THE CURRENT USED.--Storage batteries are charged with and use a _direct current_. The difference between a direct and an alternating current is, that in the first the current flows continuously over a wire in one direction, whereas in the latter it changes its direction, going, for an instant, from the north pole to the south pole, and the next instant from the south pole to the north pole, and for this reason it is said to _alternate_. MECHANICALLY-PRODUCED ELECTRICITY.--The alternating method is the natural form of flow in a current derived from mechanism, as, for instance, by means of a rotating armature. The electricity, in this case, is produced by a metallic body moving through a magnetic field, and as it passes through it takes up a certain electric impulse in one direction when the body approaches the field, and instantly reverses and flows in the opposite direction as the body recedes from the magnet, or field. 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.