ELECTRIC DIRECT CURRENT, as distinguished from the alternating current, is so-called because of the fact that it travels in one direction along a conductor. If this con ductor joins the terminals of a source of energy, as a dynamo, the current is said to flow from the positive pole of the machine along the conductor to the negative pole.
Probably the first man to detect current elec tricity was Galvani about the year 1786. To Volta (q.v.), however, is certainly due the credit of first developing a practical electro chemical cell. In the year 1800 Volta exhib ited a cell known as the "Voltaic Pile," consist ing of a series of discs, copper and zinc, alter nately separated from each other by a cloth saturated with brine; on joining wires to the end discs, quite a perceptible shock may be felt by touching with the tongue or moistened fin ger the two terminals simultaneously. This simple device was the starting point of all the electrochemical batteries of the present day. With the discovery of Volta of the laws of difference of potential between different metals when placed in contact orjoined by a fluid electrolyte began the development of very many varieties of cells, all on the same prin ciple; yet even now, the two metals he chose, zinc and copper, constitute the elements of the Daniell cell very frequently used for telegraphic purposes. The changes which would readily suggest themselves in Volta's first cell would be, increasing the amount of corroding liquid and placing the elements, .zinc and copper, in a vessel which would properly contain the fluid.
The theory as given by Gore of the electro chemical cell is as follows: "The essential cause is the stored-up and ceaseless molecular energy of the corroded metal and of the corroding element or liquid with which it unites, while contact is only a static condition; and chemical action is the process or mode by which the molecular motion of these substances is more or less transformed into heat and current" The electromotive force of chemical genera tors is small, rarely exceeding two volts per cell. This necessitates a large number of cells connected in series; that is, the positive ter minal of one connected with the negative ter minal of the adjoining cell, the electromotive force thus produced being the product of the electromotive force of one cell by the number of cells. By connecting the two positive and
the two negative terminals of two rows of cells, an increased quantity of current can be ob tained, at the potential of one row. The first method is called joining battery cells for inten sity, and the second for quantity. It is known that the energy generated in a chemical cell is produced by the consumption of zinc. The cost of this energy must necessarily be high, as both the zinc and the chemicals are expensive, so that the use of current electricity was quite limited until the introduction of the dynamo electric machine, which might be called the me chanical method of transforming energy from some source, such as a steam-engine, into cur rent electricity, as contrasted with the chemical method.
In the year 1831 Faraday discovered and an nounced the principle of electromagnetic in duction. This opened up the field of what might be called the commercial generation of current electricity. The principle discovered by Faraday, which forms the basis of all dynamo electric machines, is that if a wire is moved in a magnetic field, so as to cut the lines of force, a current will be generated in the wire, and it is upon this principle that all dynamo electric machines depend for their action. The con verse of this law he also announced, namely, that when an electric current is applied to the dynamo by some external source such as a bat tery or another dynamo, the machine will fur nish mechanical power. Hence a dynamo elec tric machine may be considered either as a generator or as a motor.
All dynamos consist of two essential parts, one, the field magnet, which is usually station ary, and the other, the armature on which the copper conductors are mounted and which re volves on a shaft between the poles of the field magnet. This armature is so arranged as to cut the lines of force flowing between the mag netic poles. The lines of force are imaginary lines flowing from the north pole to the south pole of any magnet. They can easily be traced by placing a piece of paper above the mag net and sprinkling on this paper iron filings.