In the accompanying illustration of the Galvanic Pile a disc of copper is at the bottom and a disc of zinc at the top. The latter, P, is the positive pole; the former, N, the negative. When the wires are united the current is closed, and no sign of disturbance can be detected, although the action, of course, is proceeding within the pile. The opposite kinds of electricity neutralize each other, and if a continuous supply were not kept up the electricity would disappear; but as it is, a powerful current is obtained, and if the wire be divided a spark will be observed. [Illustration: Fig. 222.—Bunsen Battery.] There are many forms of galvanic batteries. The Trough Battery or Cruickshank has been mentioned. There is Wollaston’s Pile, Bunsen’s Battery, Grove’s Battery, and Daniell’s, called the “Constant” Battery. In this last a porous earthenware cell is placed within a cylinder of copper; in the cell a rod of zinc is inserted, the cell being filled with diluted sulphuric acid,—one part of acid to ten parts of water,—and in the outer cylinder is a solution of sulphate of copper. [Illustration: Fig. 223.—Daniell’s Battery.] [Illustration: Fig. 224.—Grove’s Battery.] The cut above illustrates Daniell’s Battery (fig. 223) with connectors. 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.