The steel case is closed by a cover of the same material, which is welded all around so as to be perfectly air tight. This feature is excludes all atmospheric air with its contamina tion of carbonic acid.
The Edison cell of to-day is a very highly developed piece of electrochemical apparatus, hearing ample witness to the genius of its inventor. The fact of its being built of nickel plated steel throughout gives it a ruggedness and a finished appearance which can hardly be matched by other types. If operated under favorable conditions it will last for a long time, often upwards of 1,000 cycles of charge and discharge.
It possesses on the other hand certain ap parently inherent properties which very much limit its application, and which absolutely ex clude it from many of the fields to which storage batteries arc applied; under the heading of characteristics these points will be more fully brought out.
Electro-Chemical Equations of the Lead Sulphuric Acid Battery.— The oldest and most generally used type of storage battery employs as its active materials peroxide of lead for the positive plate, spongy metallic lead for the negative, and fnr the electrolyte an aqueous solution of sulphuric acid, of about 25 to 40 per cent A cell of this kind is diagrammatically shov n in Fig. 5, in which A and B are the positive and negative elates, respectively, each with a coating or rake, C, of active material, held in contact with it, while both plates are immersed in the sulphuric acid electrolyte, held in a suitable containing jar.
Between sulphuric acid and metallic sponge lead there exists a strong chemical affinity, which constitutes the basic operating force of the lead cell.
So soon as the internal reactions are per mitted to talce place, by the closing of the external circuit, the sulphuric acid, }LSO., be gins to split up, SO, going to the negative plate, there combining with the sponge lead to form lead sulphate, PbSO4, and in so doing liberating at the negative plate a quantity of minute nega tive charges carried by the individual SO4 °ions° (as these disengaged groups are termed); and at the same time the hydrogen, Rh pro ceeds to the positive plate, there combining with one part of the oxygen of the lead fer oxide to form water, and giving up an equiva lent quantity of minute positive charges, carried on the individual H °ions."' Simultaneously the
Pb0, which results from this action, unites with another part of' H2SO4 to form lead sulphate and a second molecule of water.
The reaction may be represented as follows: Pb+21-1,SO4±Pb0,=---PbSO4+2H,0-1-PbSa.
The discharge thus results in the formation of lead sulphate at both plates, the disappear ance of sulphuric acid, and the formation of water at the positive. It may be said, in brief, that these reactions are held in checic by the fact of the charge upon the negative plate acting to repel the negatively charged SO. ions; while the charge upon the positive plate in like manner repels the positively charged H ions. Immediately the respective charges upon the two plates are lessened by closing the ex ternal circuit (i.e., the potential difference be tween them is reduced), the equilibrium between electrical and chemical forces ceases and the ions rush to the respective plates and discharge their minute unit charges, thereby maintaining the potential difference of the cell nearly constant.
But if there be applied to the cell terminals a potential difference slightly greater than that of equilibrium, the chemical forces are over balanced by the electrical, with the result that these various internal movements are reversed, so that the original condition of the °working parte) is restored, as indicated in Fig. 5.
During charge there are thus restored to the electrolyte the two parts of H,SO4, ab sorbed by the plates in discharge, and simul taneously there are removed the two parts of water given off in discharge; at all times there lemains a surplus of H,SO4 in solution.
It is to be noted that in charge these re actions are quite complete, so that the active materials are entirely converted into metallic lead and peroxide of lead, respectively; but that on discharge they are never in practice carried to the theoretical limit, and a large proportion, usually above 60 per cent, of the metallic lead and lead peroxide remains even after a cell is completely discharged. The fully charged condition, however, is not accomplished with 100 per cent efficiency; the charge (in ampere hours) must exceed the discharge by 10-15 per cent, the loss manifesting itself by the evolution of hydrogen and oxygen.