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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1903 Excerpt: ...surface be as great as possible. Now, the working surface of a plate of any particular size may be increased by providing it with very many ridges and cavities such as shown in Fig. 157. The chemical processes which take place in the accumulator are by no means as simple as we have hitherto assumed them to be. We have supposed that the water only in the cell is decomposed, and that the acid serves merely for the purpose of improving the conductivity of the water. The sulphuric acid really has an influence on the chemical process, and it has been found that the proportion of the quantity of water to that of the acid is of great importance. The complete chemical changes which take place during the charging and discharging of an accumulator are too complicated to be described here. The main facts are these:--During charging, spongy lead is formed at one plate which is termed by cell makers the negative plate; whilst at the other plate, called the positive., a dark red oxide called lead peroxide is formed. On discharging the cell both the lead and lead peroxide are changed to lead sulphate. Under normal conditions an accumulator should never be discharged so that its E.M.F. falls lower than 1.80 to 1.83 volts. For the purpose of charging, a higher pressure is necessary. Soon after the beginning of the charge the E.M.F. of a cell rises to two volts, and when completely charged becomes 2.5 to 2.7 volts. When gas is evolved from both plates of the accumulator, the end of the charge is indicated, and that no more oxygen is being absorbed by the positive plate. The rising of the voltage during charging is firstly due to the chemical change of the plates, and then for two other reasons. There are formed bubbles of hydrogen and oxygen, which increase the back E.M.F. o...
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