262 OXIDATION AND REDUCTION ELEMENTS, chap.
behaves similarly, and may be used to fill the rills of the lead plates. The cuts are uow made so deep that the lead plate has the appearance of a sort of framework (or grid), with the spaces filled with the mixture described. These plates are " formed " against an ordinary plate. An objection to such plates is that the substance of the electrode does not hold together well, and cannot withstand high current strengths. The passage of the current is accompanied by chemical processes, which take place with change of volume ; if these occur quickly, pieces of the filling material break off from the plates, and fall to the bottom of the containing vessel. [In the Tudor process the positive grid is constructed by pickling it in a bath of sulphuric acid containing nitric acid, and then obtaining the stratum of lead peroxide by electro- lysis, using an ordinaiy plate as cathode. The negative plate is an open grid, pasted with litharge, but not reduced before leaving the factory.]
The liquid in the accumulator must be free from certain impurities, such as chlorine, nitrates, and foreign metals, for if these are present the lead plates are violently attacked ; [the presence of arsenic is particularly detrimental]. If foreign metals are present, local currents are set up, provided that the metal deposited on the lead plate during the charging is more negative (as, e.g., copper) than lead. It is particularly important that the water used in filling up the vessel to replace that lost by evaporation should be entirely free from chlorides and nitrates.
In charging an accumidator, a certain current density, which depends on the kind of plates, should not be exceeded. Formerly, 06 ampere per square decimetre was recommended. According to more recent experience, the current density may safely be raised to 1 amp./sq. dm., and, with the best plates (prepared by the Plants method), it may even be as high as 2*6 amp./sq. dm. In discharging, too, the current density must be maintained within the same limits. At the beginning of the charging, the back electromotive force (the pole tension) has a value of about 207 volts, which rises slowly to 220 volts,
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