Page:Popular Science Monthly Volume 92.djvu/328

312 terminal, at which the current emerges after passing through all the negative plates, during charging, and at which the current enters, on its return path on discharge, is called the cathode.

The conditions in operating a storage cell are most exacting, and serious complications are liable to follow violation of the rules. Thus, it must neither be charged nor discharged too quickly, there being safe average rates in both cases. It should never be over-charged or over-discharged beyond the safe limits prescribed. The voltage at full charge should not exceed 2.6 volts, and its working pressure not more than 2 volts. On normal discharge, it should not be permitted to fall below 1.75 volts. The discharge capacity of a storage cell (its current rate) is measured in ampere-hours, which are found by multiplying the current in amperes by the number of hours at which it can flow at not less than 1.75 volts. In order, however, to standardize, the normal discharge rate is assumed to be at 8 hours. In the main, the ampere-hour capacity decreases with increase in the current rate.

The ideal storage battery or accumulator is an electrolytic cell in which electrical energy may be stored as chemical energy until ready for use. It must be capable of returning at any time all, or any part, of the electrical energy put into it; and when discharged, the cell must be in its original condition. It is, therefore, necessary to find a perfectly reversible chemical reaction whose direction and energy relation is perfectly controlled by the electric current. That is, no chemical action should take place except that which necessarily accompanies the flow of useful current when on charge or discharge; and secondly, the quantity of material whose chemical composition is changed should be proportional to the quantity of electrical energy passed through the cell.

The nearness of the plates to each other and the large surface obtained by using a number of plates, cause the resistance of the cell to be very small. The greater the number and size of the plates in a cell, the larger the current that can be sent through it without injury to the cell.

The commercial importance of such storage cells is due in part to their extremely small resistance, and to the fact that they can be renewed not by means of costly chemicals, but by a current obtained from a dynamo, driven by engine or water-power.



HEN a dry battery of an electric door became exhausted and there was no sal ammoniac on hand to renew it, it was decided to try in its stead some hyposulphate soda. After removing the cardboard wrappers from the cells, a number of perforations were made in the zinc casing, then the cells were placed into wide-mouthed fruit jars, into each of which was put several spoonfuls of the salt and enough water to fill them within 1 in. of the top. If the water should cover the cells, it will cause a short circuit. Upon testing the battery, while the cells were still in the jars, it was discovered to be fully as powerful as one composed of new dry cells. Then the experiment was tried of using it just as it was—a new type of wet battery. It has proved so effective that after a year of use it has required no other attention than that of replacing the water evaporated. Thus, at no expense, excellent results have been obtained, and the cost of several dry cells saved.—

COMMON tin oil can may be made rust proof if it is given a coat or two of paint, especially on the bottom. It is worth while, occasionally when through painting, to wipe off the paint brush on the bottom of the old oil can, just before the brush is laid away.—