On charging a cell, the positive plate - that is to say, the plate connected to the positive terminal of the dynamo - becomes the colour of wet chocolate, and the negative plate that of slate. The deeper the chocolate colour, the more thoroughly charged is the cell, and after a little experience, it is an easy matter to decide roughly the amount of charge a cell possesses. The specific gravity of the dilute acid also varies with the charge, and by means of a hydrometer this can be ascertained. Thus there are three ways of determining the charge - firstly, by the colour of the plates; secondly, by the reading of the hydrometer; and thirdly, by the E.M.F. of the cell, which can be ascertained by means of a voltmeter, as will be hereafter explained.

Fig. 620. - View of Accumulator-cell.

As I have said, each cell gives at the end of its discharge 1.9 volts, so that if an E.M.F. of 100 should be required, 53 accumulator-cells in series would be necessary. By the expression "in series" is meant that the cells should be joined up in a row so that the current from the end cell must pass through the rest - that is to say, the positive of one cell must be connected to the negative of the next, and its positive to the negative of the next, and so on, as shown in Fig. 621. But supposing one cell gave 1.9 volts and l0 amperes, the 53 cells connected in series, although giving a pressure of 100.7 volts, would still yield a eurrent of only 10 amperes. If the same number of cells are connected in "parallel" or'•multiple", we should have a different result, viz. 53 cells, each giving 10 amperes = 530 amperes, at 1.9 volts. Connecting "in parallel" is merely joining all their positive terminals together on to one wire, and all their negatives on to another. Each cell can then give its ten amperes to the circuit, but the pressure between the two wires is only 1.9 volts, since the voltage has not been, as in the previous case, augmented by the cells being in "series".

Fig. 621. - Accumulator-cells connected in Strict.

As an analogy to explain the above more clearly, imagine three pipes full of water, and of equal dimensions, say 100 feet high and 6 inches in diameter. Now, if we join these "in series", we have a pipe 300 feet high by 6 inches diameter, and we get so many gallons of water with the head of 300 feet, but if we join these side by side, or "in parallel", we only get a third the pressure, but we have three diameters of 6 inches, each giving us water.

Although, in order to avoid confusion, I described cells just now as having each only two plates, they generally consist of many such, as shown in figs. 620 and 621. The number of plates in cells, however, is determined merely by the-output required. If, for instance, 2 plates give 10 amperes, 4 plates would give 20 amperes, and so on. The number of plates is increased in preference to increasing their size, as the latter method would render them weaker. No matter how many plates there are in a cell, there are only two terminals, all the positive plates being joined together "in multiple" and all the negative "in multiple", usually by means of lead lugs east on the plates and fused together. These plates are interleaved, so that, on looking into a cell, one sees first a +, then a - plate, then another positive, and so on. There is always, however, one more negative plate than positive, as it has been found that by having a - plate on each side of a + plate, the positive plates, which are the more delicate, are better preserved.

The plates of accumulators, to be commercially useful, must be strong, and must offer large surfaces to the acid, or electrolyte (as it is sometimes termed). In order to achieve this, some of the first makers take a solid lead plate about half an inch thick, and cut or cast fine grooves in it on each side to the depth of (say) one-eighth of an inch; others take a ribbon of lead about half an inch wide by one-thirty-second of an inch thick, and form a plate of about a foot square with it, by bending it back continually on itself.

The surface of the lead plates after charging becomes spongy, and this spongy surface constitute- their capacity; the plate offering the most surface has the-greatest capacity. This peculiar surface is generally obtained in the first place by chemical and electrical treatment, and if the plates are used judiciously, the depth of this spongy lead should increase, until, after very many years, the entire plate should become spongy and the cell attain its maximum capacity, after which it will fall to pieces. The procuring of such a spongy surface by pasting prepared lead on to the plate, is obviously a less substantial method than the production of the spongy surface from the plate itself.

The selection of a cell can be easily made with confidence, if the two points I have mentioned are kept in view, viz. a maximum of strength and a maximum of surface. Few, if any, makers exceed the size of about 12 inches square for their plates.

In stationary work in small installations, the plates are set up in glass boxes, which permit of the plates being readily examined; but in electricity-supply works, where the cells are very large, or in cases where they are subjected to rough treatment, such as in tramway or yacht work, lead boxes are used with an outer casing of teak.

The connections from cell to cell, in small installations, are generally made with brass bolts and nuts, clamping the lugs together tightly. All are then varnished to protect them from the acid. In large installations, such as electneity-supply works, these lugs are often burned together with lead.

The capacity of cells is described in ampere-hours, that la to say, so many amperes for so many hours; for instance, if a set of cells is described as having 600 ampere-hours at 100-volts pressure, it would be understood that, at that pressure. 600 amperes could be obtained for one hour, or 300 for 2 hours. I mention this figure to make the point clear, but have now to add that the 600 ampere-hours (or A.H. as it is often written) are conditional on a maximum discharge of (say) 60 amperes not being exceeded, so that the quickest method of discharging the cells permissible, would be 60 amperes for 10 hours. The voltage of 100 would only indicate the number of cells, since one cell giving a minimum of 1.9 volts would discharge at 60 amperes for 10 hours, and fifty-three in series would also discharge 00 amperes for 10 hours, but (as stated above) at a pressure of 100 volts.