Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/49

about 1190, or according to the recommendation of the maker. About five volumes of water ought to be taken to one volume of acid. After mixing, allow to cool for two or three hours. The strong acid ought to be free from arsenic, copper, and other similar impurities. The water ought to be as pure as can be obtained, distilled water being best; rain water is also good. If potable water be employed, it will generally be improved by boiling, which removes some of the lime held in solution. The impurity in ordinary drinking water is very slight; but as all cells lose by evaporation and require additions of water from time to time, there is a tendency for it to increase. The acid must not be put into the cells till everything is ready for charging. (5) A shuntwound or separately-excited dynamo being ready and running so as to give at will 2'6 or 2’7 volts per cell, the acid is run into the cells. As soon as this is done, the dynamo must be switched on and charging commenced. The positive terminal of the dynamo must be joined to the positive terminal of the battery. If necessary, the + end of the machine must be found by a trial cell made of two plain lead sheets in dilute acid. It is important also to maintain this first charging operation for a long time without a break. Twelve hours is a minimum time, twenty-four not too much. The charging is not even then complete, though a short interval is not so injurious as in the earlier stage. The full charge required varies with the cells, but in all types a full and practically continuous first charge is imperatively necessary. During the early part of this charge the density of the acid may fall; but after a time ought to increase, and finally reach the value desired for permanent working. Towards the end of the ‘ ‘formation” vigilant observation must be exercised. It is important to notice whether any cells are appreciably behind the others in voltage, density, or gassing. Such cells may be faulty, and in any case they must be charged and tended till their condition is like that of the others. They ought not to go on the discharge circuit till this is assured. The examination of the cells before passing them as ready for discharge includes :—(a) Density of acid as shown by the hydrometer, (b) Yoltage. This may be taken when charging or when idle. In the first case it ought to be from 2‘4 to 2’6 volts, according to conditions. In the second case it ought to be just over 2 volts, provided that the observation is not taken too soon after switching off the charging current. For about half an hour after that is done, the e.m.f. has a transient high value, so that, if it be desired to get the proper e.m.f. of the cell, the observation must be taken thirty minutes after the charging ceases, (c) Eye observations of the plates and the acid between them. The positive plates ought to show a rich dark brown colour, the negatives a dull slate blue, and the space between ought to be quite clear and free from anything like solid matter. All the positives ought to be alike, and similarly all the negatives. If the cells show similarity in these respects, they will probably be in good working order.

As to management, it is important to keep to certain simple rules, of which these are the chief:—(1) Never discharge below a potential difference of I'SS (or in rapid discharge, 1'8) volt. (2) Never leave the cells discharged, if it be avoidable. (3) Give the cells a special full charging once a month. (4) Make a periodic examination of each cell, determining its e.m.f., density of acid, the condition of its plates, and freedom from growth. Any incipient growth, however small, must be carefully watched. (5) If any cell shows signs of weakness, keep it off discharge till it has been brought back to full condition. See that it is free from any connexion between the plates which would cause shortcircuiting; the frame'or support which carries the plates sometimes gets covered by a conducting layer. To restore the cell, two methods can be adopted. In private installations it may be disconnected and charged by one or two cells reserved for the purpose ; or, as is preferable, it may be left in circuit, and a cell in good order put in parallel with it. This acts as a “milking” cell, not only preventing the faulty one from discharging, but keeping it supplied with a charging current till its P.c. is normal. Every battery attendant should be provided with a hygrometer and a voltmeter. The former enables him to determine from time to time the density of the acid in the cells; instruments specially constructed for the purpose are now easily procurable, and it is desirable that one be provided for every 20 or 25 cells. The voltmeter should read up to about 3 volts and be fitted with a suitable connector to enable contacts to be made quickly with any desired cell. A portable glow lamp should also be available, so that a full light can be thrown into any cell; a frosted bulb is rather better than a clear one for this purpose. He must also have some form of wooden scraper to remove any growth from the plates. The scraping must be done gently, with as little other disturbance as possible. By the ordinary operations which go on in the cell, small portions of the plates become detached. It is important that these should fall below the plates, lest they shortcircuit the cell, and therefore sufficient space ought to be left between the bottom of the plates and the floor of the cell for these “ scalings ” to accumulate without touching the plates. It is desirable that they be disturbed as little as possible till their increase seriously encroaches on the free space. It sometimes happens that brass nuts or bolts, &c., are dropped into a cell; these should be removed at once, as their partial solution would greatly endanger the negative plates. The level of the liquid must be kept above the top of the plates. It may sometimes be necessary to replenish the solution with some dilute acid, but strong acid must never be added.

The chief faults are buckling, growth, sulphating, and disintegration. Buckling of the plates generally follows excessive discharge, caused by abnormal load or by accidental shortcircuiting. At such times asymmetry in the cell is apt to make some part of the plate take much more than its share of the current. That part then expands unduly, as explained later, and curvature is produced. The only remedy is to remove the plate, and press it back into shape as gently as possible. Growth arises generally from scales from one part falling on some other ; say, on the negative. In the next charging the scale is reduced to a projecting bit of lead, which grows still further because other particles rest on it. The remedy is, gently to scrape off any incipient growth. Sulphating, the formation of a white hard surface on the active material, is due to neglect or excessive discharge. It often yields, if a small quantity of sulphate of soda be added to the liquid in the cell. Disintegration is due to local action, and there is no ultimate remedy. The end can be deferred by care in working, and by avoiding strains and excessive discharge as much as possible.

Accumulators contain only three active substances— spongy lead on the negative plate, spongy peroxide of lead on the positive plate, and dilute sulphuric acid between them. It will be shown later that Accumua fourth substance, sulphate of lead, is formed ^JposJ" on both plates during discharge, and also, though to a small extent, during repose. Spongy lead looks much like ordinary lead. Its colour is a blue gray, density probably similar to that of lead, 11'3, and its electric resistance ’0000195 ohm between opposite faces of a cubic centimetre. Sp>ongy peroxide of lead is a dark brown crystalline powder, with a density of 9’28. Its electric conductivity has been determined by J. Shields {Chemical News, 65, 87). The hydrated peroxide prepared by purely chemical methods had a specific resistance of 5’59 x 109 c.g.s. units ( = 5’59 ohms) ; but a similar hydrate, prepared electrolytically, gave slightly higher figures, 6'78 x 10!l* c.g.s., or 6’78 ohms. Both forms of this substance conduct metallically, and neither of them electrolytically. Sulphate of lead is a white powder, with a density of 6’3. Its electric conductivity is so low that it may be classed as a non-conductor. The densities of these three solids being known, it is easy to follow the changes in volume which accompany the formation of sulphate of lead on either plate. One hundred volumes of lead (on the negative plate) form 290 volumes of sulphate, and will therefore require three times as much space as before the action, while 100 volumes of peroxide (on the positive plate) form 186 volumes of sulphate, and will therefore

fill nearly twice as much space as before. These expansion figures are not only important in themselves, but also