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30 throw light on the well-known tendency of the negative plate to run down before the positive. In a given discharge the quantities of sulphate of lead formed on the two lead plates are the same, but the expansion in the lead plugs (and consequent reduction of porosity) is 60 per cent, greater than in the positive plugs. Sulphuric acid is used in the dilute state. The usual density employed is 1*210, which has an electric conductivity of 0*73 c.g.s. units, or a specific resistance of 1*37 ohm for a centimetre cube. The relative conductivity (the reciprocal of resistance) is shown in Fig. 10. From this and the datum just given, the specific resistance of any strength of acid can be found approximately.

There are three kinds of local action :—(1) In all the practical forms of accumulator the positive plate contains two conductors in contact—the lead grid or plate and the peroxide it is intended to supLocal port. There exists between them a difference action of potential ( = 2 volts), and, as the acid touches both, local currents must flow. The course of these will be from a point in the positive grid to the acid, thence to the adjacent peroxide, and back through it to the grid beneath. Such currents involve the formation of sulphate from both the grid and the peroxide, with two definite results : {a) a diminution of the available energy existing as peroxide, and (&) an attack on the lead - supporting surface below. In an early form of accumulator, Gladstone and Tribe found 7 per cent, of sulphate was formed in two hours. Fortunately the sulphate of lead thus formed is insoluble, and covers the free lead surface. Consequently the action becomes very slow after the first hour or so, and would in fact stop altogether if the film of sulphate remained intact, and so prevented access of acid to the lead surface. But in the daily changes of temperature, &c., the strains are great enough to cause some conducting communication, and the local action goes on. This progressive action explains the advantage Plante derived from periods of repose in forming, since it brings a greater quantity of the lead within the reach of the next operation. It also explains the danger which waits on the positive grid or plate, and ultimately leads to its destruction. (2) Local action will arise on the negative plate if a more electro-negative conductor settle on the lead. This sometimes happens because of impurities in the materials used for the paste or existing in the acid. In either case they will find their way to the negative grid or support, and lead to loss of energy and the evolution of hydrogen gas. (Swinburne, Journ. Inst. Elec. Eng. 1886.) Even where impurities do not exist in the materials, they are often introduced by small bits of metal being carelessly dropped into the cell. A not unusual habit of engineers is to scrape copper connecting-wires in close proximity to the cells, thus endangering their capacity and life very considerably. Besides this chance of local action on the negative plate, acid of density 1*210 acts directly on the finely - divided lead. Swinburne also drew attention to this in 1886, and indicated its effect on the capacity of the cells, which may be seriously reduced. (3) There is a local action on each of the plates, whenever the acid in one part of the cell differs in density from that in another. This often happens. There is a constant tendency for the acid to get stronger at the bottom of the cell, because during charge the stronger acid brought to the positive plate tends to fall, and in discharge the weaker acid diffusing from the porous plugs of both plates tends to rise. Further still, during charge the acid is strongest in the inner parts of the spongy material, and differences of 25 to 30 per cent, may exist between the inner and outer parts of a plug. A similar difference may exist during discharge, but in this case the acid is strongest on the outside and weakest in the interior. The following table gives the e.m.f. between a plate of peroxide in weak and another in strong acid 3 corresponding values are also given for lead plates in acid of different strength (Gladstone and Hibbert, Journ. Inst. Elec. Eng. 1892):—

The lead in the weaker acid is + to the other; with the peroxide plates the case is reversed. With these figures it is easy to picture the local currents flowing between an inner and an outer part of a single plug during the discharge, owing to the different strength of acid then existing. *

The importance of these three kinds of local action arises not so much from the magnitude of the actions they set up as from the special parts of the plates affected by them. Sulphate of lead is formed in each case, but not in the course or path of the regular current. On the positive plate the local action forms sulphate along the interface between the plugs and the grid, and may effectively isolate a large proportion of the peroxide. On the negative plate, when due to deposited copper, <kc., it cannot be so definitely followed, but may even give an outside surface coating of sulphate. That arising on both plates by reason of variable acid strength in the

plugs cannot be localized, but must necessarily be out of the regular lines of flow of the working currents. It is partly by virtue of these special localizations of the chemical changes that an accumulator seems to be seriously damaged by a rest, whether charged or discharged, and it is a further consequence that subsequent charges only slowly get at this deep-seated sulphate, so that many cycles of work are required to bring back the cell to its earlier value as a working machine. The electromotive force of a cell varies with the strength of the acid, as may be seen from Fig. 11, taken Electrofrom Gladstone and Hibbert’s paper. The motive observations with very strong acid were very force. difficult to obtain; but one good experiment made with