History of the Atlantic Cable & Undersea Communications
from the first submarine cable of 1850 to the worldwide fiber optic network
A.A. Clokey: Submarine Cable Telegraphy (1936)
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History of the Atlantic Cable & Undersea Communications |
A.A. Clokey: Submarine Cable Telegraphy (1936) |
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SUBMARINE CABLE TELEGRAPHY CABLE CONSTRUCTION Submarine telegraph cables are specially insulated and armored conductors that are laid on ocean bottoms to provide wire communication between points separated by great expanses of water. Non-loaded cables usually consist of a single copper conductor which may be either solid, stranded, or a combination of both, to afford greater flexibility, surrounded by a thick gutta‑percha insulation. This insulated conductor or “core” is protected from mechanical injury by several layers of tanned jute and a layer of steel armor wire as shown in Fig. 1.
The completed diameter of cables laid in deep water is about ¾ in. to 1½ in., depending upon the size of conductor and thickness of gutta-percha. Sections of cable laid in shallow water are built up with additional layers of armor wire and jute to a diameter of as much as 4 in. to provide additional mechanical protection against abrasion due to wave action and injury from ship’s anchors. Loaded cables, Fig. 2, are of similar construction except that the central conductor is surrounded with a thin spirally wound tape or wire of an alloy having high magnetic permeability at very low magnetizing forces. The addition of this loading material effects a marked reduction in attenuation of the signaling currents at higher speeds and diminishes signal distortion. The second conductor shown in the section of the shore end is used to carry the receiving ground connections out to deep water in order to obtain greater freedom from disturbances created by power circuits and natural causes.
The electrical and mechanical properties of types of core frequently used are shown in Tables 1 and 2.
OPERATION OF NON-LOADED CABLES The high resistance and electrostatic capacity of long non-loaded submarine cables (longest about 3500 nautical miles) distort and attenuate the signaling impulses to such extent as to preclude the use of ordinary land line methods of operation. Such cables are usually duplexed, using the modified form of bridge duplex, shown schematically in Fig 3, in which the usual bridge inductance coil is replaced by condensers which offer high impedance to slowly varying currents due to differences in potential of the earth connections at the two ends of the cable. Satisfactory operation requires that the current in the receiving arm of the bridge, due to imperfect balance, shall not exceed one-sixth of the received signaling currents. This necessitates maintaining a balance accurate to about 1 part in 10,000 or better between the cable and the artificial line.
Signals in the cable Morse code are transmitted from a perforated tape, Fig. 4, by a transmitter and group of relays, arranged to apply either positive or negative battery or ground to the apex of the bridge. In many cases each signaling impulse is immediately followed by a shorter period during which the cable is grounded to improve the shape of the received signals. This is called “curbing,” and such signals are referred to as “curbed signals.”
Received signals for manual translation are recorded on a moving paper tape by a siphon recorder. This is essentially a d’Arsonval galvanometer in which the movements of the coil are mechanically transmitted to a small-bore (0.010-in) glass siphon tube that acts as an ink writing pen and traces the signals on a paper tape moving beneath the siphon point; see Fig. 5. The writing point of the siphon is rapidly vibrated in a plane perpendicular to the plane of the record tape by means of an electromagnetic vibrator which is connected with the siphon by a slender silk fiber. This is necessary to reduce the friction between siphon and paper to a minimum and results in the record being a series of closely spaced dots instead of a continuous line. The distorted received signals, which would otherwise be difficult to interpret, are “shaped” or “corrected” by proper adjustment of the shunted receiving condenser and the magnetic shunt, or inductance, and its associated series resistance shown in Fig. 3, and by means of altering the natural frequency of oscillation of the moving system of the recorder. The received signals, after being thus shaped, appear on the siphon recorder tape as shown in Fig. 4.
The received signals on long non-loaded cables operating at commercially economical speeds are generally too feeble to operate a siphon recorder or relay directly, and some form of amplification becomes necessary. Mechanical amplifiers commonly designated “magnifiers” are used for the purpose. A frequently used type, shown schematically in Fig. 6, is known as the Heurtley magnifier. It employs a moving coil which swings two 0.0003-in.-diameter platinum wires into and out of close proximity to two similar stationary wires, all of which are electrically heated. These two pairs of wires form two arms of a balanced Wheatstone bridge which has a battery connected across one diagonal and a siphon recorder or relay connected across the other. As the coil moves in response to a signal of one polarity one of the moving wires is brought nearer to its associated stationary wire, which raises the temperature of both wires and consequently increases their electrical resistance, while the separation between the other moving wire and its associated heated wire is increased, thereby lowering their temperature and electrical resistance. This unbalances the bridge and causes current to flow in one direction through the siphon recorder or relay. Movement of the moving wires in the other direction in response to a signal of opposite Polarity also unbalances the bridge, but in a way to reverse the direction of current in the siphon recorder.
Repeaters for joining two sections of non-loaded cable are usually of the regenerative type, shown in Fig. 7 and are similar in principle of operation to the regenerative repeaters used in land lines. The received signals are “magnified” or “amplified” and used to control a moving-coil relay similar in construction to a siphon recorder in which the siphon is replaced by a delicate contact “tongue” which moves between two other contacts that are continuously vibrated or moved to reduce friction. This relay controls the operation of two more rugged relays which repeat the signals into the regenerator circuits.
Printers for non-loaded cables are similar to the multiplex printers employed on land lines with certain modifications which are necessitated by the different types of code used. The operating speed of a non-loaded cable is determined by the length and construction of the cable itself, by the nature and magnitude of any extraneous interference, and by the kind of apparatus used and the precision with which the apparatus and balance adjustments are made and maintained. The speed is almost inversely proportional to the square of the cable length. The approximate speed in letters per minute of duplexed cables, using magnifiers and siphon recorders or moving-coil relays, may be determined by the formula Speed lpm = K/krl² in which k is the capacity in farads and r the resistance in ohms per nautical mile, l is the length of the cable in nautical miles, and K is the speed constant which varies between 550 and 700 depending upon the length and type of cable and apparatus used. A speed constant of 660 represents an average value for cables from 1000 to 2000 nautical miles long, using Heurtley magnifiers and siphon recorders or relays. OPERATION OF LOADED CABLES Cables in which the loading is distributed uniformly from end to end have not thus far been successfully duplexed and are, therefore, operated simplex, or in only one direction at a time. Taper loaded cables, which comprise two non-loaded terminal sections joined to a main loaded portion through several intermediate sections having progressively greater inductance, may be balanced by specially constructed artificial lines to provide satisfactory duplex operation. The received signals are amplified sufficiently by a vacuum-tube amplifier to control a modified form of multiplex printing system. The channel speed is usually between 250 and 300 letters per minute, and the number of channels varies between 2 and 8 depending upon the construction of the cable. One uniformly loaded cable, approximately 2300 nautical miles in length, operates at 65 cycles per second and provides 5 one-way channels, each operating at 312 letters per minute. Another cable consisting of two cable sections, respectively 1344 and 2023 nautical miles long, and two land line terminal sections joined by 3 intermediate regenerative repeaters, operates simplex, or in one direction at a time, at a speed of 100 cycles per second which provides eight 300-letter‑per-minute channels. One duplexed taper loaded cable 1341 nautical miles long is capable of being operated at a speed of 70 cycles per second in both directions simultaneously with a 4-channel multiplex system. PICTURE TRANSMISSION A special method of transmitting pictures, known as the Bartlane System, is used on submarine cables where the comparatively low transmission frequencies preclude the economical use of conventional facsimile or picture-transmission methods. This system provides for the transmission of black and white and a predetermined number of intermediate tone or shade values. The picture to be transmitted is scanned by a photoelectric apparatus which selects the shade value most nearly corresponding to the shade of each successively exposed area of the picture, and automatically perforates a transmitting tape with a code combination representing the selected shade. The perforated tape is passed through the regular transmitter on the cable in the same way as any ordinary transmitting tape. A corresponding perforated tape, prepared by a receiving perforator, is used to operate the picture-reproducing apparatus. Each square inch of picture surface is broken up by the scanning apparatus into about 3600 squares 1/60 in. on a side. As each of these small areas is transmitted separately over the cable it requires about 2¼ min to transmit a square inch of picture surface on a circuit operating at 1500 letters per minute. POWER AND MAINTENANCE Power for submarine cable transmission is generally supplied by storage batteries which rarely exceed 100 volts. Storage batteries are also used to supply current for operating the local circuits of magnifiers, moving-coil relays, and siphon recorders, while generators are used for supplying energy to the local circuits of printers and associated apparatus. Faults in submarine cables are located by means of insulation resistance, capacitance, and conductor resistance measurements made with a special form of bridge and a sensitive galvanometer in a manner which is similar in principle to the tests made in locating faults on land lines. BIBLIOGRAPHY General References: Theory of the Submarine Telegraph and Telephone Cable, H. W. Malcolm. Bann Brothers, Special References: Submarine cable telegraphy, J.W. Milnor. Trans. A.I.E.E. Vol. 41, 1922.The Newfoundland-Azores high speed duplex cable, J.W. Milnor and G. A. Randall. Trans. A.I.E.E., Vol. 50, 1931. Printing telegraphs on non-loaded ocean cables, H. Angel. Trans. A.I.E.E., Vol. 46, 1927. Direct printing over long non-loaded submarine telegraph cables, M.H. Woodward and A.F. Connery. Trans. A.I.E.E., Vol. 51, 1932. The application of vacuum tube amplifiers to submarine telegraph cables, A.M. Curtis. Bell System Tech. J., Vol. 6, 1927. Automatic printing equipment for long loaded submarine telegraph cables, A.A. Clokey. Bell System Tech. J., Vol. 6, 1927. The loaded submarine telegraph cable, O.E. Buckley. Bell System Tech. J., Vol. 4, 1925. A non-rotary regenerative telegraph repeater, A.F. Connery, Trans. A.I.E.E., Vol. 48, 1927. Modern photo telegraphy, R.C. Walker. Wireless World, Vol. 30, 1932. Allison A. Clokey’s US Patents:
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Last revised: 17 November, 2011 |
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