By 1868, the new Atlantic cable had slashed communication time bewteen the U.S. and Europe from days to just minutes. In its earliest days, not many used the cable, since transmitting international messages was expensive. Yet the cable was a technological triumph. This article explains some of the scientific insights that had been necessary to make the cable viable.
An exceedingly interesting lecture upon the Atlantic cable, its construction and operation, was given by Mr. Cromwell F. Varley, Electrician to the Atlantic Telegraph Cable Company, before the Royal Institution of Great Britain, a short time ago. The lecture was illustrated by the use of two artificial telegraph cables; one representing the Atlantic cable, the other a cable similar in size and length, but forty times slower in its action. The latter, therefore, represented a cable 13,000 miles long, such as would be needed to connect England with Australia. The artificial Atlantic cable was composed of eleven fine wires, whose resistance, when taken together was equal to the real cable. The phenomena of electro-static induction in this artificial cable, were obtained by interposing large condensers between the resistance coils. The second cable included within its circuit eleven U tubes of glass, filled with a solution of zinc sulphate, two parts to ninety-eight of water. The metallic poles immersed in this solution were of amalgamated zinc; it having been demonstrated that electrodes like these are entirely free from polarization when immersed in the zinc sulphate solution. By simply turning a switch all the condensers could be detached from these tubes.
Between each pair of tubes was placed a galvanometer. These galvanometers were similar to the ordinary reflecting instruments used upon cables, except that the mirrors were polished lenses, instead of being plane glass; and the tubes in which the mirrors were placed had glass ends and were filled with pure water, which diminished the oscillations so that the needle came to rest in a fraction of a second. Each mirror had a diameter of half an inch, weighed with its magnet two grains, and was suspended with a triple filament of silk, one-sixteenth of an inch long.
It was, of course, impossible within the limits of a single lecture, to do more than touch upon a few of the very many interesting questions which suggested themselves in connection with this subject. On opening, the lecturer spoke of the skill displayed in the mechanical contrivances for manufacturing and laying the cable, and regretted that a more accurate description of them had not been published. He also alluded to the lecture of Sir William Thompson, delivered in that theatre not long before, upon the method employed for testing the insulation of the cable and also its conducting power. Mr. Varley then proceeded by calling the attention of the audience to the fact that while light and radiant heat traverse space with a definite velocity, this fact is not true of electricity. Luminous waves pass through space for millions upon millions of miles without varying in length. If, for example, the surface of Sirius could be suddenly veiled and thus concealed for ten minutes, its brilliancy, as seen at the distance of the earth, would suffer no diminution for twenty years, after which time its sudden disappearance for ten minutes would take place. Electricity, on the other hand, manifests itself at one end of the longest cable at the precise instant when it leaves the other, but with an intensity too feeble to be measured. At the end of a certain time, however, depending upon the cable itself, the quantity transmitted rapidly increases and approaches continually a limit which it never attains. Hence, to attribute to electricity a velocity as definite as that of light is a mistake. It becomes important, therefore, to define what is meant by the velocity of electricity; since if this be not done, a confusion is inevitable on account of the peculiar mode in which this force is transmitted. As just stated, whenever a current enters one end of a cable, signs of electricity appear instantly at the opposite end. And reasoning from this, the velocity might be viewed as sensibly infinite. But the quantity transmitted increases gradually without ever reaching its maximum. A diagram, on which this curve of increase was plotted, the abscissas representing units of time of duration a, (the value of which depends upon the dimensions of the cable) and the ordinates the units of force of the current after it has traversed the cable, was employed to illustrate this point. And if a be allowed to represent the time at the end of which the transmitted current is one-thousandth of the battery current, then the curve shows that at the end of the time represented by 4a, the current has reached one-fourth of its maximum force; it attains one half of this maximum at the end of the time 6a, and ninety-eight hundredths at 20a. By interrupting the connection with the battery after a certain time, and measuring the strength of the current, the distant end of the cable, a curve of decrease may be obtained. This curve shows that the decrease of the current is more rapid than its increase. Since, therefore, the decrease of the current is relatively gradual, care must be taken to allow one signal to be dissipated before sending a second. Two conditions ate consequently required in a cable: First, its power of transmitting electricity must be at a maximum; and second, it must lose its charge as promptly as possible.
The present Atlantic cable is composed of seven copper wires imbedded in a layer of gutta percha and wood tar, and surrounded by four concentric envelopes of gutta percha, which forms the insulating medium. The use of seven wires increases greatly the flexibility of the cable, and so lessens the liability to rupture. The weight of this cable is as follows: copper 300 pounds, and gutta percha 400 pounds, to the nautical mile (2,029 yards).
The telegraph cable is a long Leyden jar, one coating of which is connected with the earth, the other with the source of electricity, and which receives a charge the instant it is made to communicate with the battery. After a short time this charge becomes definite, and then may be represented by the hypothenuse of a right-angled triangle, being greatest at the battery end and diminishing to zero at the end which is connected with the earth. When such a cable is immersed in the ocean, the copper conductor forms the interior coating, the gutta percha the insulator, and the water the exterior coating. The artificial cables used in the lecture had both their extremities connected with the earth. Upon depressing the signal key, a battery is introduced into the circuit between one end and the earth, and the current is set in motion. By employing two keys one of which unites the positive or copper pole with the cable, and the negative or zinc pole with the earth, and the other inverts this order, the current may be alternated at pleasure. A third key makes connection either with the battery or with a small Geissler's tube, which serves to show the discharge of the cable. Whenever one of the keys first mentioned is pressed down the current enters the cable; and, after surmounting the first resistance, may pursue two routes; one through the first condenser, and the other through the resistance coils and the rest of the circuit. If ten galvanometers be distributed upon a long cable at equal distances, it may readily be shown that the instant the connection with the battery is made the current has much more force than after a few moments. The lecturer illustrated this fact by means of his long artificial cable. The ten galvanometers were placed one above the other, so that the ten images thrown from the mirrors upon the screen by the electrical light were in a vertical line. To assist the mind in fixing this progression of the wave, the galvanometers had names assigned them. The upper image represented England, the lower Australia, its antipodes. Between these extremes were the following nine stations: Gibraltar, Malta, Suez, Aden, Bombay, Calcutta, Rangoon, Singapore and Java. The upper station was not provided with a galvanometer, the current being so powerful there as to reverse the magnetism of the needle and vitiate the results. All the condensers of the cable were connected together and charged by communicating with a Daniell battery of 800 cells. The discharge was accompanied with a brilliant flash and a strong detonation.
The artificial Atlantic cable was first experimented with. The condensers were detached, a Geissler tube, whose resistance to the passage of the current was scarcely overcome by 400 Daniell cells, was inserted at the Newfoundland end to indicate the passage of the current there. When the current at this point attained to one half the power of the whole battery, the tube was illuminated by its discharge. On introducing the condensers, the current did not appear in the tube until after three or four seconds; but now, when the battery connection was broken at the English end, the current continued to discharge itself at the Newfoundland end for several seconds. This experiment was varied by charging the cable till the maximum charge was attained as indicated by the Newfoundland tube; then on breaking connection at the English end and inserting a second Geissler tube in place of the battery, it was far more illuminated than the first one, thus demonstrating that the charge is greatest at the battery end of a cable. Moreover, the English tube continued bright for some seconds after the Newfoundland tube had ceased to shine.
The experiments with the second and longer cable were equally striking. The electric light thrown upon the screen from the mirrors of the ten galvanometers gave ten brilliant disks in a vertical line. When the signal key was depressed the image representing Gibraltar instantly responded by moving across the screen, six feet or more to the right, and Malta began to move. In a few seconds the force of the current had diminished perceptibly at Gibraltar, indicating that this part of the line began to be charged. The other disks followed each other successively to the right, and at the end of fourteen seconds the impulse had reached Australia, though its maximum effect there was not obtained for an entire minute.
Connection with the battery in England being now broken, and the end of the cable being put in communication with the earth, Gibraltar instantly moved to the left for an equal distance, thus showing a strong retrograde current. In this movement it was speedily followed by Malta, Suez and Aden. Bombay, however, returned only to the zero line, where it remained, the cable discharging itself impetuously through both extremities. Several minutes were required for this discharge to take place perfectly so as to allow the disks to return to their original positions, and permit a repetition of the experiment. Several signals were then sent through the cable consecutively at intervals of five seconds; but these waves were distinguishable only as far as Aden, beyond which they ran into each other and became confused.
The lecturer then referred to the various methods thus far employed for freeing the cable from these charges, after each signal. In 1853-4, he conceived the idea of sending a negative current after a positive one, a proceeding now generally adopted on submarine lines. In 1856 he invented another method, which consisted in sending a strong positive current, definite in force and duration, and then a feeble current, also positive, carrying the signal. This was quite a step forward. In 1858 Sir William Thompson proposed to employ three currents, equal in duration, but irregular in strength and alternate in direction. The use of this method increased still more the rapidity of transmission. In 1863, Mr. Varley found, by experiments upon his artificial cable, that a still further increase was attainable by sending in succession four or five currents, all of the same strength but variable in duration. For example, if five currents be sent as follows: first, a positive current, then a negative current of longer duration, a positive current much less in length, a negative current still shorter, and finally, a very short positive current, thus producing a series of positive and negative waves from one end of the line to the other, there will result at the Australian end of the cable a small and perfectly distinct positive wave, and the line will be left free of charge and ready for a new signal.