Alkaline solutions cannot be used with lead, because they dissolve it; but they may be employed with some other metals, as iron, which would absorb H at one plate and form peroxide at the other, producing a battery of rather low E.M.F.

In charging a number of cells, it is necessary so to arrange them in series and in arc as to distribute them on the same principles as ordinary battery cells when a number are used together. So many must be ranged in series that the number multiplied by the E.M.F. is so roach below the charging E.M.F. as allows the required rate of current to pass; that is to say, E (or 2.25) x n x 1 - 25=E.M.F. of source, assuming that this is to exceed the counterforce of the battery by 1/4. So many must be ranged in multiple arc as brings the united resistance to such a ratio to the available E.M.F. as will permit the intended rate of current to pass; such rate being well below the proper working density suited to the area of the plates.

It is of the utmost importance that all cells to be worked together shall be fairly equal, for, as in a chain, the capacity of a combination is that of the weakest link; if some cells become inactive in discharge, they are not merely useless, they begin to take charge in the opposite direction and oppose their E.M.F. If several sets in multiple arc differ in E.M.F., which will occur if their conditions differ, some of the sets will not get charged, or if left so connected when the source is not acting, they will be reversed, and the charge be wasted.

The charge should not be carried to more than § of the capacity. The discharge should not be carried farther than f of the charge actually stored; and the battery should not be charged, if avoidable, long before it is intended to be used.

Each cell should be occasionally tested as to its condition, in order to discover any derangement or accidental short circuit, and any cell showing unusual evolution of gas should receive immediate attention.

Galvanometers should always be kept in circuit, to give warning as to what is going on; and automatic cut - outs are very useful in case of a failure of the source, or other accident. Such appliances are easily made with an electromagnet inserted in the circuit, or in a shunt circuit, with a permanent steel magnet for the armature, mounted on a spring, and completing the circuit only when held down : the attraction of the armature for the core will hold it down ordinarily; but if a reverse current arises, the armature leaves the magnet, breaks the circuit, and can be made to ring a bell to call attention.

It is asserted by some that secondary batteries will return 90 Per cent. of the energy stored. It is quite likely that 90 Per cent. of the electricity, reckoned - in coulombs, might be obtained, provided the battery were used not long after charge. But what is really important is, the energy depends upon the E.M.F. as well as the current; and as the E.M.F. of charge must exceed that of discharge, and may probably exceed it by }, here is an inevitable loss, which cannot be defined for all cases, because it will depend upon the ratio of the external and internal resistances. This only gives the loss upon actual storage; there is to be added that lost in the act of charge, and that carried away in the escaping gases. When all these are considered, it is almost certain that, on the average, the use of storage batteries means the loss of 50 per cent. of the energy as compared with direct working. This means doubling the cost, irrespective of the value and expense of the battery itself. (J. T. Sprague, Eng. Mech.)

Henry Greer, of New York, gives a good detailed account of the construction of the various secondary batteries before the public, in his pamphlet on the ' Storage of Electricity.'