­tential difference. a low resistance. a high resistance.

5. Make up and write all possible questions to these sen­tences:

1. Two spiral springs, one at the top and the other at the bottom, carry the current into and out of the coil. 2. They serve also to keep the coil in position between the poles of the magnet.

6. Copy these sentences putting "mounted", "a hairspring", "circuit", "an ammeter", "a voltmeter", "across" in the blanks:

1. ... restrains the rotation of the coil which is ... be­tween the two poles of a permanent magnet. 2. ... is connected in series to the ... . 3. ... has a high resistance and it is con­nected ... .

7. Analyse conditional sentences and translate them into Russian:

115

a) 1. If I see him I shall ask him. 2. If I have time, I read. 3. If I knew I should ask him. 4. Had I seen him, 1 should have asked him. 5. Were he in the shop he would buy this book.

b) 1. Had he taken into account the properties of the substance under investigation, he would have been careful when working with it. 2. Should the oscillator burst into free oscillations, the frequency would be very unstable. 3. If one knew the dimensions of the body, he would easily calculate its volume. 4. Had it not been for a large size of this body, we should have already weighed it on our pan. 5. If a compass needle were sensitive enough, it would swing back and forth as the waves went on. 6. Should the flying feather stop, all friction would disappear.

Laboratory Exercises

8. Listen and read the following word combinations

moving-coil ammeter, the delicateness of their construc­tion, the restrained motion, the rotating armature, zero position, restoring force, a long pointer, fixed scale reading amperes, the resultant magnetic field, the resulting increase, reaching a point, just balanced, potential difference, a low resistance, additional resistance, in series.

9. Listen and repeat in pauses:

The ammeter or voltmeter has basically a moving-coil system. The principle of their operation is essentially the same as that of the electric motor. A coil of fine copper wire is mounted between two poles of permanent magnet. Its rotation is restrained by a hairspring. A long pointer is fast­ened to the coil. A fixed scale reading amperes or volts is at the end of a pointer. The ammeter is connected in series and voltmeter in parallel.

10. Listen to these questions on the above text and answer them:

1. What system does the ammeter or voltmeter have? 2. What is the principle of their operation? 3. Where is a coil of fine copper wire mounted? 4. Where is a long pointer fastened?

11. Ask questions using the model.

116

Model: A voltmeter is connected across. How is a voltmeter connected?

1. An ammeter is connected in series. 2. The rotation of a coil is restrained by a hairspring. 3. The resulting increase in force reaching a point is just balanced by the restoring force of the hairspring.

12. Listen to this short dialogue and learn it by heart:

— Are you familiar with such measuring instruments as an ammeter or voltmeter?

--Certainly, I am. For example, an ammeter is an in­strument which measures the current in electric circuits. Can you give reasons for using a voltmeter?

--Why not? If it is necessary to measure potential dif­ference a voltmeter is used. By the way, what is the main unit of p.d.?

— The volt is. As for the current, it is measured in units called amperes. Do you know how they are connected to the circuit?

—It depends on an instrument. Whenever an ammeter or voltmeter is connected to a circuit to measure electric current or p.d., the ammeter must be connected in series and voltmeter in parallel.

13. Suggested topics for oral narration:

1. Describe the moving-coil ammeter.

2. Compare and contrast the construction of the volt­meter and the ammeter.

3. Draw a diagram showing how to connect an ammeter

and voltmeter in a circuit.