What a storage battery does—The lead battery and the Edison battery—Economy of tungsten lamps for storage batteries—The low-voltage battery for electric light—How to figure the capacity of a battery—Table of light requirements for a farm house—Watt-hours and lamp-hours—The cost of storage battery current—How to charge a storage battery—Care of storage batteries.
For the man who has a small supply of water to run a water wheel a few hours at a time, or who wishes to store electricity while he is doing routine jobs with a gasoline engine or other source of power, the storage battery solves the problem. The storage battery may be likened to a tank of water which is drawn on when water is needed, and which must be re-filled when empty. A storage battery, or accumulator is a device in which a chemical action is set up when an electric current is passed through it. This is called charging. When such a battery is charged, it has the property of giving off an electric current by means of a reversed chemical action when a circuit is provided, through a lamp or other connection. This reversed action is called discharging. Such a battery will discharge nearly as much current as is required originally to bring about the first chemical action.
There are two common types of storage battery—the lead accumulator, made up of lead plates (alternately positive and negative); and the two-metal accumulator, of which the Edison battery is a representative, made up of alternate plates of iron and nickel. In the lead accumulator, the "positive" plate may be recognized by its brown color when charging, while the "negative" plate is usually light gray, or leaden in color. The action of the charging current is to form oxides of lead in the plates; the action of the discharging current is to reduce the oxides to metallic lead again. This process can be repeated over and over again during the life of the battery.
Because of the cost of the batteries themselves, it is possible (from the viewpoint of the farmer and the size of his pocketbook) to store only a relatively small amount of electric current. For this reason, the storage battery was little used for private plants, where expense is a considerable item, up to a few years ago. Carbon lamps require from 3½ to 4 watts for each candlepower of light they give out; and a lead battery capable of storing enough electricity to supply the average farm house with light by means of carbon lamps for three or four days at a time without recharging, proved too costly for private use.
The Tungsten Lamp
With the advent of the new tungsten lamp, however, reducing the current requirements for light by two-thirds, the storage battery immediately came into its own, and is now of general use.
Since incandescent lamps were first invented scientists have been trying to find some metal of high fusion to use in place of the carbon filament of the ordinary lamp. The higher the fusing point of this filament of wire, the more economical would be the light. Edison sought, thirty years ago, for just the qualities now found in tungsten metal. Tungsten metal was first used for incandescent lamps in the form of a paste, squirted into the shape of a thread. This proved too fragile. Later investigators devised means of drawing tungsten into wire; and it is tungsten wire that is now used so generally in lighting. A tungsten lamp has an average efficiency of 1¼ watts per candlepower, compared with 3½ to 4 watts of the old-style carbon lamp. In larger sizes the efficiency is as low as .9 watt per candlepower; and only recently it has been found that if inert nitrogen gas is used in the glass bulb, instead of using a high vacuum as is the general practice, the efficiency of the lamp becomes still higher, approaching .5 watt for each candlepower in large lamps. This new nitrogen lamp is not yet being manufactured in small domestic sizes, though it will undoubtedly be put on the market in those sizes in the near future.
The Fairbanks Morse oil engine storage battery set