Text. The accumulator

If a large current is required for several hours for exam­ple, an accumulator or secondary cell is used. The most com­mon form is the lead-acid accumulator, which has a plate containing lead peroxide (chocolate-brown in colour) and a plate containing lead (slate-grey in colour), both plates being immersed in sulphuric acid (Fig. 7a). The lead peroxide is the positive pole; the lead is the negative pole. No depolar­izer is necessary. Further, the accumulator can easily be recharged when run-down, whereas the primary cells are of little value thereafter.

After the accumulator has been used for many hours, chemical changes occur, the density of the acid drops and the e.m.f. falls too. In this condition the accumulator will be damaged if it is used further. The cell is therefore "re­charged" by passing an electric current through it in the op­posite direction to the way in which it supplied current when

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it was used. This is achieved by connecting the positive terminal of a direct voltage supply to the positive pole of the accumulator, and the negative terminal to the negative pole (Fig. 7b). A rheostat R is essential in the circuit to control the amount of current passing through the accumulator, which is registered by an ammeter A. After a time, as determined by testing the specific gravity of the acid with a bulb hydrometer, the accumulator is recharged and is again ready for use.

The capacity of an accumulator is a measure of the electric charge it can drive around an external circuit once it has been fully charged. This should logically be measured in coulombs, but it is more usual to use a larger unit, called the ampere-hour. This is the amount of electric charge which moves when a steady current of 1 ampere flows for 1 hour.

The lead-acid accumulator is very efficient. The chief disadvantages are the high weight-to-capacity ratio, the corrosive nature of the acid, and the gradual falling away of the active material from their electrodes, limiting the life to only a few years. The chief merit is the constancy of the e.m.f. over the greater part of the discharge period. More modern forms of accumulator have a lower weight-to-capacity ratio and a longer life, but they do not show the same constancy of e.m.f. during discharge. The lead-acid type is therefore still in common use.

Listen and read words and word combinations to be remembered:

lead [led] свинец essential [i'senjal] сущест-

peroxide [pe'roksaid] перекись венный

charge [tja:dj] заряжать circuit ['so:kit] электрическая -

immerse [i'me:s] погружать цепь

value ['vaelju:] величина register ['redjista] регистрировать

drop [drop ] падать plate [pleit ] пластинка

damage ['daemidj] повреждать determine [di'te:min] onpeделять

efficient [i'fijent] эффектив- capacity [ka'paesiti] элект-

ный рическая емкость

supply [sa'plai] подавать external [eks'tə:nl] наружный

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secondary cell вторичный элемент, аккуму лятор lead-acid accumulator свинцово-кислoтый аккумулятор

chocolate-brown in colour темно-шоколадного цвета

to run down истощаться, слабеть

after a time после того как

specific gravity удельный вес

bulb hydrometer шариковый гидрометр

to drive around проводить вокруг (по)

steady current постоянный электрический ток

weight-to-capacity ratio отношение веса к электрической ем- кости

Give Russian equivalents to the following words without using a dictionary:

accumulator [a'kju:mjuleita] passing ['pa:sing]

example [ig'za:mpl] recharge [,ri'tja:dj]

containing [kan'teining] logically ['lodjikali]

positive ['pozitiv] coulomb ['ku:lom]

pole [poul] ampere ['aempea]

negative ['negativ] corrosive [ka'rousiv]

polarize ['poularaiz] active ['aektiv]

depolarizer [di'poularaiza] electrode [i'lektroud]

opposite ['opazit] limiting ['limiting]

voltage ['voultidj] constancy ['konstansi]

rheostat ['ri:oustaet] period ['piariad]

control [kan'troul] modern ['moden]

Models