These currents of static induction are proportional in intensity to the force of the battery and the length of the wire, whilst an inverse relation is true as regards the length of the conductor with the ordinary voltaic current.
Professor Wheatstone proved, by actual experiment, 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 he considers due to the uniform and continual dispersion of the statical electricity with which the wire is charged along its whole length.
It was mainly owing to the retardation from this cause that communication through the Atlantic cable was so exceedingly slow and difficult.
I will now endeavor to show why the new cable will not be liable to this difficulty, to anything like the same extent.
I have alluded to the resistance offered by the conductor of a telegraph-cable to the passage of an electric current, and to the retardation of this current by static induction. The terms retardation and resistance are not considered technically synonymous, but are intended, as electrical terms, to designate two very different forces. The resistance of a wire, as we have seen above, is proportional to its length, and inversely to its diameter. It is overcome by increasing the number of cells in the battery, or, in other words, by increasing the intensity or force of the current. The retardation in a telegraphic cable, on the contrary, is proportional to the length of the conducting-wire and the intensity of the battery. In the former case, by increasing the electrical force you overcome the resistance; while in the latter, by augmenting the electrical force you increase the retardation.
From the foregoing law it will be seen that there are two ways of lessening the resistance upon telegraphic conductors,—one by reducing the length, and the other by increasing the area of the section of the conducting-wire. Now, as already remarked, the copper conducting-wire in the old cable weighed but ninety-three pounds to the mile, while in the new cable it weighs five hundred and ten pounds to the mile, or more than five times as much. If, then, by comparison, we estimate the resistance in the old Atlantic cable to have been equal to two thousand miles of ordinary telegraph-wire, the increased size of the conducting-wire of the new cable reduces the resistance to one-fifth that distance, or four hundred miles. And while it required two hundred cells of battery to produce intensity sufficient to work over the two thousand miles of resistance in the old cable, it will require but one-fifth as much, or forty cells, to overcome the four hundred miles of resistance in the new cable. The retardation which resulted from the intense current generated by two hundred cells will be also proportionately reduced in the comparatively small battery of forty cells. Thus we perceive, that, while the length of the cable is, electrically and practically, reduced to one-fifth of its former length, the retardation of the current is also decreased in the same proportion. Therefore, if, with the old cable, three words per minute could be transmitted, with the new cable we shall be able to transmit five times as many, or fifteen words per minute. This is not equal to our Morse system on the land-lines, which will signal at the rate of thirty-five words per minute, still less to the printing system, which can signal at the rate of fifty words per minute; but, even at this rate, the cable would be enabled to transmit in twenty-four hours one thousand despatches containing an average of twenty words apiece. Mr. Field, however, claims for the cable a speed of only twelve words per minute, which would reduce the number of despatches of twenty words each that could be transmitted in twenty-four hours to eight hundred and sixty-four. We will suppose, however, that the cable transmits only five hundred telegrams per day; this number, at ten dollars per message, would give an income of five thousand dollars per diem, or one million five hundred and sixty-five thousand dollars per annum. Quite a handsome revenue on an outlay of about one million of dollars!
The only instrument which could be used successfully in signalling through the old cable was one of peculiar construction, 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 this 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 and dashes 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.
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, held a key in his hand communicating with a local instrument in the office, which he pressed down or raised, according to the deflection of the needle; and another operator deciphered the characters thus produced upon the paper. This mode of telegraphing was, of necessity, very slow, and it will not surprise the reader that the fastest rate of speed over the cable did not exceed three words per minute. Still, had the old cable continued in operation a few months longer, experience and practice would have enabled the operator to transmit and receive with very much greater facility. On our land-lines, operators of long experience acquire a dexterity which enables them not only to transmit and receive telegrams with wonderful rapidity, but to work the instruments during storms, when those of less experience would be unable to receive a dot. There is no occupation in which skill and experience are more necessary to success than in that of telegraphing, and at the time the Atlantic cable was laid no experience had been obtained upon similar lines, or with the instruments employed. Now, however, the company can avail itself of experienced operators from lines of nearly equal length, and who will require no time for experimenting, but may commence operations as soon as the two ends of the cable are landed upon the shores of Europe and America.
In the old cable the copper wire was covered but three times with gutta-percha, while in the new it is covered four times with the purest gutta-percha and four times with Chatterton's patent compound, by which the cable is rendered absolutely impenetrable to water. The old cable was covered with eighteen strands of small iron wire, which, as they had no other covering, were directly exposed to the action of the water. The new is covered with thirteen strands, each strand consisting of three wires of the best quality, and covered with gutta-percha, to render it indestructible in salt water. By this new construction, it has double the strength of the old cable, at the same time that it is lighter in the water, a very important matter in laying it across the ocean.
The risk of loss in laying the new cable would be very much diminished by the fact that it would be of such strength, that, even if broken, it could be recovered, as has been done in the Mediterranean; and besides, the principal and most expensive materials, copper and gutta-percha, being indestructible, would have at all times a market value.