All chemical action is accompanied by electrical phenomena. The electricity furnished by the galvanic combinations in use is derived from a chemical action which takes place in the elements. These are of various forms. Reduced to its simplest condition, a galvanic combination consists of two substances, one of which can be acted on chemically, while the other has merely the conducting property ; and also of a material—a fluid, usually— which can excite the chemical action. There is, then, a generating plate or element, usually of zinc, a conducting plate or element of copper or carbon, usually, and a fluid or semi-solid which acts on the zinc, setting up the chemical action. When such elements with the exciting fluid are placed in a glass or earthen vessel, the whole combination is called a galvanic cell or couplet, and when several of these are united they form a battery (Fig. 3).

Fig. 3. Galvanic Battery.

Galvanic cells may have a single or two fluids : the single fluid is not constant ; in the two-fluid, there are arrangements, partly mechanical, chiefly chemical, for securing constancy in the current. In Fig. 4 we have two metals—zinc, z, and copper, c—united by the wire, M, placed in a glass vessel containing dilute sulphuric acid. The chemical action in such a cell consists in the formation of the sulphate of zinc, the water being decomposed, and hydrogen appearing at the surface of the copper. Such an arrangement may be defined to be a means of making a difference in potential between two points.

Fig. 4. A Cell.

Electricity flows from the higher to the lower potential. The point where the chemical action is taking place—the surface of the zinc—is the higher potential, and hence "the current" passes from this through the liquid to the lower potential, the copper, which is the conducting plate. Now, as the current always flows from the higher to the lower potential, it follows that, outside of the element, the copper becomes positive and the zinc negative, for the current passes through the "conjunctive wire" from the former element to the latter. It follows that such an "element," or "cup," can not furnish a constant current. The chemical action soon rises to the maximum, the sulphuric acid combines with the zinc, and hence the exciting fluid is soon nothing more than a saturated solution of zinc sulphate, while the hydrogen accumulates on the copper plate. The chemical action, therefore, quickly subsides, and the hydrogen-bubbles hinder the passage of the current by the conducting plate. Elements of this kind are usually employed to run faradic machines, but they are not suited for galvanic batteries, since the tension of the current varies so much in a short time, and the action soon ceases.

Fig. 5. Daniell's Element.

The two-fluid cells, in which the chemical action is less violent, and mechanical devices prevent the polarization of the hydrogen, are alone suited to medical uses. Only those ascertained by experience to be adapted to medical purposes can be referred to. One of the earliest two-fluid cell batteries constructed was that of Daniell (Fig. 5). The arrangement of the parts in this cell will illustrate the principles concerned in such galvanic combinations. The zinc and copper elements and the two liquids are separated by a porous cup of unglazed earthenware. The zinc, z, is outside, and is a cylinder having a cleft; about it is diluted sulphuric acid (1 part to 16 of water). The copper, c, is contained in the porous cup, and is surrounded by a saturated solution of copper sulphate, which is kept at the point of saturation by a quantity of the crystals packed around the copper element. The polarization of the hydrogen is prevented by chemical means, for in the decomposition of copper sulphate the hydrogen combines with oxygen to form water, while metallic copper is deposited on the copper element. The Daniell, as modified by the celebrated Dr. Remak, of Berlin, has been more widely used for medical purposes than any other cell, and still maintains its superiority. In this arrangement made by Siemens and Halske, of Berlin, and known under their name, besides the porous cup, a quantity of papier-maché, or paper-pulp, is packed in between the elements, and, while the copper is surrounded by copper sulphate in solution, only water is used with the zinc. In the decomposition, copper is deposited on the copper element and sulphuric acid diffuses through into the zinc compartment. This cup is remarkable for the uniform tension of the current, for constancy, and for economy. A "gravity battery," composed of zinc and copper elements, zinc sulphate solution about the zinc, and copper sulphate solution about the copper, and separated merely by the specific gravity of the respective solutions, is now much employed in telegraphy, and to a considerable extent in medical practice. Zinc and carbon are now utilized in two-fluid as in one-fluid cells.