A high tension current is capable of forcing its way against considerable resistance, whereas, a low tension current must have its path made easy.
Production of the Electric Current.—To produce a steady flow of water in a pipe two conditions are necessary. There must first be available a hydraulic pressure, or, as it is technically called, a “head” of water produced by a pump, or a difference of level or otherwise.
In addition to the pressure there must also be a suitable path or channel provided for the water to flow through, or there will be no flow, however great the “head,” until something breaks down under the strain. In the case just cited, although there is full pressure in the water in the pipe, there is no current of water as long as the tap remains closed. The opening of the tap completes the necessary path (the greater part of which was already in existence) and the water flows.
For the production of a steady electric current two very similar conditions are necessary. There must be a steadily maintained electric pressure, known under different aspects as “electromotive force,” “potential difference,” or “voltage.” This alone, however, is not sufficient. In addition, a suitable conducting path is necessary. Any break in this path occupied by unsuitable material acts like the closed tap in the analogous case above mentioned, and it is only when all such breaks have been properly bridged by suitable material, that is, by conductors, that the effects which denote the flow of the current will begin to be manifested.
The necessary electromotive force or voltage required to cause the current to flow may be obtained:
1. Chemically; 2. Mechanically; 3. Thermally.
In the first method, two dissimilar metals such as copper and zinc called elements, are immersed in an exciting fluid or electrolyte.
When the elements are connected at their terminals by a wire or conductor a chemical action takes place, producing a current which flows from the copper to the zinc. This device is called a cell, and the combination of two or more of them connected so as to form a unit is known as a battery. The word battery is frequently used incorrectly for a single cell. That terminal of the element from which the current flows is called the plus or positive pole, and the terminal of the other element the negative pole.