Man's physical universe

xanabras

ELECTRON TRANSFER 593

current of electricity ceases because this layer of gas is a nonconductor

and also because the electrode now becomes a hydrogen electrode

instead of the metallic electrode that it was before and therefore has a

different potential. The formation of such a layer of gas around an

electrode is called polarization.

Consider, for example, a simple cell consisting of a zinc rod and a

carbon rod dipping into a solution of ammonium chloride. Zinc is

above carbon in the electromotive series and goes into solution to form

zinc ions, thus giving up electrons. In the solution there are ammonium

ions, (NH4)+, and hydronium ions, (H80)+. Inasmuch as the hydronium

ions accept electrons

more readily

than ammonium ions, the hydronium

ions accept electrons when

the electrodes are connected by a

metallic conductor and form water,

H2O, and hydrogen gas, H2. This

cell quickly polarizes as a result.

The ordinary, general-purpose

dry cell consists of a can of sheet

zinc acting as one electrode and a

carbon rod acting as the other

electrode. The two electrodes are

in contact with a solution of ammonium

chloride and zinc chloride.

This cell is, therefore, substantially

Paper Felt-

Carbon

Fig. 282.

DRY CELL

NH4 Cl + MnO,

•Zinc

Diagram of a dry cell.

the same as the one mentioned above, but the carbon rod in this

case is surrounded by a moist mixture of granulated carbon, solid

ammonium chloride, and manganese dioxide. The manganese dioxide

acts as a depolarizer, preventing the formation of hydrogen.

In

1940 there was announced the development of a portable flashlight,

smaller than a man's head, which is so powerful that it is possible

to read a newspaper by its light half a mile away.

It produces a beam

of light of 180,000 candle power.

The dry cell, like most chemical cells which require a depolarizer, is

irreversible; i.e., it cannot be charged by running a current into it,

and it must be thrown away when it is exhausted. The lead storage

battery is an example of a reversible cell. It consists of an electrode of

lead and another of lead peroxide, Pb02, in contact with sulfuric acid.

During discharge both electrodes become coated with lead sulfate,

and the sulfuric acid in the solution is used up in the process, thus

decreasing the chemical dissimilarity and the potential difference. Inasmuch

as the solution of sulfuric acid decreases in specific gravity

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