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Page:Popular Science Monthly Volume 24.djvu/268

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THE POPULAR SCIENCE MONTHLY.

great fall in the insulation resistance, and a slight fall in the apparent resistance of the copper conductor, between the two stations; but messages can be still transmitted, as a part only of the whole current, inversely proportional to the resistance of the fault, escapes into the ocean. If one office insulates the cable, and the other measures the resistance, the fault acts like a fault that is caused by the fracture of both the copper wire and the gutta-percha, but little of the copper core being exposed.

The fifth kind of fault corresponds almost exactly in behavior to a fault caused by fracture of the copper conductor and gutta-percha, in which a considerable portion of the length of copper wire remains exposed to the water. The resistance will vary still less; and there will be a total absence of the feeble currents which result when the copper and iron of a cable are broken and separated by salt water.

Submarine or ocean telegraphy holds a very prominent place in electrical engineering, and the instruments used in it are interesting. In instructing pupils a very curious apparatus is used. It is the artificial or dummy cable, consisting of a number of "resistance-coils," and condensers so arranged as to reproduce all the phenomena and all the practical difficulties that are presented by a real ocean-cable. With a good instructor, this piece of apparatus is of very great service, inasmuch as all kinds of imperfections can be readily and correctly imitated in any part of the circuit.

Still greater interest, perhaps, attaches to the apparatus for showing the retardation that a current experiences in traversing a long cable. This apparatus consists of a series of "resistance-coils," "rheostats," and condensers, having small receiving instruments at a dozen different points in the circuit, representing as many different offices on the line. The receiving instruments are similar to the mirror portion of Sir William Thomson's mirror galvanometer. In this a ray of light falls upon a very small mirror attached to a small magnet; and this rotates around a vertical axis when acted upon by a current that circulates in a coil of wire. These magnets, with the mirrors attached moving one after the other, indicate the time taken in charging the whole length of the circuit.

I. The Storage of Electricity.—Another principal branch of electrical engineering, promising much in the near future, is the great French discovery of the storage of electrical energy. It is among the most important inventions of the last thirty years. The electrical storage of energy must not be confounded with the storage of electricity. An electrical storage-battery is an apparatus for transforming electricity; in it electrical energy is no longer produced directly, but changes its properties. A given source furnishes a certain volume or quantity of electricity, at a certain pressure or tension. In certain instances, it may be important to increase one of these prop-