Referring to the main objection in storage batteries - that storage involves a loss of some 50 per cent. - Prof. Lodge says there are many cases where the convenience of storage outweighs the evil of waste altogether, as, e. g. (1) where the power of the source would be otherwise so completely wasted that every fraction of it stored is clear gain - terrestial water-power for instance; (2) where regularity and continuity of supply are needed, while the source of power is irregular and fitful - wind and wave power; (3) where the available source is weak but continuous, while the supply is needed only for a short time at intervals-small water-power for instance, or a small steam-engine which can be used during the daytime to store up a supply of current for light at night. The following remarks on what tnay be termed the practical history of storage batteries will be useful to many, but there are not a few electricians who will ask whether, after all, Planters old form of secondary battery is not quite as useful as any of the newer and patented forms.
The first form of manufacture consisted in rolling up sheets of lead and composition with trousering to keep them separate. The difficulties found were that the coatings would not adhere, but became detached in large flakes; that the trousering got corroded through and permitted short circuiting; and that, free circulation of fluid being impossible, the acid became exhausted in some places and concentrated at others, and thus every sort of irregularity began. Now regularity or uniformity is of the most vital and fundamental importance in any form of battery. If any part of a plate is inactive, that part is better away; if any plate in a cell is inactive, it is better away; and if any cells of a battery are inactive, they are infinitely better away. The rolling or coiling up of the sheets being found awkward in practice, and liable to detach the coatings, flat plates came to be used, then perforated plates, and then cast grids; these last having such large hole space that they held enough composition, and held it securely enough, to enable the trousering or intermediate porous material to be dispensed with.
This was an evident step in advance; free circulation of the liquid became possible, and could be assisted by stirring; there was nothing to corrode except the plates themselves, and the composition, being in the cells or holes of the grid, might be reasonably expected to adhere. So far, expectation was not altogether belied. The adhesion was not perfect, it was true, and pieces of composition sometimes fell out of the holes, especially if too powerful currents were passed through the cell, but still it was much better than it had been; and if the plates were filled, properly formed, and fairly treated, the composition adhered extremely well and securely. The cir culation of the liquid was not automatically perfect either, but mechanical agitation could be readily applied; without it the acid near the bottom of the cell's tended to become more concentrated than that near the top, not by reason of gravitation undoing diffusion, which is impossible, but because during each charging fresh acid is formed, and in .great part falls to the bottom in visible streams.
Another great advantage was that some amount of inspection of the plates became possible, and experience as to the actual behaviour and appearance of the plates began to be accumulated.
The main difficulty now experienced was how to keep the plates from touching. They might be put in wooden frames, or elastic bands might be stretched round each of them, and if they would only keep fiat it was impossible they should touch unless the composition should drop out of the holes. Sometimes the composition did drop out of a hole, and bridge across the interval between two plates, but the more common and more fatal experience was that the plates would not keep straight. In a few months the positives were found to swell, and as they swelled to buckle - to buckle and twist into every variety of form, so that elastic bands, wooden frames, and every other contrivance failed altogether to prevent short-circuiting. The cause of the buckling is of course irregular and one-sided swelling, and the cause of swelling is apparently the gradual peroxidation and sulphating of the material of the bars of the lead grid, which occupy less room as metallic lead than as oxide. As the bars swell they press on the inclosed composition, occasionally driving it out, but more frequently, and with properly made and treated plates universally, distending themselves and stretching the whole medial portion of the plate.
The edge or frame of the grid is stronger than the middle bars, and is not so easily stretched; in a good and uniformly worked plate it does stretch, and an old positive plate is gome 1/4 in. bigger every way than a. new one, but if one face of the plate is a trifle more active than the other, it is very plain that the most active side will tend to become convex; and buckling once begun very easily goes on. To cure it two opposite plans have been tried: one is to leave the plates as free and unconstrained as possible, hanging free it may be from two points, thin, and with crinkled or crimped margins to allow for expansion; the other is to make them thick and strong, with plentiful ribs for stiffness, and besides to clamp them up to one another as tightly as may be, and thus in mechanical ways to resist buckling and distortion. I do not know that anyone could say for certain beforehand which of these two plans would be likely to answer best, but practice is "beginning to reply in favour of the latter, and well-braced plates of fair thickness show no unmanageable tendency to buckle. It must be remembered that no material can buckle with a force greater than that necessary to restore it to flatness, and this force in the case of lead is very moderate.