26+ OXIDATION AND REDUCTION ELEMENTS, chap.
amounts, for instance, to 140 ampere-hours when the discharge takes 10 hours, and only to 100 ampere-hours when the ceU is discharged iu 3 hours. A discharged element should not l>e kept in this condition for any length of time, because the lead sulphate formed easily sets to a hard mass, which can only with difl&culty be transformed during charging.
It is of interest to learn what is the economic value or the so-called efiSciency of an accumulator. This can be judged, on the one hand, by the number of ampere-hours which the element can give compared with the number lequired for chaining it. On the other hand, the eflBciency may be determined by the quantity of energy, generally estimated in watt-hours at 3600 coulombs, which the element can yield compared with that absorbed by it during charging. According to the former method, the efficiency amounts to from 82 to 94 per cent., whilst by the latter method it is from 75 to 83 per cent, [and is frequently as low a3 60 per cent.], depending on the current density. If an accumulator be left for some time unused, it spontaneously loses part of its charge, i.e. its efficiency diminishes.
By means of Thomson's rule we calculate that the electro- motive force of an accumulator is ||3?8 = 1886 volts. On the basis of Helmholtz's theorem the temperature coefficient of the electromotive force must/ be taken into account ; this amounts, however, to only 2 to 4 millionths per degree, and so the con^ction dees hot exceed 0*001 volt. Now, the electromotive force of an accumulator during the principal reaction is, on the average, 1*9 volts, i.e. slightly higher than the calculated value, and even more so at the beginning of the principal reaction when the electromotive force may be 08 high as 2 volts. The reason for this difference is that the sulphuric acid of the element is more concentrated than was assumed by Thomsen in his calculation. Streintz (JO) found that the electromotive force of an accumulator at the beginning of the principal process is given by the formula —
£ = 1-850 + 0-917 (S - 1),
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