Читать книгу Lessons in Wireless Telegraphy - Alfred Powell Morgan - Страница 6

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If a piece of zinc is dipped in dilute sulphuric acid, the zinc will be attacked by the acid and replace hydrogen in it, the hydrogen appearing as bubbles on the zinc and passing off as a gas.

FIG. 6. Simple Voltaic Cell

If the zinc is connected by means of a wire, W, with a strip of copper, C, dipping in the same solution, the zinc will still to continue to dissolve but the hydrogen bubbles will now form on the surface of the copper strip as well as on the zinc. It will be found that the wire W becomes heated. If the copper and zinc are connected to a galvanometer it will show the presence of an electric current passing through the circuit. The cell may be considered as a sort of chemical furnace in which fuel is burned to drive the current. The zinc is the fuel. The copper is merely present to "pick up" the current and takes no part chemically.

If a number of such simple cells are properly united, the zinc of one being joined to the copper of the next and so on, a battery is formed. The current flows from the copper, called the positive pole, through the wires (when they are joined) to the zinc or negative pole and back to the copper through the solution.

The electricity generated by the cells exerts a certain pressure or tendency to pass through the wires. This tendency is called the potential. The potential is measured in volts. The potential (also called the electromotive force) in the case of the Voltaic Cell just described is 1.07 volts. If the copper strip is replaced with one of graphite or carbon, the voltage will rise to 1.73 volts.

After a cell has been in action for a short time, the positive plate (copper or carbon, as the case may be) becomes covered with a film of hydrogen. The cell is then said to be polarized. The film of gas bubbles partially shields the plate from contact with the liquid. When the plate becomes in this condition, the current is much feebler than when it is clear.

The most effective way of removing the hydrogen is to add some chemical to the sulphuric acid solution which will combine chemically with the hydrogen as soon as it appears. The usual substance is bichromate of potash. The voltage of the battery will rise to 2.2 volts and the polarization be stopped when bichromate of potash is added. The bichromate of potash enters into chemical action with the sulphuric acid and forms chromic acid. Such cells are usually termed chromic acid cells.

One of the principal disadvantages of a cell such as that just described lies in the fact that the zinc is continuously consumed whether the cell is in action or not and in order to prevent its rapid waste must be lifted out of the solution and washed each time after using.

Various methods have been devised for overcoming this objection, the most prominent of the resulting cells being known as the Fuller, Gordon and Edison-Lalande Cells.

FIG. 7. Edison Cell.

The liquid excitant of the Gordon and Edison-Lalande cells is a strong solution of sodium hydroxide. The positive pole of these cells is a block of compressed copper oxide and the negative a pair of zinc plates. In the Gordon cell the positive is enclosed in a porous chamber.

FIG. 8. Dry Cell.

One of the best known forms of cell is the dry cell. It consists of an outer shell of zinc forming the negative electrode and a central rod of carbon as the positive. The active agent of the cell is a paste composed principally of sal ammoniac lining the interior of the zinc shell. The depolarizing agent of the cell is manganese dioxide mixed with crushed carbon and packed tightly around the carbon rod. The cell is not as its name implies perfectly dry inside, but the chemicals are in paste form. The cell is sealed at the top by a bituminous compound making the cell air tight and portable. Dry cells are only successful for intermittent work, that is, where they are not required to deliver a heavy current continuously. They deteriorate after long standing because the moisture evaporates. Dry cells, however, are a very convenient source of current where the demand is not too great and portability is desired.

The cells so far described are all of the type known as primary cells.

SECONDARY CELLS.

The storage cell or secondary cell is made up of plates of lead, or an alloy of lead, cast in the form of a grid or framework of bars. The spaces formed in the plate by the little bars are filled with a paste of lead oxide. The paste for the positive plates are made of red lead while litharge is used for the negatives.

FIG. 9. A Storage Battery Grid.

The positive and negative plates are placed alternately in a bundle with a wooden or rubber separator between, there always being one more negative plate than positive. The negative plates are all connected in parallel at one end of the cell by means of lead connecting strips. The positive plates are connected at the other end. The plates are placed in a jar, usually glass or hard rubber, and covered with a dilute sulphuric acid solution.

The storage cell is then connected to a dynamo, the positive pole of the cell being connected to the positive pole of the dynamo and the current allowed to flow through until the plates are formed, that is to say, until the paste in the positive changes to peroxide of lead and that in the negative to spongy lead. When the cell is disconnected it will give out a current of its own lasting until it becomes discharged. The charging and discharging must be repeated several times before the cell really becomes efficient.

FIG. 10. Storage Cells.

What is effected in the storage cell is really the storage of chemical energy and not the storage of electricity, for, properly speaking, the energy is put into the form of chemical affinity and there is in reality no more electricity actually in the cell at the end of a charge than there is when the cell is discharged.

The storage battery is the most convenient means of absorbing electrical energy at one time or place and using it at another time or place.

Storage cells are very often employed in wireless stations for emergency purposes so that in case the dynamo supplying current fails the station will not be thrown out of operation.

The voltage of a storage cell is about two volts.