paragraph of Art. 80. These laws only relate to the useful chemical action, and do not include the waste of "local" actions (Art. 166) due to parasitic currents set up by impurities.

LESSON XV.- Voltaic Cells

179. A good Voltaic Cell should fulfil all or most of the following conditions:

1. Its electromotive-force should be high and constant.

2. Its internal resistance should be small.

3. It should give a constant current, and therefore must be free from polarization, and not liable to rapid exhaustion, requiring frequent renewal of the acid.

4. It should be perfectly quiescent when the circuit

is open.

5. It should be cheap and of durable materials.

6. It should be manageable, and if possible, should

not emit corrosive fumes.

No single cell fulfils all these conditions, however, and some cells are better for one purpose and some for another. Thus, for telegraphing through a long line of wire a considerable internal resistance in the battery is no great disadvantage; while, for producing an electric light, much internal resistance is absolutely fatal. The electromotive-force of a battery depends on the materials of the cell, and on the number of cells linked together, and a high E.M.F. can therefore be gained by choosing the right substances and by taking a large number of cells.. The resistance within the cell can be diminished by increasing the size of the plates, by bringing them near together, so that the thickness of the liquid between them may be as small as possible, and by choosing liquids that are good conductors.

180. Classification of Cells. Of the innumerable forms of cells that have been invented, only those of first importance can be described. Cells are sometimes classified into two groups, according as they contain one or two fluids, or electrolytes, but a better classification is that adopted in Art. 177, depending on the means of preventing polarization.

CLASS I.. - WITH MECHANICAL DEPOLARIZATION.

(Single Fluid.)

The simple cell of Volta, with its zinc and copper plates, has been already described. The larger the copper plate, the longer time does it take to polarize. Cruickshank suggested to place the plates vertically in a trough, producing a more powerful combination. Dr. Wollaston proposed to use a plate of copper of double size, bent round so as to approach the zinc on both sides, thus diminishing the resistance, and allowing the hydrogen more surface to deposit upon. Smee, as we have seen, replaced the copper plate by platinized silver, and Walker suggested the use of plates of hard carbon instead of copper or silver, thereby saving cost, and at the same time increasing the electromotive-force. The roughness of the surface facilitates the escape of hydrogen bubbles. By agitating such cells, or raising their kathode plates for a few moments into the air, their power is partially restored. The Law cell, used in the United States for open-circuit work, is of this class: it has a small rod of zinc and a cleft cylinder of carbon of large surface immersed in solution of salammoniac.

CLASS II. WITH CHEMICAL DEPOLARIZATION.

In these cells, in addition to the dilute acid or other excitant to dissolve the zinc, there is added some more

powerful chemical agent as a depolarizer. Amongst depolarizers the following are chiefly used: - Nitric acid, solutions of chromic acid, of bichromate of potash, of bichromate of soda, of nitrate of potash, or of ferric chloride; chlorine, bromine, black oxide of manganese, sulphur, peroxide of lead, red lead, oxide of copper. Most of these materials would, however, attack the copper as well as the zinc if used in a zinc-copper cell. Hence they can only

be made use of in zinccarbon or zinc-platinum cells. Nitric acid also attacks zinc when the circuit is open. Hence it cannot be employed in the same single cell with the zinc plate. In the Bichromate Cell, invented by Poggendorff, bichromate of potash is added to the sulphuric acid. This cell is most conveniently made up as shown in Fig. 100, in which a plate of zinc is the anode, and a pair

Fig. 100.

of carbon plates, one on each side of the zinc, joined together at the top serve as a kathode. As this solution would attack the zinc even when the circuit is open, the zinc plate is fixed to a rod by which it can be drawn up out of the solution when the cell is not being worked.

To obviate the necessity of this operation the device is adopted of separating the depolarizer from the liquid into which the zinc dips. In the case of liquid depolarizers this is done by the use of an internal porous cell or partition. Porous cells of earthenware or of parchment

paper allow the electric current to flow while keeping the liquids apart. In one compartment is the zinc anode dipping into its aliment of dilute acid: in the other compartment the carbon (or platinum) kathode dipping into the depolarizer. Such cells are termed two-fluid cells. In the case of solid depolarizers such as black oxide of manganese, oxide of copper, etc., the material merely needs to be held up to the kathode. All solid depolarizers are slow in acting.

CLASS III. - WITH ELECTROCHEMICAL DEPOLARIZA

TION.

It

When any soluble metal is immersed in a solution of its own salt-for example, zinc dipped into sulphate of zinc, or copper into sulphate of copper - there is a definite electromotive-force between it and its solution, the measure of its tendency to dissolve. If a current is sent from metal to solution some of the metal dissolves; if, however, the current is sent from solution to metal some more metal will be deposited (or "plated ") out of the solution. But as long as the chemical nature of the surface and of the liquid is unchanged there will be no change in the electromotive-force at the surface. follows that if a cell were made with two metals, each dipping into a solution of its own salt, the two solutions being kept apart by a porous partition, such a cell would never change its electromotive-force. The anode would not polarize where it dissolves into the excitant; the kathode would not polarize, since it receives merely an additional thickness of the same sort as itself. This electrochemical method of avoiding polarization was discovered by Daniell. It is the principle not only of the Daniell cell, but of the Clark cell and of others. For perfect constancy the two salts used should be salts of the same acid, both sulphates, or both chlorides, for example. 181. Daniell's Cell. - Each cell or "element" of

Daniell's battery has an inner porous cell or partition to keep the separate liquids from mixing. The outer cell (Fig. 101) is usually of copper, and serves also as a copper kathode. Within it is placed a cylindrical cell of unglazed porous ware (a cell of parchment, or even of brown paper, will answer), and in this is a rod of amalgamated zinc as anode. The liquid

spare

in the inner cell is dilute sulphuric
acid or dilute sulphate of zinc;
that in the outer cell is a saturated
solution of sulphate of copper
("blue vitriol "), some
crystals of the same substance
being contained in a perforated
shelf at the top of the cell, in
order that they may dissolve and
replace that which is used up
while the battery is in action.

Fig. 101.

When the circuit is closed the zinc dissolves in the dilute acid, forming sulphate of zinc, and liberating hydrogen; but this gas does not appear in bubbles on the surface of the copper cell, for, since the inner cell is porous, the molecular actions (by which the freed atoms of hydrogen are, as explained by Fig. 266, handed on through the acid) traverse the pores of the inner cell, and there, in the solution of sulphate of copper, the hydrogen atoms are exchanged for copper atoms, the result being that pure copper, and not hydrogen gas, is deposited on the outer copper plate. Chemically these actions may be represented as taking place in two stages.

Zn + H2SO

=

ZnSO + H2

Zinc and Sulphuric Acid produce Sulphate of Zinc and Hydrogen.