Page:Text-book of Electrochemistry.djvu/278

 and then quickly to about 2*5 volts. When the charging is nearly complete, persulphuric acid is fonned at the peroxide plate, and this partially decomposes into sulphuric acid and oxygen. The oxygen evolves (the accumulator " boils "), and when this occurs it shows that the accumulator is fully charged ; at the same time hydrogen ia evolved at the other plate; according to Darrieus (19\ the former plate is then permeated by persulphuric acid, and the latter has absorbed hydrogen. During the discharge the comparatively small quantities of these substances are used up, and this causes the electromotive force to sink to about 2 volts. Thereafter the principal reaction is —

Pb (spongy) + PbOa + 2H2SO4 + aj = 2PbS04 + 2H2O + aq,

which evolves, according to the measurement of J. Thomseu, 43,500 cal. for every gram-equivalent of spongy lead which is transformed. During the course of this reaction, the electro- motive force falls slowly to 19 volts, and then more quickly to 1-8 volts, provided that the discharge does not take place too fast. If the discharge is carried out very rapidly, the electromotive force, after a certain time, falls to a lower value than that mentioned, and this is probably due to a kind of polarisation, inasmuch as the chemical action cannot, so to say, keep up with the electrical process. When the voltage has been reduced to 1*8, no more current should be drawn from the cell, as this is apt to spoil it. If the discharge, however, be carried further, the electromotive force very rapidly decreases. This shows that by slowly dis- charging an accumulator more electricity (calculated in ampere-hours, at 3600 coulombs) can be obtained than when it is quickly discharged; for in the latter case the voltage more quickly reaches the value 18, i.e. after a shorter number of ampere-hours. amperes per sq. dm. of the positive plate) which an accumulator can yield determines its capacity. This is, therefore, greater for weak currents than for strong ones ; it

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