15. The Production of Electricity by Contact.- All electricity is basically the same, no matter how produced. Static electricity gains its name from the manner in which it is produced; i. e., by rubbing together two insulating materials, such as a glass rod and piece of silk. When two different materials are rubbed together and then separated, they are both found to be electrically charged. One of the materials is charged to the opposite polarity from the other. This is caused by the electrons of one of the substances being rubbed off and gathered by the other. Since the materials are insulators, the charge does not flow off and they remain electrically charged. The charge is therefore said to be "static." If touched by another substance they will discharge to it a portion of their charge. If touched to the earth they will give up their entire charge. Electricity produced in this manner has very little value because of the inefficiency of its generation, although it may be very annoying when produced where it is not wanted, as in the case of belt driven machinery, or in nature in the form of lightning.

16. The Production of Electricity by Chemical Action.- Electricity can be generated by chemical means. Examples of electricity generated in this manner are shown in the use of (so called) dry batteries, wet batteries and storage batteries.

Two different substances, such as copper, zinc, carbon, etc., immersed in a dilute solution of acid, constitute an electric cell. The two solid substances are called "plates" and the acid solution is called the "electrolyte." The plates extend above the solution for making external connections. If the two plates are connected together by means of a conductor a current will flow through the connection. The energy is furnished by the action of the acid on one of the plates, which is eaten away as the current circulates. The current will continue until a plate is entirely eaten away or the active element of the acid is used up. In order to renew the cell it is necessary to replace the consumed plate and the acid solution. A common example of such a cell is the ordinary "dry" cell with zinc and carbon plates. The acid solution, instead of being liquid, is a paste formed by impregnating absorbent material with the acid solution. If the direction of current through such a cell is reversed, neither the acid nor the plate will be restored, so that such a cell can not be used as a storage battery. This type of electric cell is called a "primary cell."

In certain types of cells the original condition can be restored, after the cell has been discharged, by forcing current through it in the opposite direction to that in which it delivers current. In such a cell neither plate is eaten by the acid, but the chemical compositions of the plates change, and, although the acid is weakened as the cell discharges, it is restored to its original strength when the cell is recharged. This type of cell is known as a "storage cell." The commercial name "storage battery" is derived from the fact that several cells are arranged together to form a "battery" of cells. A storage battery does not actually store electricity, but stores energy by chemical means which is readily changed to electrical energy when the proper external connections are made to the battery.

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