Page:Text-book of Electrochemistry.djvu/280

 XV.

��ACCUMULATORS.

���FiQ. 48.

��where S is the specific gravity of the sulphuric acid used. In practice, sulphuric acid of 20 to 24 per cent, with specific gravity varying from 1'144 to 1'173 is employed. [In the Tudor cell acid of specific gravity 1*20 is used.]

It is easy to see why the electromotive force of an accumulator increases with the concentration of the sulphuric acid. Suppose we have two ac- cumulators, A and B, filled with 25 per cent, and 15 per cent, sulphuric acid respectively, and suppose that they are so connected that their electromotive forces are opposed to each other, as shown in Fig. 48. Just as the concentra-'

tion of a layer of 25 per cent, sulphuric acid tends by diffusion to come into equilibrium with a layer of 15 per cent, acid with which it is in contact, so in the system depicted a current arises which tends to establish the same equilibrium, i.e. the common concentration of 20 per cent. Now, since on the discharge of an accumulator water is formed and sulphuric acid disappears, whilst the opposite reaction takes place during charging, the accumulator A must discharge in order to establish the equilibrium, and at the same time this, charges B. It can easily be found thermo- dynamicaUy how much work can be gained by transferring 18 grams of water from B to A, and 98 grams of sulphuric acid from A to B (see p. 75). Dolezalek (f?i) has ascer- tained in this way the electromotive force of the above combination, and from this calculated how the electromotive force of an accumulator changes with varying concentration of the sulphuric acid. The calculation is in good agreement with the result found by Sti-eintz,

On account of the many advantages possessed by accu- mulatoi-s, they have almost entirely replaced in practice all the older galvanic elements, with the exception of those of Leclanche [and of Uaniell], which are more suitable for cases when only a weak current is required, and that for only a

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