In Bunsen’s Battery (or the Zinc-Carbon Battery), which is very like the “Daniell” arrangement, as will be seen from the plates (figs. 222, 223), the porous cell has a prism of carbon immersed in it, and is apparently a modification of the powerful “Grove” Battery (fig. 224). This consists of slips of platinum, h, placed in porous cells, g, each cell being surrounded by a glass cylinder. The outer (glass) cells are filled, or nearly filled, with diluted sulphuric acid; nitric acid is used in the porous cells, and a platinum plate inserted. The chemical action of the Grove cell is thus explained by Professor Stewart: “The zinc dissolves in the dilute sulphuric acid, and during this process hydrogen gas is given off. But this hydrogen does not rise up in the shape of bubbles; it finds its way into the porous vessel which contains the strong nitric acid. It there decomposes the acid, taking some oxygen to itself, so as to become water (hydrogen and oxygen forming water), and thereby turning the nitric into nitrous acid, which shows its presence by strong orange-coloured fumes.” By this decomposition of the nitric acid the polarization of the platinum (due to hydrogen) is avoided. The porous cell, while keeping the liquids apart, does not interfere with the chemical action.
A great number of cells are used in the Grove Battery; perhaps even a hundred may be employed.
Smee’s Battery consists of a plate of platinized silver, S, with a bar of wood to prevent contact with the zinc on each side, Z. These are immersed in a glass jar, A, which contains dilute sulphuric acid. The current is obtained by metallic communication with the binding-screws on the top. This battery has much the same general arrangement as Wollaston’s—the position of the plates being, however, reversed; in the latter there are two negative plates to one positive. In Smee’s Battery there are two positive (zinc) plates to one negative plate.
Fig. 225.—Smee’s Cell.
Fig. 226.—Smee’s Battery.
It will now be understood how an electric current is produced; the electricity passing through the cells, etc., to wires, confers certain properties upon the wires, and we can ascertain the effect of the current by means of a Galvanometer, an instrument used to detect the strength and direction of electric currents. The current will evolve heat and light; it will excite muscular action, and will decompose substances into their constituent elements. The deflection of the magnetic needle by the electric current is considered the best evidence of its power; it is on this that the Galvanometer is based.
We can perform a few simple experiments with the current. Suppose, for instance, that a piece of fine wire be fixed between the pole wires of the battery; it will be heated “white hot.” Or if two carbon points be approached in a glass of water, as in the illustration (fig. 227), they will emit a brilliant light in the fluid from the voltaic arc which has given us the electric light. The current is the passage of electricity along the wire, and continues until the working power or “potential” of one conductor is equal to that of the other. When they become equal of course the action ceases, as there is equilibrium. But when an apparatus like the galvanic battery is brought to bear so that the force of electricity from one conductor is made always greater than that of the other conductor, we have a continuous flow while the action of the battery goes on. One view of the principle is thus expressed by Professor Gordon:[16]
“If two metals be placed near together, but not in contact, in a liquid which acts chemically more upon one than upon the other, the metals become charged, so that the one least acted on is of higher potential than the one most acted on. The difference of potential produced depends only upon the nature of the metals and of the liquid, and not on the size or position of the plates. As soon as the difference of potential has reached its constant value the chemical action ceases.