This is unquestionably the most feasible route for telegraphic communication between America and Europe; and, though the longest by several thousand miles, it would afford the most rapid means of communication, owing to the great superiority of aërial over subaqueous lines.
No limit has yet been found to aërial telegraphing; for, by inserting transferrers into the more extended circuits, renewed energy can be attained, and lines of several thousands of miles in length can be worked, if properly insulated, as surely as those of a hundred. The lines between New York and New Orleans are frequently connected together by means of transferrers, and direct communication is had over a distance of more than, two thousand miles. No perceptible retardation of the current takes place; on the contrary, the lines so connected work as successfully as when divided into shorter circuits.
This is not the case with subaqueous lines. The employment of submarine, as well as of subterranean conductors, occasions a small retardation in the velocity of the transmitted electricity. This retardation is not due to the length of the path which the electric current has to traverse, since it does not take place with a conductor equally long, insulated in the air. It arises, as Faraday has demonstrated, from a static reaction, which is determined by the introduction of a current into a conductor well insulated, but surrounded outside its insulating coating by a conducting body, such as sea-water or moist ground, or even simply by the metallic envelope of iron wires placed in communication with the ground. When this conductor is presented to one of the poles of a battery, the other pole of which communicates with the ground, it becomes charged with static electricity, like the coating of a Leyden jar,—electricity which is capable of giving rise to a discharge current, even after the voltaic current has ceased to be transmitted.
Professor Wheatstone experimented upon the cable intended to unite La Spezia, upon the coast of Piedmont, with the Island of Corsica. It was one hundred and ten miles in length, and contained six copper wires one-sixteenth of an inch in diameter, individually insulated, and each covered with a coating of gutta-percha one-twelfth of an inch in thickness. The cable was coiled in a dry pit in the yard, with its two ends accessible. The ends of the different wires could be united, so as to make of all these wires merely one wire six hundred and sixty miles in length, through which the electric current could circulate in the same direction. This current was itself furnished by an insulated battery formed of one hundred and forty-four Wheatstone's pairs, equal to fifty of Grove's. In the first series of experiments, it was proved, that, if one of the ends of the long wire, whose other end remained insulated, were made to communicate with one of the poles of the battery, the wire became charged with the electricity of that pole, which, so long as it existed, gave rise to a current which was made evident by a galvanometer: but, in order to obtain this result, the second pole of the battery must communicate with the ground, or with another long wire similar to the first.
In a second series of experiments, Professor Wheatstone interposed three galvanometers in the middle and at the ends of the circuit, determining in this manner the progress of the current by the order which they followed in their deviation. If the two poles of the battery were connected by the long conductor of six hundred and sixty miles, the precaution having been taken to divide it into two portions of equal length, it was observed, on connecting the two free extremities of these two portions in order to close the circuit, that the galvanometer placed in the middle was the first to be deflected, whilst the galvanometers placed in the vicinity of the poles were not deflected until later.
By a third series of experiments, Wheatstone, with the galvanometer, has shown that a continuous current may be maintained in the circuit of the long wire of an electric cable, of which one of the ends is insulated, whilst the other communicates with one of the poles of a battery whose other pole is connected with the ground. This current is due to the uniform and continual dispersion of the statical electricity with which the wire is charged along its whole length, as would happen to any other conducting body placed in an insulating medium.
It was owing to the retardation from this cause that communication through the Atlantic Cable was so exceedingly slow and difficult, and not, as many suppose, because the cable was defective. It is true that there was a fault in the cable, discovered by Varley, before it left Queenstown; but it was not of so serious a character as to offer any substantial obstacle to the passage of the electric current.
As everything pertaining to the actual operation of the Atlantic Cable has been studiously withheld from the public, until it has come to be seriously doubted whether any despatches were ever transmitted through it, we presume it will not be out of place here to give the actual modus operandi of this great wonder and mystery.
The only instrument which could be used successfully in signalling through the Atlantic Cable was one of peculiar construction, by Professor Thompson, called the marine galvanometer. In this instrument momentum and inertia are almost wholly avoided by the use of a needle weighing only one and a half grains, combined with a mirror reflecting a ray of light, which indicates deflections with great accuracy. By these means a gradually increasing or decreasing current is at each instant indicated at its due strength. Thus, when this galvanometer is placed as the receiving instrument at the end of a long submarine cable, the movement of the spot of light, consequent on the completion of a circuit through the battery, cable, and earth, can be so observed as to furnish a curve representing very accurately the arrival of an electric current. Lines representing successive signals at various speeds can also be obtained, and, by means of a metronome, dots, dashes, successive A-s, etc., can be sent with nearly perfect regularity by an ordinary Morse key, and the corresponding changes in the current at the receiving end of the cable accurately observed. The strength of the battery employed was found to have no influence on the results; curves given by batteries of different strengths could be made to coincide by simply drawing them to scales proportionate to the strengths of the two currents. It was also found that the same curve represented the gradual increase of intensity due to the arrival of a current and the gradual decrease due to the ceasing of that current. The possible speed of signalling was found to be very nearly proportional to the squares of the lengths spoken through. Thus, a speed which gave fifteen dots per minute in a length of 2191 nautical miles reproduced all the effects given by a speed of thirty dots in a length of 1500. At these speeds, with ordinary Morse signals, speaking would be barely possible. In the Red Sea, a speed of from seven to eight words per minute was attained in a length of 750 nautical miles. Mechanical senders, and attention to the proportion of the various contacts, would materially increase the speed at which signals of any kind could be transmitted. The best trained hand cannot equal the accuracy of mechanism, and the slightest irregularity causes the current to rise or fall quite beyond the limits required for distinct signals. No important difference was observed between signals sent by alternate reverse currents and those sent by the more usual method. The amount of oscillation, and the consequent distinctness of signalling, were nearly the same in the two cases. An advantage in the first signals sent is, however, obtained by the use of Messrs. Sieman's and Halske's submarine key, by which the cable is put to earth immediately on signalling being interrupted, and the wire thus kept at a potential half-way between the potentials of the poles of two counter-acting batteries employed, and the first signals become legible, which, with the ordinary key, would be employed in charging the wire.
A system of arbitrary characters, similar to those used upon the Morse telegraph, was employed, and the letter to be indicated was determined by the number of oscillations of the needle, as well as by the length of time during which the needle remained in one place. The operator, who watched the reflection of the deflected needle in the mirror, had a key, communicating with a local instrument in the office, in his hand, which he pressed down or raised, as the needle was deflected; and another operator occupied himself in deciphering the characters thus produced upon the paper. As the operator at Trinity Bay had no means of arresting the operations at Valentia, and vice versâ, and as the fastest rate of speed over the cable could not exceed three words per minute, it will not surprise the reader that the operators were nearly two days in transmitting the Queen's despatch.