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98 per cent, acid is marked by x. Streintz (Ann. Phys. be 10 per cent, a diffusion of 1 per cent, raises the e.m.f. Chem. 46, p. 449) gives for the e.m.f. the expression by 0'006 volt only (see Fig. 11).

The important practical questions concerning an acE= 1-850+ 0-917 {s - s0)

cumulator are—its maximum rate of working, its capacity between the density limits 1-055 and 1-279; (s - s0) is at various discharge rates, its efficiency, and its charge the excess of the specific gravity of the acid over that of length of life. Apart from mechanical injury, and diswater. It must be understood that the acid referred to all these depend on the way the cell is charged charge. in the foregoing curve is that existing in the pores of the and discharged. For each type and size of cell there is a spongy masses of the plates. If there be any inequality normal maximum current. Up to this limit any current may be taken, beyond it the cell may suffer. Again, it is desirable that the charge and discharge be confined between certain limits of potential difference at the terminals. Injuriously excessive currents or a discharge below a potential difference of 1"8 volt are often accom-

Fic- 12. between that and the acid outside a process of diffusion sets in between them, and the e.m.f. varies so rapidly that no exact measurement can be made. The importance of this diffusion was first pointed out by Duncan and Wiegand {Electrical World, N.Y. 1889), who took plates before and after discharge, soaked them in sulphuric acid of 1"175 density, and then transferred them to vessels of distilled water. The acid which diffused out in a given time was estimated, with the results shown in Fig. 12. The plates contained about 5 grams of acid altogether, so that about one-half diffused out in thirty minutes, a good illustration of the slowness of diffusion. It is noteworthy that the rate of diffusion is much the same for both positive and negative plates, but that the rate for a discharged is considerably less than that for a charged plate. This last difference is undoubtedly due to the formation of sulphate in the porous plugs, with the expansions indicated in the paragraph on the properties and volumes of materials. Discharge affects the rate of diffusion on the lead plate more than on the peroxide. This is in accordance with the higher rate of expansion and clogging when lead is changed to sulphate over the change from peroxide to sulphate. From what has T//77e in Mmui&s already been said Fig. 13 about the dependence of e.m.f. on acid

strength, it is obvious that wnile diffusion is proceeding there will be a corresponding change in e.m.f. Curve I., in Fig. 13, shows the rise when a positive plate, hitherto standing in 20 per cent, acid, is suddenly placed in 34 per cent.; curve II., the rise when a similar plate is taken from 20 to 58 per cent, acid (Gladstone and Hibbert, Phil. Mag. 1890).

In applying these diffusion and electromotive force curves to working conditions, it may be noticed that a given quantity of H2S04 diffusing into very weak acid produces a much greater change in the electromotive force than if it pass into stronger acid. Thus if the acid be very weak, say, 2 per cent., a diffusion of 1 per cent. H2S04 raises the e.m.f. by 0-036 volt, whereas if "the acid

Fig. A? panied by more or less disintegration. To illustrate and explain these points a connected series of observations on a given set of cells is here used, the data having been given by Ayrton, Lamb, Smith, and Woods {Journ. Inst. Elec. Eng. 1890). The cells employed contained three negative plates, weighing 17 lb 2 oz., two positive plates, weighing 11 fi) 8 oz., and acid of D206 density. The plates measured 9 by 9 inches, and were intended for maximum currents of 9 amperes in charging and 10 amperes in discharging. For reasons given in the paper, it was decided to make the potential difference at terminals the governing condition of working, the limits to be 2-4 and 1"6 volt.

Fig. 14 shows a typical discharge curve ; noteworthy points are :—(1) At the beginning and at the end there is a rapid fall in p. n., with an intermediate period of fairly uniform value. (2) When the p.n. reaches 1"6 volt the fall is so rapid that there is no advantage in continuing the action. When the P.n. 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. 15. Here

again there is a rapid variation in p. n. (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"S volt. The difficulties then disappeared,