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MIME

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Originally appearing in Volume V01, Page 134 of the 1911 Encyclopedia Britannica.
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MIME ^^^^^^^^^i\^^^^^ MI ^^^^^^^^^^^^^^6 III 11^^^U^^^^^^^^^^^^^^I1t^ ^^^4~^'lliliL3^^^^^^ I 0 10 20 30 40 60 00 70 e0 00 00 moo woo woo ewo 4000 woo 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; too volumes of lead form 290 volumes of sulphate (a threefold maximum discharging cur- io is 20 Time in minutes expansion), ifnd too 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 2o% 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., t89o). 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•o8 volt, &c. Charge and Discharge.—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 3 2.2 charging and discharging. For each type and size it 2 /5 W2 25 rent. Up to this limit any current may be taken; beyond it, the cell may suffer if discharge be continued for any appreciable time. The most important point to attend to is the voltage at which discharge shall cease. The potential difference at terminals must not fall below 1.8o volt during discharge at ordinary rates (to hours) or 1.75 to 1.7o volt for 1 or 2 hour rate. The reason underlying the figures is simple. These voltages indicate that the acid in the pores is not being renewed fast enough, and that if the discharge continue the chemical action will change: sulphate will not be formed in situ for want of acid. Any such change in action is fatal to reversibility and therefore to life and constancy in capacity. To illustrate: when at slow discharge rates the voltage is 1.8o volt, the acid in the pores has weakened to a mean value of about 2.5% (see fig. 1 1)., which is quite consistent with some part of the interior being practically pure water. With high discharge rates, something like o•t volt may be lost in the cells, by ordinary ohmic fall, ss that a voltage reading of 1.75 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., 189o), and a brief resume of their results is given below. They began by charging and discharging between the limits of 2.4 and i•6 volts. Fig. 13 shows a typical discharge curve. Noteworthy points are:—(1) At the beginning and at the end there is a rapid fall in r.n., with an intermediate period of fairly uniform value. (2) When the 2 / Wahl/. L tg LF// _ O/schsrged :u re vF /0 Amps /7 TIME /i /I ur: fron. B . // n,eg of oisch:rg e O / 2 3 3 7 o /o if 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 1.6 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- of gell there is a normal 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. 15 disproportionately large (see fig. to). 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- r e 30 2.0 l0 5 0 30 25 2 2s 0 2 .3 a s Time m M.nufres a . Wo king ? G. o/ Ca// Cgrrent 5 An• ^~ I ME H r o! Sh. ra: for, 6 7n/ ng /n oL es 5 3 3 2 ss a Further, when forming in the narrow passage its disruptive action will tend to force off the outer layers. It is evident that limitation of P.D. to 1.8 volt ought to prevent these injuries, because it pre-vents exhaustion of acid in the plugs. Fig. 15 shows the results obtained by study of successive periods of rest, the observations being taken between the limits of 2.4 and 1.8 volts. Curves A and B show the state and capacity at the beginning. After a to days' rest the capacity was smaller, but repeated cycles Discharges with 10 amperes. Charges with 9 amperes. ''+ 2 4 :..2 2.2 _ B •0 A z•o .8 • ~_ 1.8 .4 2.4 :•2 2.2 I1 _ •o. C 2.0 2~ 4 2 2 2.2 F 0 E 2.0 •8-_._ t•8 •4-_ 2 4 22 O 2.2 ..:1.__ 20 8 t.g '4 2.2 o I 2.0 •8 .4 •2 t•8 2 4 2.2 •0 • K 2.0 .a 1.8 .a 2.4 2 2.2 M 2.0 .4 •z ^ Z // 2.2 . 0 0 2•or '8 - - -~ 1'8- - - 0 1 2 8 4 5 6 7 8 9 10 11 0 1 2 8 4 5 6 7 8 9 10 11 Time in hours from beginning of discharge. Time in hours f ram beginning of charge. of work brought it back to C and D. A second rest (to days), followed by many cycles, then gave E and F. After a third rest(16 days) and many cycles, G and H were obtained. After a fourth rest (16 days) the first discharge gave I and the first charge J. Repeated cycles brought the cells back to K and L. Curves M and N show first cycle after a fifth rest (16 days) ; 0 and P show the final restoration brought about by repeated cycles of work. The numbers given by the integration of some of these curves are stated in Table III. Capacity and Efficiency under Various Conditions of Working. Discharge. Charge. Efficiency. Experiment. Am- Am- pere Watt pere Watt Quan- Energy. Hours. Hours. Hours. Hours. tity. Normal cycle. 102 201.7 104.5 230'7 97.2 87'4 Restoration too 179 103'8 228.2 96'8 85'8 after 1st rest Ditto, after 91 176.7 96.8 213'2 94.1 82'8 2nd rest . . Ditto, after 82.6 161.3 86.2 190.5 95'8 84'7 3rd rest . Discharge 56.5 110.5 86.2 190.5 65.5 581 immediately } after rest ) 56.5 110.5 71.1 158'3 79.6 69.6 Restoration 8o 156.9 83'8 184.6 95.5 85 after 8 cycles The table shows that the efficiency in a normal cycle may be as high as 87.4%; that during a rest of sixteen days the charged This discharge is here compared with the charge that preceded the rest; in the next line the same discharge is compared with the charge following the rest.accumulator is so affected that about 3o% of its charge is not available, and in subsequent cycles it shows a diminished capacity and efficiency; and that by repeated charges and discharges the capacity may be partially restored and the efficiency more completely so. These changes might be due to-(a) leakage or short-circuit, (b) some of the active material having fallen to the bottom of the cell or (c) some change in the active materials. (a) is excluded by the fact that the subsequent charge is smaller, and (b) by the continued in-crease of capacity during the cycles that follow the rest. Hence the third hypothesis is the one which must be relied upon. The change in the active materials has already been given. The formation of lead sulphate by local action on the peroxide plate and by direct action of acid on spongy metal on the lead plate explains the loss of energy shown in curve M, fig. 15, while the fact that it is probably formed, not in the path of the regular currents, but on the wall of the grid (remote from the ordinary action), gives a probable explanation of the subsequent slow recovery. The action of the acid on the lead during rest must not be overlooked. We have seen that capacity diminishes as the discharge rate increases; that is, the available output increases as the current diminishes. R. E. B. Crompton's diagram illustrating this fact is given in fig. 16. At the higher rates the consumption of acid is too rapid, diffusion cannot maintain its strength in the pores, and the fall comes so much earlier. The resistance varies with the condition of the cell, as shown by the curves in fig. 17. It may be unduly increased by long or narrow lugs, and especially by dirty joints between the lugs. It is interesting to note that it increases at the end of both charge and discharge, and much more for the first than the second. Now the composition of the active materials near the end of charge is almost exactly the same as at the beginning of discharge, and at first sight there seems nothing to account for the great fall in resistance from 0.0115 to 0.004 ohm ; that is, to about one-third the value. There is, however, one difference between charging and discharging-namely, that due to the strong acid near the positive, with a corresponding weaker acid near the negative electrode. The curve of conductivity for sulphuric acid shows that both strong and weak acid have much higher resistances than the liquid usually employed in accumulators, and it is therefore reasonable to suppose that local variations in strength of acid cause the changes in resistance. That these are not due to the constitution of the plugs is shown by the fact that, while the plugs // P. a f((YISiI~^^^^^_^ .-_iu^^^^^ ~-^C^111-\M^^^IMIIH^_ ^\\^\^^.01MI_ MEMNON - - ^rn su E^1I ^111w!w1^^ .u. 0 7 s s 2 2 C2 2 2 2 2 2 2 2 2 • r^ ^^%^^ .:11 1I~ iiIIt: 111^^^ . ^^, ^^\'PMI^^^^^^^ 2 ,f sr 0/0 132 are almost identical at end of discharge and beginning of charge, the resistance falls from 0.0055 to 0.0033 ohm. While a current flows through a cell, heat is produced at the rate of C'RX0.24 calories (water-gram-degree) per second. As a consequence the temperature tends to rise. But the change of temperature actually observed is much greater during charge, and much less during discharge, than the foregoing expression would suggest; andit is evident that, besides the heat produced according to Joule's law, there are other actions which warm the cell during charge and cool it during discharge. Duncan and Wiegand (loc. cit.), who first observed the thermal changes, ascribe the chief influence to the electrochemical addition of H2SO4 to the liquid during charge and its removal during discharge. Fig. 18 gives some results obtained by Ayrton, Lamb, &c. This elevation of temperature (due to electrolytic strengthening of acid and local action) is a measure of the energy lost in a cycle, and ought to be minimized as much as possible. Chemistry.—The chemical theory adopted in the foregoing pages is very simple. It declares that sulphate of lead is formed on both plates during discharge, the chemical action being reversed in charging. The following equations express the experimental results. Condition before discharge: +plate [!n. Liquid —plate x. PbO2 + HH20 + z. Pb After discharge: +plate Liquid —plate [(x—p). PbO2 1 + j (Y -2P). H2SO4 + [-(z—P) , Pbl p. PbSO4L L(n+2p). H2O J LP. PbSO4 J During charge, the substances are restored to their original condition: the equation is therefore reversed. An equation of this general nature was published by Gladstone and Tribe in 1882, when they first suggested the " sulphate " theory, which was based on very numerous analyses. Confirmation was given by E.Frankland in 1883, E. Reynier 1884, A. P. P. Crova and P. Garbe 1885, C. Heim and W. F. Kohlrausch 1889, W. E. Ayrton, &c., with G. H Robertson 189o, C. H. J. B. Liebenow 1897, F. Dolezalek 1897, and M. Mugdan 1899. Yet there has been, as Dolezalek says, an incomprehensible unwillingness to accept the theory, though no suggested alternative could offer good verifiable experimental foundation. Those who seek a full discussion will find it in Dolezalek's Theory of the Lead Accumulator. We shall take it that the sulphate theory is proved, and apply it to the conditions of charge and discharge. From the chemical theory it will be obvious that the acid in the pores of both plates will be stronger during charge than that outside. During discharge the reverse will be the case. Fig. 19 shows a curve of potential difference during charge, with others showing the con-current changes in the percentage of PbO2 and the density of acid. These increase almost in proportion to the duration of the current, and indicate the decomposition of sulphate and liberation of sulphuric acid. There are breaks in the P.D. curve at A, B, C, D where the current was stopped to extract samples for analysis, &c. The fall in E.M.F. in this short interval is noteworthy; it arises from the diffusion of stronger acid out of the pores. The final rise of pressure is due to increase in resistance and the effect of stronger acid in the pores, this last arising partly from reduced sulphate and partly from the electrolytic convection of SO4 (see also Dolezalek, Theory, p. 113). Fig. 20 gives the data for discharge. The percentage of PbO2 and the density here fall almost in proportion to the duration of the current. The special feature is the rapid fall of voltage at the end. Several suggestions have been made about this phenomenon. The writer holds that it is due to the exhaustion of the acid in the pores. Plante, and afterwards Gladstone and Tribe, found a possible cause in the formation of a film of peroxide on the spongy lead. E. J. Wade has suggested a sudden readjustment of the spongy mass into a complex sulphate. To rebut these hypotheses it is only necessary to say that the fall can be deferred for a long time by pressing fresh acid into the pores hydrostatically (see Liebenow, Zeits. fur Elektrochem., 1897, iv. 61), or by working at a higher temperature. This increases the diffusion inwards of strong acid, and like the increase due to hydrostatic pressure maintains the E.M.F. The other suggested causes of the fall therefore fail. Fig. 20 also shows that when the discharge current was stopped at points A, B, C, D to extract samples, the voltage immediately rose, owing to inward diffusion of stronger acid. The inward diffusion of fresh acid also accounts for the recuperation found after a rest which follows either a complete discharge or a partial discharge at a very rapid rate. If the discharge be complete the recuperation refers only to the electromotive force; the pressure falls at once on closed circuit. If discharge has been rapid, a rest will enable the cell to resume work because it brings fresh acid into the active regions. As to the effect of repose on a charged cell, Gladstone and Tribe's experiments showed that peroxide of lead lying on its lead support suffers from a local action, which reduces one molecule of PbO2 to sulphate at the same time that an atom of the grid below it is also changed to sulphate. There is thus not only a loss of the available peroxide, but a corrosion of the grid or plate. It is through this action that the supports gradually give way. On the negative plate an action arises between the finely divided lead and the sulphuric acid, with the result that hydrogen is set free: Pb + H2SO4 = PbSO4+H2. This involves a diminution of available spongy lead, or Iossof capacity, occasionally with serious consequences. The capacity of the lead plate is reduced absolutely, of course, but its relative value is more seriously affected- In the discharge it gets sulphated too much, because the better positive keeps up the E.M.F. too long. In the succeeding charge, the positive is fully charged before the negative, and the differences between them tend to increase in each cycle. Kelvin and Helmholtz have shown that the E.M.F. of a voltaic cell can be calculated from the energy developed by the chemical action. For a dyad gram equivalent (=2 grams of hydrogen, 207 grams of lead, &c.), the equation connecting them is E = H dE' 46000 Td"r where E is the E.M.F. in volts, H is the heat developed by a dyad equivalent of the reacting substances, T is the absolute temperature, and dE/dT is the temperature coefficient of the E.M.F. If the E.M.F. does not change with temperature, the second term is zero. The thermal values for the various substances formed and decomposed are:—For PbO2, 62400; for PbSO4, 216210; for H2SO4, 192920; and for H2O, 68400 calories. Writing the equation in its simplest form for strong acid, and ignoring the temperature co-efficient term, ^^^^^^^^^^^^^^^^^^^^^^^^^^^MI.^ ¢TU^^^^^^^^^^^^^^^r^^^^^.,'.~^^ ^^lI^^^^^^^^^^^^^^^^^^EMOM'd^^^^^ 1!!^^^^^M^^^^^^u^^/M.i:Euir^^^^^^^ ^!f ^^^^^^^MM711i)5ff^/G/^^^^^^^^^ 703/^^^^fERMATi''AMEEP.M^^^^^^^^^^^^ ^EAMUg^EiIl^^MIOPMM^^^^^^^^^^^EMM 11PIMPE.f^i!!lA V''iii^^^^^^^^^^I1^^/%^^ ^ioTe^^r13'yiiM^^^^^^^^^^^l^^^^-r^^^ ^^/e'iII^^^^^^1^^^^^^^^^^/2^^^^^ -.^^^^^^^^^^^^^^wMnePs^nMT^^^^^^^ ^11McrICEZEN2 ARedlCCEuf^^ii^^^^^^ REEgiii-MMMEMMMMMMEEliNUMMUMMMMMMM IAUMMMEMMEMUUEMMEMEEMEEMIIMMMEM= ^^EEIUUtfiUZa33tZ47i r'S-Z7JiMC623i3f:T.Cu>Zfi9^^^^^ 6o% 60 50 z•I za 0 I 2 3 • b 6 7 8 Time In Hours /tom beg,nnrnp of Charge or O,'aohaye la 41 4 21 MEN^^^^^^^^^^^^^^^^^^^^f^^^^^^ ^^ ^^r^^^f^^^^^^r^^^^^^^^^^^^^^ ^^^\O\^^^^^^^^^^^^^^^^^^^^^^^^^ 1 OMMMM11 MMENEEMMU^^^^^^^^^^^^^^^^ WMMMM/GGGM ft",,M.gO7.*,!-}IlINER^^^^^^^^ ^^^^^^^^^^^EMMM MOR2gWaMMMME^^^^ t&1^^^^^^^^^^^^MGM^^^^^^ MMEM^\^ ^M^^^^^^^^^^^^^ERME^^^MMMNMI^Il^ Mf:f.^r!/I^^^^^^^^ilgE M^^^^^rM^11^ mmmmommucmmummmimummmmmon. MlN^^^^^^^^11IMMMEM^^^^^\'210^^^ il^ ^!/^^^^^^^^^^E^&MM^^^MMUNCEREE RIf\\^^^^^^^^^^^^MME6CE!^^^^^1> E fF^^^^^^^^^^r^^^^^^^^^^MRMPEIMU ®L-''^^^^^^^^^^^^^^^M^^^^^^^^^ORMO 5 6 2 B 0 II 12 13 14 u 1.8 n 0P Ala 15 7010 4. 60. 4 30 PbO2 +2H2SO4 + Pb =2PbSO4+2H20 -62440—385840 +432420+136720 leaving a balance of 120860 calories. Dividing by 46000 gives 2.627 volts. The experimental value in strong acid, according to Gladstone and Hibbert, is 2.607 volts, a :very close approximation. For other strengths of acid, the energy will be less by the quantity of heat evolved by dilution of the acid, because the chemical action must take th,e H2SO4 from the diluted liquid. The dotted curve in fig. to indicates the calculated E.M.F. at various points when this is taken into account. The difference between it and the continuous curve must, if the chemical theory be correct, depend on the second term in the equation. The figure shows that the observed E.M.F. is above the theoretical for all strengths from Too down to 5 %. Below 5 the position is reversed. The question remains, Can the temperature coefficient be obtained? This is difficult, because the 228 335 285 255 130 73 Unpublished experiments by the writer give d—Er. 105=350 for acid of density 1.156. With stronger acid, a true cycle could not be obtained. Taking Streintz's value, 335 for 25% acid, the second term of the equation is Tdd—,EI, = 290 X •000335 = 0.0971 volt. The first term gives 88800 calories = 1.9304 volt. Adding the second term, 1.9304+0.0971=2.2075 volts. The observed value is 2.030 volts (see fig. io), a remarkably good agreement. This calculation and the general relation shown in fig. io render it highly probable that, if the temperature coefficient were known for all strengths of acid, the result would be equally good. It is worth observing that the reversal of relationship between the observed and calculated curves, which takes place at 5% or 6 %, suggests that the chemistry must be on the point of altering as the acid gets weak, a conclusion which has been already arrived at on purely chemical grounds. The thermodynamical relations are thus seen to confirm very strongly the chemical and physical analyses.' Accumulators in Central Stations.—As the efficiency of accumulators is not generally higher than 75%, and machines must be used to charge them, it is not directly economical to use cells alone for public supply. Yet they play an important and an increasing part in public work, because they help to maintain a constant voltage on the mains, and can be used to distribute the load on the running machinery over a much greater fraction of the day. Used in parallel with the dynamo, they quickly yield current when the load increases, and immediately begin to charge when the load diminishes, thus largely reducing the fluctuating stress on dynamo and engine for sudden variations in load. Their use is advantageous if they can be charged and discharged at a time when the steam plant would otherwise be working at an uneconomical load. Regulation of the potential difference is managed in various ways. More cells may be thrown in as the discharge proceeds, and taken out during charge; but this method often leads to trouble, as. some cells get unduly discharged, and the unity of the battery is disturbed. Sometimes the number of cells is kept fixed for supply, but the P.D. they put on the mains is reduced during charge by employing regulating cells in opposition. Both these plans have proved unsatisfactory, and the battery is now preferably joined across the mains in parallel with the dynamo. The cells take the peaks of the load and thus relieve the dynamo and engine of sudden changes, as shown in fig. 21. Here the line 12 10 zo current (shown by the FIG. 21. erratic curve) varied spas- modically from o to 375 amperes, yet the dynamo current varied from roo to 150 amperes only (see line A). At the same time the line voltage (535 volts normal) was kept nearly constant. In the late evening the cells became exhausted and the dynamo charged them. Extra voltage was required at the end of a " charge " and was provided by a " booster." Originally a booster was an auxiliary dynamo worked in series with the chief machine, and driven in any convenient way. It has de- ' For the discussion of later electrolytic theories as applied to accumulators, see Doleeaalek, Theory of the Lead Acounvaclator.veloped into a machine with two or more exciting coils, and having its armature in series with the cells (see fig. 22). The exciting coils act in opposition; the one carrying • the main current sets up an E.M.F. in the same direction as that of the cells, and helps the cells to discharge as the load rises. When the load is small, the voltage on the mains is highest and the shunt exciting current greatest. The booster E.M.F. now acts with the dynamo and against the cells, and causes them to take a full charge. Even this arrangement did not suffice to keep the line voltage as constant as seemed desirable in some cases, as where lighting and traction work were put on the same plant. Fig. 23 is a diagram of a complex booster which gives very good regulation. The booster B has its armature in series with the accumulators A, and is kept running in a given direction at a constant speed by means of a shunt-wound motor (not shown), so that the E.M.F. induced in the armature depends on the excitation. This is made to vary in value and in direction by means of four independent exciting coils, Cl, C2, C3, C4. The last is not essential, as it merely compensates for the small voltage drop in the armature. It is obvious that the excitation C2 will be proportionate to the difference in voltage between the battery and the mains, and it is arranged that battery volts and booster volts shall equal the volts on the mains. Under this excitation there is no tendency for the battery to charge or discharge. But any additional excitation leads to strong currents one way or the other. Excitation CI rises with the load on the line, and gives an E.M.F. helping the battery to discharge most when the load is greatest. C2 is dependent on the bus-bar voltage, and is greatest when the generator load is small: it opposes CI and therefore excites the booster to charge the battery. The exact generator load at which the booster shall reverse its E.M.F. from a charging to a discharging value is adjusted by the resistance R2 in series with C2. A similar resistance R3 allows the excitation of C3 to be adjusted. Very remarkable regulation can be obtained by reversible boosters of this type. In traction and lighting stations it is quite possible to keep the variation of bus-bar pressure within 2% of the normal value, although the load may momentarily vary from a few amperes up to 200 or value is so small, and it is not easy to secure a good cycle of observations. Streintz has given the following values : E 1.9223 1.9828 2.0031 2.0084 2.0105 2.078 2.2070 dT I05 140 400 350 300 250 200, 150 A too 50 earls 300. J. B. Entz has introduced an auxiliary device which enables him to use a much more simple booster. The Entz booster has no series coil and only one shunt coil, the direction and value of excitation due to this being controlled by a carbon regulator, having two arms, the resistance of each of which can be varied by pressure due to the magnetizing action of a solenoid. The main current from the generator passes through the solenoid and causes one or other of the two carbon arms to have the less resistance. This change in resistance determines the direction of the exciter field current, and therefore the direction of the boost. A photograph of the switchboard at Greenock where this booster is in use shows the voltmeter needle as if it had been held rigid, although the exposure lasted 90 minutes. On the same photograph the ammeter needle does not appear, its incessant and large movements preventing any picture from being formed. Alkaline Accumulators.—Owing to the high electro-chemical equivalent of lead, a great saving in weight would be secured by using almost any other metal. Unfortunately no other metal and its compounds can resist the acid. Hence inventors have been incited to try alkaline liquids as electrolytes. Many attempts have been made to construct accumulators in this way, though with only moderate success. The Lalande-Chaperon, Desmazures, Waddell-Entz and Edison are the chief cells. T. A. Edison's cell has been most developed, and is intended for traction work. He made the plates of very thin sheets of nickel-plated steel, in each of which 24 rectangular holes were stamped, leaving a mere framework of the metal. Shallow rectangular pockets of perforated nickel-steel were fitted in the holes and then burred over the framework by high pressures. The pockets contained the active material. On the positive plate this consisted of nickel peroxide mixed with flake graphite, and on the negative plate of finely divided iron mixed with graphite. Both kinds of active material were prepared in a special way. The graphite gives greater conductivity. The liquid was a 20% solution of caustic potash. During discharge the iron was oxidized, and the nickel reduced to a lower state of oxidation. This change was reversed during charge. Fig. 24 shows the general features. The chief results obtained by European experts showed that the E.M.F. was 1.33 volt, with a transient higher value following charge. A Flo. 24.—Edison Accumulator. cell weighing 17 8 lb had a resistance of 0.0013 ohm, and an output at 6o amperes of 210 watt-hours, or at 120 amperes of 179 watt-hours. Another and improved cell weighing 12.7 lb gave 14.6 watt-hours per pound of cell at a 2o-ampere rate, and 13.5 watt-hours per pound at a 6o-ampere rate. The cell could be charged and discharged at almost any rate. A full charge could be given in 1 hour, and it would stand a discharge rate of 200 amperes (Journ. Inst. Elec. Eng., 1904, pp. 1-36). Subsequently Edison found some degree of falling-off in capacity, due to an enlargement of the positive pockets by pressure of gas. Most of the faults have been overcome by altering the form of the pocket and replacing the graphite by a metallic conductor in the form of flakes.
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