Having explained the process of charging and discharging the cells, Dr. Lodge mentions that the reduction of the sulphate is rather troublesome, and it is desirable that no more sulphate should accumulate than is absolutely necessary. It grows more rapidly during idleness than in action. Indeed, if kept in action, these cells are very efficient; but if suffered to be idle, they rapidly degenerate. Much of the difference found in cells is due to their comparative use and idleness. The sulphate on the positive plate is less important than that on the negative, because the oxide attacks the former far more readily. The secret of the most economical use of a secondary battery lies in charging it slowly, never filling the cell beyond its capacity, and in never completely draining it; it should be discharged as slowly as practicable. The electromotive force in a small or 1/2 h. - p. cell is about 2 to 2.1 volts per hour, or about 20 amperes for 6 or 8 hours. The amount of energy is nearly uniform for this period, and then rapidly falls - a far more satisfactory condition than one in which the cell regularly diminishes in power from the commencement.

Prof. Lodge points out the extreme value of peroxide of lead, it acting better than any other known electro - negative. In an experimental cell he used 2 plates of lead. If he substituted a clean piece of platinum for the negative lead plate, the platinum acted as plus to the peroxide of lead, though whether it was really attacked by the dilute sulphuric acid he could not tell. But by substituting for the platinum a piece of copper, the latter metal was more strongly plus, and its action, as tested by the ringing of an electric bell, was not only stronger, but of longer duration. The copper was dissolved, and formed a sulphate of copper. This would proceed either until the dilute acid was saturated, and could take up no more copper, or, if the copper plate was relatively small, until it was completely dissolved. This is the principle of a Sutton cell, but with the action reversed. As tested by its ability to ring a bell, such a cell shows for some time no signs of falling of. It is not very clear why the Sutton cell is not in more general use, although possibly it is liable to run down during the night or when idle. Lead is not strongly plus to peroxide, and hence the use of a lead plate as a support for the peroxide.

Gladstone and Tribe have said that a thin coat of varnish formed on the plate. On removing the peroxidized or positive plate from the cell, and substituting for it a hydrogenized plate, a very slight, almost infinitesimal action, is perceptible. This film or scum on the lead is therefore resistance; and the result of the experiments is not due to electromotive force, and this scum is of very great Value in preventing local action on the plus plate. Using a piece of spongy lead, the scum or film takes longer to form, and the electric bell will ring for a considerable period. Lead can be rendered spongy or porous in various ways - electrically, chemically, or mechanically; and by any of these means one can obtain an almost indefinite extension of the surface of the negative or minus plate, thus postponing the period at which the scum formed on its surface renders it effective, and enabling it to outlast the positive or plus plate. In passing, he refers to the great advantage derived from the inactivity of lead in its use as a support for the peroxide.

When he took a piece of clean lead for the peroxide plate, it refused to pass a current after the first instant, and if this were not the case, they would be unable to use it as a support on account of the local action, for the plate and its peroxide coat would act as a miniature cell. Thus, if a piece of copper fell into a cell of this class, a primary battery would be formed, and holes would be eaten into the lead. The reason for this protection was the thin but insoluble film of sulphate of lead which protected the plate, and was vital to the life of the cell, for if it were soluble, a violent effervescing action would be set up. Experiments have shown that manganese is not so good as lead. It has been attempted to meet some of the defects of lead by using carbon with it, but the coat is non - adherent and troublesome. Lead is therefore, on the whole, the most suitable material at present known. The minus plates can be improved by being made spongy, or perforating or folding them. The plan adopted by Watt, of Liverpool, is to force a jet of steam under high pressure against a stream of molten lead, which is thereby deflected against a board, from which it is removed in a spongy condition.

In the Sellon - Volckmar cell, the lead is perforated, and in the process of Kabath the metal is bent or folded. The plates may again be coated with salt of lead, either electrically, as in the Plante cell ; chemically, as in Schultzer's, who prepares his plate incrusted with a thick sulphate; or mechanically, as in Faure's process. In all the varieties of coated plates tried, Lodge had not found anything much better than the Plante cell; but several other considerations had to be borne in mind in selecting secondary batteries, such as internal resistance, weight, and compactness of the cells. The 1/2 h. - p. Sellon - Volckmar - Faure cell is equivalent to 20 amperes, 2 volts in 9 hours,' and the h. - p. cell is just double that power, and equals the raising of nearly 2 million foot - lb. The percentage of quantity of available work is very good in these cells - from 80 to 90 per cent., and even more, of the charge being utilized; and it is well established that the more slowly a cell is charged the less waste is apparent, and were it practicable, the discharge should be equally slow. The next point is as to the use of the secondary battery; and the first, because most obvious, is for lighting purposes, for which the cells have many advantages.

As stores for electric power for use in lighting conveyances making short journeys, they will be highly useful. Small vessels, cabs, and tricycles can carry with them boxes giving sufficient power for some hours' lighting; and it is well known that the Pulman train between London and Brighton has for some time been lighted by secondary batteries. Another advantage is that by this means you can use for a short time a higher power than an engine can give. It is not economical to run an engine for a few hours only, and by the cells the engine can be kept going all day, and a larger horse - power can be expended in lighting energy during the evening. There is less danger of sudden extinction, for a cell is not liable to give out suddenly, as is a belt to slip from an engine. Greater steadiness of light is attainable, the secondary battery acting as a governor, and it will also be utilized as a governor, performing the function of a cistern or so - called" gasometer. Any fluctuation in the dynamo is not communicated to the lamps. If used for electric installations, the reservoir will probably be in a central place; but there are advantages in having separate boxes in each house.