Accumulator - Encyclopedia

GEOGRAPHICAL NAMES

ACCUMULATOR, the term applied to a number of devices whose function is to store energy in one form or another, as, for example, the hydraulic accumulator of Lord Armstrong (see Hydraulics, � 179). In the present article the term is restricted to its use in electro-technology, in which it describes a special type of battery. The ordinary voltaic cell is made by bringing together certain chemicals, whose reaction maintains the electric currents taken from the cell. When exhausted, such cells can be restored by replacing the spent materials, by a fresh " charge " of the original substances. But in some cases it is not necessary to get rid of the spent materials, because they can be brought back to their original state by forcing a reverse current through the cell. The reverse current reverses the chemical action and re-establishes the original conditions, thus enabling the cell to repeat its electrical work. Cells which can thus be " re-charged " by the action of a reverse current are called accumulators because they " accumulate " the chemical work of an electric current. An accumulator is also known as a " reversible battery," " storage battery " or " secondary battery." The last name dates from the early days of electrolysis. When a liquid like sulphuric acid was electrolysed for a moment with the aid of platinum electrodes, it was found that the electrodes could themselves produce a current when detached from the primary battery. Such a current was attributed to an " electric polarization " of the electrodes, and was regarded as having a secondary nature, the implication being that the phenomenon was almost equivalent to a storage of electricity. It is now known that the platinum electrodes stored, not electricity, but the products of electro-chemical decomposition. Hence if the two names, secondary and storage cells, are used, they are liable to be misunderstood unless the interpretation now put on them be kept in mind. " Reversible battery " is an excellent name for accumulators.

Sir W. R. Grove first used " polarization " effects in his gas battery, but R. L. G. Plante (1834-1889) laid the foundation of modern methods. That he was clear as to the function of an accumulator is obvious from his declaration that the leadsulphuric acid cell could retain its charge for a long time, and had the power d'emmagasiner ainsi le travail chimique de la pile voltaique: a phrase whose accuracy could not be excelled. Plante began his work on electrolytic polarization in 1859, his object being to investigate the conditions under which its maximum effects can be produced. He found that the greatest storage and the most useful electric effects were obtained by using lead plates in dilute sulphuric acid. After some " forming " operations described below, he obtained a cell having a high electromotive force, a low resistance, a large capacity and almost perfect freedom from polarization.

The practical value of the lead-peroxide-sulphuric-acid cell arises largely from the fact that not only are the active materials (lead and lead peroxide, Pb02) insoluble in the dilute acid, but that the sulphate of lead formed from them in the course of discharge is also insoluble. Consequently, it remains fixed in the place where it is formed; and on the passage of the charging current, the original PbO 2 and lead are reproduced in the places they originally occupied. Thus there is no material change in the distribution of masses of active material. Lastly, the active materials are in a porous, spongy condition, so that the acid is within reach of all parts of them.

Plante carefully studied the changes which occur in the formation, charge and discharge of the cell. In forming, he placed two sheets of lead in sulphuric acid, separating them by narrow strips Pa of caoutchouc (fig. 1). When a charging current is sent through the cell, the hydrogen liberated at one plate escapes, a small quantity possibly being spent in reducing the sur face film of oxide generally found on lead. Some of the oxygen is always fixed on the other (positive) plate, forming a surface film of peroxide. After a few minutes the current is reversed so that the first plate is peroxidized, and the peroxide previously formed on the second plate is reduced to metallic lead in a spongy state. By repeated reversals, the surface of each plate is alternately peroxidized and reduced to metallic lead. In successive oxidations, the action pene trates farther into the plate, furnishing each time a larger quantity of spongy PbO 2 on one plate and of spongy lead on the other. It follows that the duration of the successive charging currents also increases. At the beginning, a few minutes suffice; at the,' end, many hours are required.

After the first six or eight cycles, Plante allowed a period of repose before reversing. He claimed that the PbO 2 formed by reversal after repose was more strongly adherent, and also more crystalline than if no repose were allowed. The following figures show the relative amounts of oxygen absorbed by a given plate in successive charges (between one charge and the next the plate stood in repose for the time stated, then was reduced, and again charged as anode): - and so on for many days (Gladstone and Tribe, Chemistry of Secondary Batteries). Seeing that each plate is in turn oxidized and then reduced, it is evident that the spongy lead will increase at the same rate on the other plate of the cell. The process of " forming " thus briefly described was not continued indefinitely, but only till a fair proportion of the thickness of the plates was converted into the spongy material, Pb02 and Pb respectively. After this, for want of reversal was not permitted, the cell being put into use and always charged in a given direction. If the process of forming by fll reversal ff be continued, the positive plate is ultimately all converted into PbO 2 and falls to pieces.

Plante made excellent cells by this method, yet three objections were urged against them. They required too much time to " form "; the spongy masses (PbO 2 more especally) e ochanical support, and the separating strips of caoutchouc were not likely to have a long life. The first advance was made by C. A. Faure (1881), who greatly short ened the time required for " forming " by giving the plates a preliminary coating of red lead, whereby the slow process of biting into the metal was avoided. At the first charging, the red lead on the + electrode is changed to Pb02, while �,,h that on the - electrode is reduced to spongy lead. Thus one continuous operation, lasting perhaps sixty hours, takes the place of many reversals, which, with periods of repose, last as much as three months. ?;' Faure used felt as a sepa rating membrane but its "' 1;11 tiluauy?,???i i :;r / use was soon abolished by FIG. 2. - Tudor positive plate. methods of construction due to E. Volckmar, J. S. Sellon, J. W. Swan and others. These inventors put the paste not on to plates of lead, but into the holes of a grid, which, when carefully designed, affords good mechanical support to the spongy masses, and does away with the necessity for felt, &c. They are more satisfactory, however, as supporters of spongy lead than of the peroxide, since at the point of contact in the latter case the acid gives rise to a local action, which slowly destroys the grid. Disintegration follows sooner or later, though the best makers are able to defer the failure for a fairly long time. Efforts have been made by A. Tribe, D. G. Fitzgerald and others to dispense with a supporting grid for the positive plate, but these attempts have not yet been successful enough to enable them to compete with the other forms.

For many years the battle between the " Plante " type and FIG. I.

, the Faure or " pasted " type has been one in which the issue was doubtful, but the general tendency is towards a mixed type at the present time. There are many good cells, the value of all resting on the care exercised during the manufacture and also in the choice of pure materials. Increasing emphasis is laid on the purity of the water used to replace that lost by evaporation, distilled water generally being specified. The following descriptions will give a good idea of modern practice.

The " chloride cell " has a Plante positive with a pasted negative. For the positive a lead casting is made, about 0�4 inch thick pierced by a number of circular holes about half an inch in diameter. Into each of these holes is thrust a roll or rosette of lead ribbon, which has been cut to the right breadth (equal to the thickness of the plate), then ribbed or gimped, and finally coiled into a rosette. The rosettes have sufficient spring to fix themselves in the holes of the lead plate, but are keyed in position by a hydraulic press. The plates are then " formed " by passing a current for a long time. In a later pattern a kind of discontinuous longitudinal rib is put in the ribbon, and increases the capacity and life by strengthening the mass without interfering with the diffusion of acid. The negative plate was formerly obtained by reducing pastilles of lead chloride, but by a later mode of construction it is made by casting a grid with thin vertical ribs, connected horizontally by small bars of triangular section. The bars on the two faces are " staggered, " that is, those on one face are not opposite those on the other. The grid is pasted with a lead oxide paste and afterwards reduced; this is known as the " exide " negative.

The larger sizes of negative plate are of a " box " type, formed by riveting together two grids and filling the intervening space with paste. A feature of the " chloride " cells is the use of separators made of thin sheets of specially prepared wood. These prevent short circuits arising from scales of active material or from the formation of " trees " of lead which sometimes grow across in certain forms of battery.

The Tudor cell has positives formed of lead plates cast in one piece with a large surface of thin vertical ribs, intersected at intervals by horizontal ribs to give the plates strength to withstand buckling in both directions (fig. 2). The thickness of the plates is about 0.4 inch, and the developed surface is about eight times that of a smooth plate of the same size. A thoroughly adherent and homogeneous coating of peroxide of lead is formed on this large surface by an improved Plante process. The negative plate (fig. 3) is composed of two grids riveted together to form a shallow box; the outer surfaces are smooth sheets pierced with many small holes. The space between them ' 'is intersected by ribs and pasted (before riveting).

Many of the E.P.S. cells, made by the Electrical Power Storage Company, are of the Faure or pasted type, but the Plante formation is used for the positives of two kinds of cell. The paste for the positive plates is a mixture of red lead with sulphuric acid; for the negative plates, litharge is substituted for red lead. Figs. 4 and FIG. 7.

5 roughly represent the grids employed for the negative and positive plates respectively of a type used for lighting. Fig. 6 is the cross section of the casting used for the Plante positive of the larger cells for rapid discharge. Finer indentations on the side expose a large surface. Fig. 7 shows a complete cell.

The Hart cell, as used for lighting, is a combination of the Plante and Faure (pasted) types. The plates hang by side lugs on glass slats, and are separated by three rows of glass tubes inch diameter (fig. 8). The tubes rest in grooved teak wood blocks placed at the bottom of the glass boxes. The blocks also serve as base for a skeleton framework of the same material which surrounds and supports the section. Of course the wood has to be specially treated to withstand the acid.

A special non-corrosive terminal is used. A coned bolt draws the lug ends of adjacent cells together, fitting in a corresponding tapered hole in the lugs, and thus increasing the contact area. The positive and negative tapers being different, a cell cannot be connected u j) in the wrong way.

In America, in addition to some of the cells already described, there are types wslich are not found in England. Two may be described. The Gould cell is of the Plante type. A special effort is made to reduce local and other deleterious action by starting with perfectly homogeneous plates. They are formed from sheet lead blanks by suitable machines, which gradually raise the surface into a series of ribs and grooves. The sides and middle of the blank are left untouched and amply suffice to distribute the current over the surface of the plate. The grooves are very fine, and when the active material is formed in them by electro-chemical action, they hold it very securely.

The Hatch cell has its positive enclosed in an envelope. A very shallow porous tray (made of kaolin and silica) is filled with Chloride cell.  ?. .

FIG. 3. - Tudor negative plate.

FIG. 8. - Hart Accumulator.

red lead paste, an electrode of rolled sheet lead is placed on its surface, and over this again is placed a second porous tray filled with paste. The whole then looks like a thin earthen ware box with the lug of the electrode projecting from one end. The negatives consist of sheet lead covered by active material. On assembling the plates, each negative is held between two positive " boxes," the outsides of which have projecting vertical ribs. These press against the active material on the negative plates, and help to keep it in position. At the same time, the clearance between the ribs allows room for acid to circulate freely between the negative plate and the outer face of the positive envelope. Diffusion of the acid through this envelope is easy, as it is very porous and not more than -- inch thick.

Traction Cells. - Attempts to run tramcars by accumulators have practically all failed, but traction cells are employed for electric broughams and light vehicles for use in towns. There are no large deviations in manufacture except those imposed by limited space, weight and vibration. The plates are generally thinner and placed closer together. The Plante positive is not used so much as in lighting types. The acid is generally a little stronger in order to get a higher electromotive force (E.M.F.). To prevent the active material from being shaken out of the grids, corrugated and perforated ebonite separators are placed between the plates. The " chloride " traction cell uses a special variety of wood separator: the " exide " type of plate is used for both positive and negative. Cells are now made to run 3000 or more miles before becoming useless. The specific output can be made as high as 10 or 11 watt-hours per pound of cell, but this involves a chance of shorter life. The average working requirement for heavy vehicles is about 50 watt-hours per 1000 lb per mile.

Ignition Cells for motor cars are made on the same lines as traction cells, though of smaller capacity. As a rule two cells are put up in ebonite or celluloid boxes and joined in series so as to give a 4-volt battery, the pressure for which sparking coils are generally designed. The capacity ranges from 20 to ioo amperehours, and the current for a single cylinder engine will average one to one and a half amperes during the running intervals.

The tendency in stationary cells is to allow plenty of space below the plates, so that any active material which falls from the plates may collect there without risk of short-circuit, &c. More space is allowed between the plates, which means that (a) there is more acid within reach, and (b) a slight buckling is not so dangerous, and indeed is not so likely to occur. The plates are now generally made thicker than formerly, so as to secure greater mechanical rigidity. At the same time, the manufacturers aim at getting the active materials in as porous a state as possible.

The figures with regard to specific output are difficult to classify. It would be most interesting to give the data in the form of watt-hours per pound of active material, and then to compare them with the theoretical values, but such figures are impossible in the nature of the case except in very special instances. For many purposes, long life and trustworthiness are more important than specific output. Except in the case of traction cells, therefore, the makers have not striven to reduce weight to its lowest values. Table I. shows roughly the weight of given types of cells for a given output in ampere hours.

TABLE I.

These figures are true only at ordinary temperatures. In winter the capacity is diminished, in summer it is increased. The differences are due partly to change of liquid resistance but more especially to the difference in the rate at which acid can diffuse into or out of the pores: obviously this is greater at higher temperatures. The increase in capacity on warming is appreciable, and may amount to as much as 3% per degree centigrade (Gladstone and Hibbert, Journ. Inst. Elec. Eng. xxi. 441; Heim, Electrician, Nov. 1901, p. 55; Liagre, L'Eclairage slectrique, 1901,xxix. 150). Notwithstanding these results, it is not advisable to warm accumulators appreciably. At higher temperatures, local action is greatly increased and deterioration becomes more rapid. It is well, however, to avoid low winter temperatures.

Whatever the type of cell may be, it is important to attend to the following working requirements: - (1) The cells must be fully equal to the maximum demand, both in discharge rate and capacity. (2) All the cells in one series ought to be equal in discharge rate and capacity. This involves similarity of treatment. (3) The cells are erected on strong wooden stands. Where floor space is too expensive, they can be erected in tiers; but, if possible, this should be avoided. They ought to lie in rows, so arranged that it is easy to get to one side (at least) of every cell, for examination and testing, and if need be to detach and remove it or its plates. Where a second tier is placed over the first, sufficient clearance space must be allowed for the plates to be lifted out of the lower boxes. The cells are insulated by supporting them on glass or mushroom-shaped oil insulators. If the containing vessels are made of glass, it is desirable to put them in wooden trays which distribute the weight between the vessel and insulators. To prevent acid spray from filling the air of the room, a glass plate is arranged over each cell. The positive and negative sections are fixed in position with insulating forks or tubes, and the positive terminal of one cell is joined to the negative of the next by burning or bolting. If the latter method is adopted, the surfaces ought to be very clean and well pressed home. The joint ought to be covered by vaseline or varnish. When this has been done, examination ought to be made of each cell to see that the plates are evenly spaced, that the separators (glass tubes or ebonite forks between the plates) are in position and vertical, and that there are no scales or other adventitious matter connecting the plates. The floor of the cell ought to be quite clear; if anything lies there it must be removed. (4) To mix the solution a gentle stream of sulphuric acid must be poured into the water (not the other way, lest too great heating cause an accident). It is necessary to stir the whole as the mixing proceeds and to arrange that the density is 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 shunt-wound 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) Voltage. 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: - (i) Never discharge below a potential difference of I. 85 (or in rapid discharge, i. 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 short-circuiting; 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 potential difference (P.D.) is normal. Every battery attendant should be provided with a hydrometer 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 short-circuit 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. Experience shows the advisability of using distilled water for this purpose. 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 short-circuiting. 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 in Repose. - Accumulators contain only three active substances - spongy lead on the negative plate, spongy lead peroxide on the positive, and dilute sulphuric acid between TABLE II.

them. Sulphate of lead is formed on both plates during discharge and brought back to lead and lead peroxide again during charge, and there is a consequent change in the strength of acid during every cycle. The chief properties of these substances are shown in Table II.

The curve in fig. 9 shows the relative conductivity (reciprocal of resistance) of all the strengths of sulphuric acid solutions, and by its aid and the figures in the preceding moo table, the specific resistance of any given strength can be determined.

The lead accumulator is subject to three kinds of local action. First and chiefly, local action on 5eoo the positive plate, because of the contact between lead peroxide and the lead grid which supports it. Iwo In carelessly made or roughly handled cells this may be a very serious matter. It would be so in all circumstances if the lead sulphate formed on the exposed lead grid did not act as a covering for it. It explains why Plante found "repose" a useful help in "forming," and also why positive plates slowly disintegrate; the lead support is gradually eaten through. Secondly, local action on the negative plate when a more electro-negative metal settles on the lead. This often arises when the original paste or acid contains metallic impurities. Similar impurity is also introduced by scraping copper wire, &c., near a battery. Thirdly, local action due to the acid varying in strength in different parts of a plate. This may arise on either plate and is set up because two specimens of either the same lead or the same peroxide give an E.M.F. when placed in acids of different strengths. J. H. Gladstone and W. Hibbert found that the E.M.F. depends on the difference of strength. With two lead plates, a maximum of about quarter volt was obtained, the lead in the weaker acid being positive. With two peroxide plates the maximum voltage was about o�64, the plate in stronger acid being positive to that in weaker. The electromotive force of a cell depends chiefly on the strength of the acid, as may be seen from fig. io taken from Gladstone and Hibbert's paper (Journ. Inst. Elec. Eng., 1892). The observations with very strong acid were difficult to obtain, though even that with 98% acid marked X is believed to be trustworthy. C. Heim (Elek. Zeit, 1889), F. Streintz (Ann. Phys. Chem. xlvi. p. 449) and F. Dolezalek (Theory of Lead Accumulators, p. 55) have also given tables.

It is only necessary to add to these results the facts illustrated by the following diffusion curves, in order to get a complete clue to the behaviour of an accumulator in active work. Fig. i 1 shows the rate of diffusion from plates soaked in I. 17 5 acid and then placed in distilled water. It is from a paper by L. Duncan and H. Wiegand (Elec. World, N.Y., 1889), who were FIG. IO.

26 25 23 e t So .y FIG. 9.

the first to show the importance of diffusion. About one half the acid diffused out in 30 minutes, a good illustration of the slowness of this process. The rate of diffusion is much the same for both positive and negative plates; but slower for discharged plates than for charged ones. Discharge affects the rate of diffusion on the lead plate more than on the peroxide plate. This is in accordance with the density values given in Table I. For while lead sulphate is formed in the pores of both plates, the consequent expansions (and obstructions) are different; ioo volumes of lead form 290 volumes of sulphate (a threefold 30 20 10 5 10 15 2 T'me in mrnutes. FIG. II.

expansion), and ioo volumes of peroxide form 186 volumes of sulphate (a twofold expansion). The influence of diffusion on the electromotive force is illustrated by fig. 12. A cell was prepared with 20% acid. It also held a porous pot containing stronger acid, and into this the positive plate was suddenly transferred from the general body of liquid. The E.M.F. rose by diffusion of stronger acid into the pores. Curve I. in fig. 12 shows the rate of rise when the porous pot contained 34% acid; curve II. was obtained with the stronger (58%) acid (Gladstone and Hibbert, Phil. Mag., 1890). Of these two curves the first is more useful, because its conditions are nearer those which occur in practice.

At the end of a discharge it is a common thing for the plates to be standing in 25% acid, while inside the pores the acid may not exceed 8% or io%. If the discharge be stopped, we have conditions somewhat like fig. 12, and the E.M.F. begins to rise. In one minute it has gone up by about o � 08 volt, &c.

The most important practical questions concerning an accumulator are: - its maximum rate of working; its capacity at various discharge rates; its efficiency; and its length of life. Apart from mechanical injury all these depend primarily on the way the cell is made, and then on the method of 58 ° charging and discharging.

For each type and size c of cell there is a normal maximum discharging cur rent. Up to this limit any current may be taken; beyond it, the cell may suffer if discharge be con like o�i volt may be lost in the cells, by ordinary ohmic fall, so that a voltage reading of I�75 means an E.M.F. of a little over 1�8 volt, and a very weak density of the acid inside the pores. Guided by these figures, an engineer can determine what ought to be the permissible drop in terminal volts for any given working conditions. Messrs W. E. Ayrton, C. G. Lamb, E. W. Smith and M. W. Woods were the first to trace the working of a cell through varied conditions (Journ. Inst. Elec. Eng., 1890), and a brief resume of their results is given below.

They began by charging and discharging between the limits of 2.4 and 1.6 volts.

Fig. 13 shows a typical discharge curve. Noteworthy points are: - (i) At the beginning and at the end there is a rapid fall in P.D., with an intermediate period of fairly uniform value. (2) When the FIG. 13.

P.D. reaches 1.6 volt the fall is so rapid that there is no advantage in continuing the action. When the P.D. had fallen to I. 6 volt the cell was automatically switched into a charging circuit, and with a current of 9 amperes yielded the curve in fig. 14. Here again there is a rapid variation in P.D. (in these cases a rise) at the beginning and end of the operation. The cells were now carried through the same cycle several times, giving almost identical values for each cycle. After some days, however, they became more and more difficult to charge, and the return on discharge was proportionately less. It became impossible to charge up to a P.D. of 2.4 volts, and finally the capacity fell away to half its first value. Examination showed that the plates were badly scaled, and that some of the scales had partially connected the plates. These scales were cleared away and the experiments resumed, limiting the fall of P.D. to 1.8 volt. The diffi FIG. 14.

culties then disappeared, showing that discharge to 1.6 volt caused injury that did not arise at a limit of 1.8. Before describing the new results it will be useful to examine these two cases in the light of the theory of E.M.F. already given.

(a) Fall in E.M.F. at beginning of discharge. - At the moment when previous charging ceases the pores of the positive plate contain strong acid, brought there by the charging current. There is consequently a high E.M.F. But the strong acid begins to diffuse away at once and the E.M.F. falls rapidly. Even if the cell were not discharged this fall would occur, and if it were allowed to rest for thirty minutes or so the discharge would have begun with the dotted line (fig. 13). (b) Final rapid fall. - The pores being clogged by sulphate the plugs cannot get acid by diffusion, and when 5% is reached the fall in E.M.F. is disproportionately large (see fig. Jo). If discharge be stopped, there is an almost instantaneous diffusion inwards and a rapid rise in E.M.F. (c)The rise in E.M.F. at beginning and end of the charging is due to acid in the pores being strengthened, partly by diffusion, partly by formation of sulphuric acid from sulphate, and partly by electrolytic carrying of strong acid to the positive plate. The injurious results at 1.6 volt arise because then the pores contain water. The chemical reaction is altered, oxide or hydrate is formed, which will partially dissolve, to be changed to sulphate when the sulphuric acid subsequently diffuses in. But formed in this way it will not appear mixed with the active masses in the electrolytic paths, but more or less alone in the pores. In this position it will more or less block the passage and isolate some of the peroxide.