1. A plate containing, a plate being immersed...

An accumulator has a plate containing lead and a plate containing lead peroxide, both plates being immersed in acid.

2. Achieved by connecting; determined by testing...

He learned English by reading much. The charge of the accumulator is determined by testing the specific gravity of the acid with a bulb hydrometer.

Exercises

1. Listen, read and translate the text.

2. Answer the following questions in a written form:

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

1. What accumulator is very efficient? 2. What plates does the accumulator contain? 3. When do chemical changes occur in the accumulator? 4. What does a rheostat control? 5. Why is a bulb hydrometer used?

10. Answer these questions using the model.

Model: When is an accumulator used?

If a large cur­rent is required an accumulator is used.

1. When does the density of the acid drop and the e.m.f. fall in the accumulator? 2. When does an accumulator be­come damaged? 3. When does an accumulator become re- charged?

11. Listen to this dialogue and learn it by heart:

— What is an accumulator?

— It's an electric device.

— Do you know where it is used?

— Certainly I do. It is used for supplying current to the electrical motors.

— Is it complicated in construction?

— No, it isn't. The most common form is the lead-acid accumulator. It contains two plates being immersed in sulphuric acid.

— What is used to control the amount of current passing through it?

— The rheostat is used. The current is registered by an ammeter.

— How is the charge determined?

— By testing the specific gravity of the acid with a bulb hydrometer.

— What must one do when the density of the acid drops and the e.m.f. falls?

— One must recharge the accumulator by passing electric current through it in the opposite direction.

12. Say this in English:

1. Аккумулятор — это электрический прибор. 2. Он используется для подачи тока к моторам. 3. Он содержит две пластины, помещенные в раствор серной кислоты. 4. Чтобы аккумулятор стал источником тока, его надо зарядить. 5. В аккумуляторе во время зарядки электрическая

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энергия превращается в химическую, а при работе накопленная химическая энергия снова переходит в электри­ческую. 6. При продолжительной работе плотность кислоты падает и э.д.с. снижается. 7. Элемент следует перезаря­дить.

13. Suggested topics for oral narration:

1. What distinguishes an accumulator from a simple cell? Describe one form of accumulator.

2. Describe briefly the changes which take place in a lead-acid accumulator during charging and discharging. Which is more valuable, a voltmeter or a hydrometer, to find out the state of charge of such a cell?

Additional Material

Read the text and get its central idea. Pay attention to: when the voltage differs from the e.m.f. and why; what the internal resistance depends on.

Battery Electromotive Force and Resistance

The e.m.f. of a given battery depends upon several factors and does not remain absolutely constant for all conditions of charge or discharge. A so-called "standard cell" main­tains an e.m.f. which is very nearly constant only if very little current is ever drawn from the cell. When a battery is used to supply considerable current for a time its e.m.f. does not remain constant.

The voltage between the terminals of a battery when cur­rent is flowing through it differs from the e.m.f. because of the internal voltage drop. The resistance which may be considered as causing the internal voltage drop is called the internal resistance. As far as external relations are con­cerned,¹ a battery may be considered to have its e.m.f. and its internal resistance separate. When current is being drawn from a battery, the terminal voltage is less than the e.m.f. or Vt=E—Iri, where Vt is the voltage between terminals in volts, E is the e.m.f. of the battery in volts, I is the current in amperes, and ri is the internal resistance in ohms.

When a battery is being charged the terminal voltage is greater than the e.m.f. or Vt > E + Iri

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The internal resistance of a given battery is not constant but depends upon several factors such as temperature, condition of the electrolyte, value of the current, and whether the battery is being charged or discharged. For a battery which has a rated current of 1 amp. or more the internal res­istance drop due to the current required by a commercial voltmeter is negligible. Consequently the terminal voltage, measured when the battery is supplying no current except that required by the voltmeter, may be considered to be the same as the e.m.f. of the battery.

Note

1. As far as external relations are concerned — когда это касается внешних зависимостей

UNIT 17