The wire from C, which is the copper pole of the voltaic battery, is connected with the instrument A; the electric current is then transmitted along the wire D to the receiving instrument B; thence it is transferred to the earth-plate E, passes through the earth to the corresponding plate E´, which is connected with Z, the zinc pole of the battery. When a communication is returned from B to A, a similar arrangement is made; the wires connected with the instruments being so arranged as to bring into action a voltaic battery at B, and to throw out of circuit the one at A; for the connection with the battery is only made when the transmitting instrument is worked.
Since all the electric telegraphs in different parts of the world are connected with the earth, as one portion of the circuit, it might be supposed that the various currents would mingle, and occasion a confusion of messages; but it must be borne in mind that no electric current is formed until a communication be made from one pole of a voltaic battery to the other, and as such communication can only be completed through the insulated wire, the earth-currents cannot mingle, but each one passes to the proper terminus of its respective battery. The accompanying diagram and explanation may serve to remove the difficulty of understanding why the two circuits are maintained quite distinct.
The letters A B represent the wires making communications between the batteries D and E, and the telegraph instruments I O at the receiving station. The electricity from the copper end of the battery D would be conducted along A through the instrument I, and by the wire K to the earth-plate H. It would be then transmitted through the earth on its return to the battery, in the direction of the arrows, to the other earth-plate G, and thence it would find its way to the zinc pole of the battery D, and complete the circuit. In the same manner, the electric current from the copper end of the battery E would be transmitted through the wire B, and would complete its current also by means of the earth-plates G H, and would traverse the course indicated by the arrows, and return to the zinc end of E. Though both electric currents traverse the same wire from the instruments I O to the earth-plate H, and are thence transmitted through the earth to a single plate, G, at the transmitting station, there is no mingling of currents, the electric current of each battery being kept as distinct as if separate wires were used both for the transmitted and the return current. It would, indeed, be as impossible for the separate currents transmitted from the two batteries to be mingled together, as it would be for the written contents of two letters enclosed in the same mail-bag to intermix.[8]
The length of telegraph lines at present laid down by the several telegraph companies in Great Britain, exceeds 10,000 miles. To complete those lines required 40,000 miles of wire, and there are 3,000 persons engaged in the transmission of telegraphic intelligence.
In North America there is a direct communication from New York to New Orleans, a distance of 2,000 miles, through the whole length of which wires messages can be transmitted without any break. Wires have also been suspended on lofty posts across the Indian Peninsula, where no railways have been yet laid down. Lines of insulated wire, partly submerged in the sea, partly buried underground, and partly suspended on posts in the air, place London and Vienna in direct communication; and other telegraph lines are in the course of construction, which will unite London with Africa: and a complete net-work of telegraph wires is spreading over the face of Europe.
It will not be long before this system of communication is connected with a similar one in America. The failure of the cable already laid down has confirmed the opinion of the author, expressed in papers read at meetings of the British Association for the Advancement of Science, and in his work on Electricity, that the conducting wire should be sufficiently strong to be self-protective, without requiring an external coating of iron wire rope. A conducting copper wire, a quarter of an inch in diameter, covered with gutta percha and tarred hemp, would be more flexible and stronger than the combined cable; and it being a much better conductor of electricity, the rapidity of transmission would be greatly increased.
The effect of the establishment of competing telegraph companies in England has been to diminish the charge for transmitting messages, in some instances to one-fifth of the rate formerly demanded; and when further experience in the construction of telegraphic lines, and the adoption of more rapidly transmitting instruments, have facilitated and improved the means of communication, we may anticipate that correspondence by Electric Telegraph will in a great measure supersede the transmission of letters by post.