BATTERY CELLS.

622. Battery Cells.-A battery cell is a combination in which electromotive force is produced by chemical action. The simple cell of Volta is the earliest type, but it has important practical defects.

An ideal cell will have:

1. Small resistance.

2. Large electromotive force.

3. A constant electromotive force whatever the current. 4. No local action or wasteful chemical action.

623. Resistance of Battery Cells.-When the electrode plates are large and close together the resistance of the cell is small. While if the plates are very small the resistance of the cell may be so great that even when the poles are short-circuited or connected by a short copper wire offering very little resistance, the current will be extremely small.

Cells from which large currents are to be obtained must therefore have large plates separated by a comparatively thin layer of electrolyte.

624. Local Action.-If commercial zinc is used in a Voltaic cell hydrogen gas will be given off at the surface of the plate as soon as it is placed in the acid and before it is connected with the copper plate.

This is accompanied by a corresponding wearing away of the zinc and formation of zinc sulphate, which goes into solution. This wasting of the zinc is called local action and is due to impurities. Suppose that a particle of iron or carbon imbedded in the surface of the zinc is in contact both with the zinc and acid; it forms a minute Voltaic cell, in which the current flows from the iron or carbon to the zinc and through the acid from zinc to iron again, as indicated in the figure, and zinc is eaten away near the Local action. impurity and hydrogen set free at its surface.

FIG. 351.

To prevent local action the zinc surface is freshly amalgamated with mercury, which dissolves the zinc, covers up the impurities, and presents a homogeneous surface to the acid.

625. Polarization.-When the poles of a simple Voltaic cell are connected by a wire, the current does not remain constant but rapidly decreases in strength.

This weakening of the current is due to polarization. The

hydrogen set free at the copper electrode forms a sort of gaseous layer over the plate which interferes with the action of the cell in two ways. In the first place, the resistance of the cell is increased, for the flow of electricity is interfered with by the bubbles of gas. In the second place, the electromotive force of the cell is diminished, for the hydrogen layer is much more like zinc in its relation to the acid than is the copper which it covers. This difficulty is most effectively met by the use of two electrolytes.

626. Primary and Secondary Battery Cells.-Cells such as the Voltaic cell in which the current is obtained from the chemicals of which the cell was originally constructed are known as primary cells, while cells in which the chemical state necessary for the production of a current is produced by sending through the cell a current from some outside source for a certain length of time, are known as secondary batteries, storage cells, or accumulators.

A few of the cells most commonly used in practice will now be considered.

Primary Battery Cells.

627. The Daniell Cell.-One of the first and most useful two fluid cells was devised by Daniell in 1836. It consists of a copper electrode immersed in a solution of copper sulphate and an electrode of amalgamated zine immersed in dilute sulphuric acid, the two being separated by a partition of porous earthenware. In figure 352 the copper electrode with its solution is represented as contained in a cup of porous earthenware surrounded by the zinc and dilute acid.

When the circuit is closed, the positively charged zinc atoms pass into solution form

ing zinc sulphate with the negative SO, ions, FIG. 352.-Daniell's cell. while the positively charged hydrogen ions

(H) in the acid move toward the copper plate, passing through the porous cup by diffusion and forming sulphuric acid (H2SO1) with the negative SO, ions from the copper sulphate, and dis

placing the positive copper ions (Cu) which give up their charges and are deposited on the copper plate.

In the dilute acid

In the copper sulphate

Thus zinc is dissolved and zinc sulphate formed, copper sulphate is used up and copper deposited on the copper electrode. There is no hydrogen layer formed on the copper and consequently no polarization. The electromotive force of this cell is about 1.08 volts.

628. Gravity Cell.-A form of Daniell cell which has been. extensively used in telegraphy and is still much used where a

small constant current of electricity is required is the gravity cell, so called because the liquids are kept separate by gravity alone, the denser copper sulphate solution resting at the bottom of the cell, while the lighter acid or zinc sulphate solution floats above it.

If the gravity cell stands without being used the copper sulphate diffuses gradually up into the acid above and copper is

FIG. 353.-Gravity or crow-foot cell. deposited on the zinc, causing extensive local action. A small current, sufficient to balance the diffusion, should always be kept flowing while the cell is set up.

629. The Bunsen and Bichromate Cells. In these cells also, zinc in dilute sulphuric acid forms one electrode. In the Bunsen cell the positive pole is a plate of carbon immersed in strong nitric acid. Its electromotive force is about 1.90 volts.

The fumes from this cell are pungent and corrosive and it should never be used in a laboratory room with other instruments, but always under a well ventilated hood or out of doors.

The bichromate cell is like the Bunsen, except that the nitric acid around the carbon electrode is replaced by a solution of bichromate of potassium.

This cell is quite free from fumes, but must be taken down when not in use, on account of diffusion through the porous cup and local action on the zinc. It is a useful cell, however, and has an electromotive force of 2.0 volts.

630. Leclanché Cell. This very useful form of cell has a zinc and a carbon electrode. The carbon is packed in a porous cup with a mixture of fragments of carbon and black oxide of manganese; the zinc electrode is in a strong solution of ammonium chloride (sal ammoniac) which surrounds the porous cup.

The hydrogen which would polarize the carbon electrode combines with oxygen from the manganese dioxide and forms water. But as the depolarizing agent is

in the solid form its action is slow, so that this cell polarizes rapidly. It is extensively used, however, for open-circuit work, such as for bells, annunciators, and clocks, where a steady current is not required. It is entirely free from injurious or disagreeable fumes, there is but little local action, and no trouble from diffusion, so that the cells may stand set up for a year or two without attention, and ready for use at any instant. Its electromotive force is about 1.40 volts.

631. Modifications of the Leclanché Cell.-A variety of forms of Leclanché cells have been introduced by different makers. In some forms the manganese oxide and carbon are compressed into a solid electrode which is used in the ammonium chloride solution without any porous cup.

632. Dry Cells.-The so-called dry cells are ordinarily a form of Leclanché cell. The outer cylindrical cup forms the zinc electrode which is lined with thick absorbent paper and packed with the pulverized manganese dioxide and carbon mixture surrounding the central carbon rod. The whole is saturated with ammonium chloride solution and sealed with pitch to keep it from drying out.

633. Edison-Lalande Cell.-In this form of cell, devised by Lalande and improved by Edison, the positive plate is a tablet of compressed black oxide of copper, while the negative plate is of zinc. These are immersed in a strong solution of caustic potash, which is covered with a thick layer of heavy oil to prevent evaporation and the creeping up of the solution on the sides and plates. The plate of copper

oxide acts both as electrode and depolarizer, the hydrogen which is set free at that pole reducing the copper oxide to metallic copper.

When the cell is exhausted both plates as well as the liquid must be renewed.

This cell does not polarize and may be used where a steady current is required and where a Daniell or gravity cell would have too much resistance. It has the further advantage that it is quite free from local action and may be left standing without deterioration when no current flows. Its electromotive force is low, being about 0.75 volt.

Secondary Cells.

634. Grove's Gas Battery.-The English physicist Grove showed that when in the decomposition of water long electrodes were used, extending to the tops of the tubes in which the gases were collected, as in figure 355, on changing the switches ss' to the dotted positions, thus disconnecting the battery B and

H

FIG. 355.

simply joining the two electrodes together through a galvanometer G, a current was obtained which was in the opposite direction to the decomposing current. At the same time a gradual recombination of the hydrogen and oxygen took place until these gases had entirely disappeared.

Long electrodes were necessary since each electrode must pass through the surface where the gas and electrolyte meet.

635. Planté Cell. If a current of electricity is sent through a cell consisting of two plates of sheet lead in dilute sulphuric acid it becomes polarized, one plate becoming oxidized while hydrogen is set free at the surface of the other, reducing any