consists of restoring the chemicals in the cell, discharging involves the breaking down of the active chemicals.
The degree of charge in a secondary cell can be ascertained by measuring the specific gravity of the electrolyte. As the cell discharges, more and more water is formed and the lighter the electrolyte becomes.
The device used to measure specific gravity is the “hydrometer.” It is nothing than a glass tube with a float inside it. The float has a graduated scale reading from 1,100 near the top to 1,300 near the bottom.
In testing a cell, a portion of the electrolyte is drawn into the glass tube, and the float rises in the liquid. (See Fig. 14.) If the liquid is heavy the float will rise farther out of the liquid than if it is light. The lighter the electrolyte, the more water is in it.
A hydrometer reading between 1,250 and 1,300 indicates that the cell is at least half charged. A reading of less than 1,150 indicates that electrolyte is almost all water, that no further discharge should be allowed, and that the cell should be put on charge.
THE EDISON BATTERY
While the lead type of storage cell is very common, another type of battery, the Edison battery, has many advantages over the lead type which makes it particularly adapted to use on board ship. The Edison cell employs an alkaline electrolyte, a solution of potassium hydroxide, to which flake nickel is added, instead of the sulphuric acid solution used in the lead cell. The advantage of this electrolyte is that it has no appreciable wearing effect on the plates, and so the Edison cell will outlast many lead cells.
The electrodes of the Edison cell are more complex than the lead electrodes--the positive plate consists of tubes into which the active ingredients, nickelous hydroxide and flake nickel are packed, securely held in place on a steel grid (framework). The negative plate consists of a steel grid divided into compartment s for the insertion of strips of perforated nickel-plated steel ribbons. The active materials are pure iron which has undergone several chemical processes, mixed with 6% of yellow oxide of mercury. This is placed on the perforations during the “formation” process.
