Page:Popular Science Monthly Volume 3.djvu/417

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the same inventors have been much more extensively used, the former requiring only one wire, and the latter two.

All public telegraphs have now for many years been worked by voltaic currents; the magneto-electric system, which was tried on some lines, having been found to involve a needless expenditure of labor.
Fig. 1.
PSM V03 D417 Insulators.jpg
According to Mr. Culley, engineer-in-chief to the post-office, the battery which had been adopted by the authorities of that department is a modified Daniell's, consisting of a teak trough, divided into cells by plates of glass or slate, and well coated with marine glue, each cell being divided into two by a slab of porous porcelain. The zinc plates measure four inches by two, and the copper plates, which are very thin, are four inches square. The zinc hangs at the upper part of its cell, which is filled with dilute solution of sulphate of zinc. The copper cell is filled with a saturated solution of sulphate of copper, and crystals of this salt are placed at the bottom. The expenditure in sulphate of copper is about a pound and a half for each cell per annum.

The wires for land-telegraphs are commonly of what is called galvanized iron, that is, iron coated with zinc, supported on posts by means of glass or porcelain insulators, so contrived that some part of the porcelain surface is sheltered from rain, and insulates the wires from the posts, even in wet weather. Wires thus suspended are called air-lines.

Underground wires are, however, sometimes employed. They are insulated by a coating of gutta-percha, and are usually laid in pipes, an arrangement which admits of their being repaired or renewed without opening the ground except at the drawing-in boxes. There is less leakage of electricity from subterranean than from air lines, but their cost is greater, and they are less suited for rapid signalling on account of the retardation caused by the inductive action between the wire and the conducting earth, which is similar to that between the two coatings of a Leyden jar.

The early inventors of electric telegraphs supposed that a current could not be sent from one station to another without a return-wire to complete the circuit. Steinheil, while conducting experiments on a