. A text book of physics, for the use of students of science and engineering . eated for a number oftimes it will be found that the storing capacity of the cell becomesgreater. This is generally carried out not merely by letting the celldischarge, but by reversing the current in it until the kathode isieduced to lead. This reduced lead forms a soft layer upon theplate, known as spongy lead, and with repeated reversals of currentthis spongy lead forms a deeper and deeper layer, and the storagecapacity of the cell increases. This process is called forming theplate. LXX SECONDARY CELLS 917 Chargi
. A text book of physics, for the use of students of science and engineering . eated for a number oftimes it will be found that the storing capacity of the cell becomesgreater. This is generally carried out not merely by letting the celldischarge, but by reversing the current in it until the kathode isieduced to lead. This reduced lead forms a soft layer upon theplate, known as spongy lead, and with repeated reversals of currentthis spongy lead forms a deeper and deeper layer, and the storagecapacity of the cell increases. This process is called forming theplate. LXX SECONDARY CELLS 917 Charging. When the plates have been properly formed the processes ofcharging and discharging may be represented approximately by thefollowing equations : Positive + S04 + 2H20 = Pb02 + 2H,S04. Negative + H2 = Pb + H2S04. Positive + H2S04 + H2 = PbS04 + 2H20. Negative + H2S04 + 0 = PbS04 + H20. It will be seen that during the process of charging, sulphuric acidis formed, and consequently the density of the electrolyte rises. Fig. 8f>:i. Secondary cells. During discharge, sulphuric acid disappears and the density of falls. Observation of the density of the acid givesbhe best indication of the condition of the cell. It should notexceed 1-21 when the cell is fully charged, nor fall below 1-15 when>.t is discharged. 918 MAGNETISM AND ELECTRICITY chap. The secondary cell of this type has a remarkably constant electro-motive force of about 2-1 volts throughout the greater period of itsdischarge. It is usually constructed of a series of parallel lead plates,alternate plates being connected to one of two lead strips at the topwhich form the leads for the current (Fig. 853). Owing to the highconductivity of the electrolyte, and the large area of the plates andtheir closeness together, the internal resistance is very small, andhence considerable currents can be obtained from them. For a100 volt supply circuit a batte
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