Lessons in Wireless Telegraphy

by Alfred Powell Morgan

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.