To convert this alternating phase into what is called a direct current, certain dynamos are provided with a commutator, and the function of this commutator, which has two oppositely-disposed fingers contacting therewith, is to so divert the alternating impulses that they will go over the wire in one direction only.
Current from Batteries.—Currents derived from batteries do not have the alternating flow. Instead, the movement is in one direction only, and it is in connection with this method of producing electricity that the terms positive and negative are found convenient in describing the current, and the action of the mechanism operated by it.
Primary Battery.—The primary battery is one which generates an electric current. It comprises one or more pairs of plates, of which zinc and copper are examples, although other couples are found to be equally serviceable.
Two metals, or materials, such as carbon and zinc, are selected, which are termed electrical opposites, or which are positive-negative to each other, and when such couples are immersed in an electrolyte a current will be set up between the two plates, if a wire is attached to each plate, and the outer ends of these wires brought together, a continuous current will flow through the wire.
The electrolyte is a solution of water, with a small amount of sulphuric acid. Numerous acid solutions are made, and salt, or saline solutions are also frequently employed.
Secondary, or Storage Batteries.—These are also called Accumulators, because they are so constructed that they will accumulate a certain charge. The term Secondary is used to indicate the idea that they receive their charge from an outside source, in distinction from a Primary, which generates its own current.
After the secondary is once charged it then begins to work on its own account, the same as a primary battery.
Reversal of Currents.—When a storage battery is being charged from an outside source of electricity, the current flows within the battery in one direction; but the moment the outside source is discontinued, and the battery itself is connected up with mechanism, it becomes a source of energy, but the current output is in the opposite direction.
The foregoing suggestions and features of explanation are thought to be desirable, in view of the following statements which pertain to the operation of machines of this type.
Charging.—One of the most important things in the care and handling of machines of this character, is the charging of the batteries. The utmost caution must be exercised to prevent derangement of the batteries.