Text 5. Measurement of Electric Current and Measuring Devices

Ohm, ampere, volt and watt are units of measurement of electric quantities. The ohm is a unit for measurement of resistance. The ampere is a unit of rate of an electric current flow. The ampere indicates the amount of electric current. It is equal to 3,000 million electrostatic charges per second.

The volt is defined as the potential difference across a conductor when electric current of one ampere dissipates one watt of power. It was named in honour of the Italian physicist Alessandro Volta, who invended the so-called Voltaic pile, a forerunner of the battery, which produced a steady electric current. Volta had determined, that the most effective pair of dissimilar metals to produce electricity was zinc and silver. In the 1880s, the International Electrical Congress, now the International Electrotechnical Commission (IEC), approved the volt for electromotive force or potential difference.

Nominal voltages of familiar sources:

• single-cell, rechargeable alkaline battery: 1,2 volt;

• transistor-transistor logic (TTL) power supply: 5 volts;

• automobile electrical system: 12 volts (nominal);

• household mains electricity: 230 volts in Europe, 120 volts in North America, 100 volts in Japan;

• lightning: varies greatly, often around 100 megavolts.

The watt is the SI derived unit of power, equal to one joule per second.

A human being climbing a flight of stairs is doing work at a rate of about 200 watts; a trained athlete can work at up to 900 watts for short periods. An automobile engine produces work at a rate of around 100,000 watts. A typical household incandescent light bulb uses 40 – 100 watts. The watt is named after James Watt for his contribution to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889. In an electrical circuit, the watt is a very convenient unit. A kilowatt-hour (kwh) is a unit of energy, that would be transferred at a constant rate of one kilowatt for one hour. This unit is often used in the content of power plants and home energy bills.

Electric quantities are measured by special instruments. These instruments are called measuring devices. Current is measured by an ammeter, voltage (electrical potential difference) by a voltmeter and power by a wattmeter.

Ammeters measure the current flowing in a circuit and normally have scales graduated in amperes, milliamperes or microamperes. The ammeter has a low resistance coil to avoid absorption of an appreciable amount of power.

The galvanometer is an instrument used to indicate the presence, strength and direction of very small current in a circuit.

Voltmeters measure the potential difference between two points in a circuit. The scale of a voltmeter is calibrated in volts, millivolts and microvolts. A voltmeter is connected in parallel across the points of a circuit to measure the difference of potential. The resistance of the operating coil must be as high as possible¹ to limit the amount of consumed current.

A wattmeter combines three instruments in one. It has two separate coils. One of them has comparatively high resistance, equivalent to a voltmeter coil, another one is of low resistance.

Recently there was created a device measuring three important electrical quantities: electrical current, voltage and electrical resistance. This multifunctional instrument is called multimeter.

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1. as high as possible – якомога вище

Exercise 52. Translate the following word combinations.

Multifunctional measuring device, potential difference across a conductor, electricity production, nominal voltage, household mains electricity, derived unit of power, human being, electrical circuit, constant rate, special instrument, graduated scale, operating coil.

Exercise 53. Fill in the gaps with the words given in brackets below and translate the sentences.

(Scales, watt, direct, ohm, alternating, absorption, electrostatic charges, high, amperes).

1. The … is a unit for measurement of resistance. 2. The ampere is equal to 3,000 million … per second. 3. The … is a unit of power developed by an electric device. 4. The ammeter has a low resistance coil to avoid … of an appreciable amount of power. 5. The resistance of the voltmeter operating coil must be as … as possible to limit the amount of consumed current. 6. The multimeter is designed to measure electrical quantities of both … and … current. 7. Ammeters measure the current flowing in a circuit and normally have … graduated in … .

Exercise 54. Translate the following universal words. Mind the difference in pronunciation and spelling in English and Ukrainian.

Ohm, ampere, volt, watt, electric, million, electrostatic, potential, physicist, battering, effective, pair, metal, zinc, international, congress, commission, electrotechnical, nominal, transistor, logic, automobile, second, train, athlete, period, typical, association, energy, special, instrument, voltmeter, normally, absorption, galvanometer, microvolt, microampere, kilowatt, parallel, limit, equivalent, multifunctional.

Exercise 55. Make word combinations using the words given in columns A and B. There may be more then one word combination.

A	B
electrical measuring potential appreciable low electrostatic analogue	amount difference quantity device current resistance charges

Exercise 56. Translate the following questions into English and answer them.

1. Для вимірювання якого параметру електричного струму використовується ампер? 2. Чому дорівнює 1 ампер? 3. В яких одиницях вимірюється напруга? 4. На яку напругу розраховані електричні побутові прилади в Європі? 5. Для чого слугує амперметр? 6. Чому амперметр має котушку низького опору?

7. Яким приладом вимірюється електрична потужність? 8. Чому вольтметр повинен мати великий опір? 9. На честь якого вченого назвали одиницю потужності? 10. Як називається одиниця електричного опору? 11. В яких одиницях вимірюється сила струму?

Exercise 57. Complete the sentences by putting the verbs in brackets into correct Active or Passive tense forms.

1. A kilowatt-hour is a unit of energy that (transfer) at a constant rate of one kilowatt per one hour. 2. Alessandro Volta (develop) the so-called Voltaic pile, which (produce) a steady electric current. 3. The ammeter (use) to measure the flow of current through a conductor. 4. The scale (calibrate) to read milliamperes or microamperes, depending on the magnitude of the current. 5. The measuring instrument (connect) in parallel. 6. We may (connect) a number of cells in series to produce a large voltage. 7. Electric current (flow) only through a closed circuit. 8. If we (open) a circuit anywhere the electric current (stop) flowing. 9. The generator (create) the electromotive force. 10. Mechanical energy (convert) into electrical energy by the generator.

Exercise 58. Make the following sentences interrogative.

1. A voltmeter is connected in parallel across the points of a circuit to measure the difference of potential. 2. Ammeters measure the current flowing in a circuit. 3. A wattmeter combines three instruments in one. 4. Modern digital multimeters measure the main electrical quantities, temperature and frequency. 5. A number of factors will determine the resistance. 6. The ammeter had a low resistance coil.

UNIT II. ELECTRICITY TRANSMISSION.