Great Facts / A Popular History and Description of the Most Remarkable Inventions During the Present Century

by Frederick C. Bakewell

The successful laying down of so frail a cable, after many failures, affords good ground for hoping that, with the experience already gained, subsequent efforts will prove more satisfactory and much less expensive than this first attempt to establish telegraphic communication with America. The most questionable part of the problem has, indeed, been already solved; for the transmission of electric signals, through that length of submerged wire, was at one time doubted; and though the communication through the present cable has ceased, it has sufficiently established the fact, that telegraphic communication with America is a practicable undertaking.

The excellent insulation obtained by means of gutta percha covered wires has caused a return to the original plan of burying the wires in trenches in the ground. The British and Submarine Telegraph Company make all their communications by that means; the number of coated wires required being enclosed in iron tubes, and laid in the ground along the common roads. That plan is, however, attended with considerable disadvantages. In the first place, the cost of the coated copper wire is more than quadruple that of galvanized iron wire; and though copper, compared with iron, offers only one-seventh part the resistance to the transmission of electricity, yet the thin wire employed is scarcely equal in conducting power to the galvanized iron wire usually supported on posts. The quantity of electricity transmitted is therefore less, and the comparative intensity of it is greater.

Another difficulty attending the use of insulated wires buried in the ground arises from a very peculiar condition of electrical conduction, that could scarcely have been anticipated. The wire, coated with gutta percha, and surrounded externally with water or with moist earth, becomes an elongated Leyden jar; the gutta percha representing the glass, the wire the inside coating, and the water the conducting surface outside. Thus, when electricity is transmitted through such a medium, a portion of the charge is retained after connection with the battery has been broken. This effect increases with the length of the wire and the intensity of the current; and it materially interferes with the working of many telegraph instruments. In some experiments with the copying telegraph at the Gutta Percha Works in the City Road, it was found that through a circuit of 50 miles of wire immersed in water, the mark made by electro-chemical decomposition on paper had a tendency to become continuous; so that instead of ceasing to mark, when the varnish interrupted the current, a line was drawn continuously on the paper, though the stronger marks where the current passed were sufficient to make the writing legible. The retention of the charge was also shown still more remarkably by the explosion of gunpowder by the electricity retained in the wire half a minute after connection with the battery had been broken. It is owing to the retention of the electricity by the wire that the slowness with which the messages through the Atlantic cable were transmitted is to be attributed, and not to the length of the cable. The rate of one word a minute was the average speed of transmission when the first messages were sent through the wire. The effect of the retardation of the electric current is comparatively insignificant and were it not for the peculiar action of the surrounding water, the messages might have been transmitted twelve times faster than they were.

The cost of constructing a telegraphic line has greatly diminished with the increased facilities of insulating the wires, and since the expiration of patents, which conferred a monopoly on certain plans of doing so. The cost to the Great Western Railway Company for a line of six wires to Slough, was £150 per mile, with comparatively low and slender posts and very imperfect insulation. The cost of the same number of wires at the present day would not be one-half that sum, with thicker wires and better insulation.

It is customary in England to restrict the suspension of telegraphic wires to railways, from the notion that the protection of railways is necessary to prevent wilful damage to the wires; and as the Electric Telegraph Company have made exclusive arrangements with all the railway companies out of London, the competing telegraph companies have preferred to lay their wires underground rather than incur the supposed risk of damage to the wires if suspended from posts on common roads, though by this means the cost of construction is at least quadrupled. The protection which railways afford is, however, more imaginary than real, for any one inclined to interrupt the communication could easily do so; and if on common roads proper precautions were taken in fixing the posts, and a heavy penalty were imposed on wilful offenders, the common roads and open fields would, there can be little doubt, offer as safe a course for the telegraphic wires as railways.

The conducting power of the earth is now employed by all electric telegraph companies for one-half of every circuit. Thus, whether a communication be sent from London to Liverpool, to Edinburgh, Paris, or Brussels, the moist earth serves to complete one-half of the communication. In the telegraphic circuit between London and Liverpool, for example, the insulated wire is connected at each end with the earth by being soldered to a copper plate, which is buried a few feet underground, so as to insure its being always surrounded with moisture. To improve the connection of this plate with the earth, it is customary to bury with it a quantity of sulphate of copper, the solution of which surrounds the earth-plate with a better conducting liquid than water, and thus extends the connecting surface. The gas pipes or water pipes are sometimes employed for the attachment of the wires instead of an earth-plate, but the latter is generally preferred.

In arranging a telegraphic circuit, the voltaic batteries and the instruments are introduced at breaks in the telegraph wire. The course of the electric current is from the copper end of the battery through the transmitting instrument, then along the wire to the receiving instrument; from that it passes to the earth and is thus returned to the transmitting station, where it completes the circuit by being conducted from the earth-plate to the zinc end of the voltaic battery. The arrangement for completing the circuit will be more clearly understood by reference to the accompanying diagram.