chemical energy possessed, is cheaper than any other metal which could be used.
2601. Batteries, as sources of electrical energy, are used mainly in cases where a current is required very intermittently, such as in ringing bells, lighting gas, etc., or where a small but steady current is required for long periods of time, as in telegraphy and telephony, or for laboratory and testing purposes. Their general use on a large scale, as sources of electrical energy for lighting or power purposes, is prohibited, at least at present, by the comparatively great cost of the material consumed, and the expense of installation and maintenance.
For example, the bichromate battery is about the cheapest in point of cost of materials consumed, and in this the materials alone would cost about 28 cents per horsepower per hour on a large scale, while the cost of electrical energy, using dynamos, is about 5 or 6 cents per horsepower per hour, ordinarily, and in many cases is much less. The cost of material in the silver chloride battery is about $135 per horsepower per hour.
This high cost of the power does not, however, prevent batteries from being largely used for the purposes outlined above, and their practical application is an important part of electrical engineering.
CELLS.
CLASSIFICATION.
2602. The various classes of voltaic cells may be divided as follows:
Cells in Which There Is No Depolarizer.—This is the simplest form of cell, and, on account of polarization, cells of this class, commonly called open-circuit cells, are not used for other than intermittent work.
2603. Cells With a Depolarizing Electrolyte.—In this class of cells the electrolyte is of such a nature that either no hydrogen is formed or the liquid contains a sub-
