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History of the Atlantic Cable & Undersea Communications
from the first submarine cable of 1850 to the worldwide fiber optic network

1921 Key West - Havana Telephone Cable

Manufactured by Telcon for the Cuban American Telephone & Telegraph Co. and laid by CS Colonia. Three coaxial cables 100, 104, 105 nm.

1921 Key West - Havana cable
Type A Shore End Single
(see below)
Recovered in 1997
by Tom Perera

Continuously Loaded Cable. Probably the longest submarine cable system used for telephone communication is that between Key West, Florida, and Havana, Cuba. The system consists of three continuously loaded cables, each of which provides, in addition to a telephone channel, direct-current and carrier-current duplex telegraph channels. These cables are from 100.2 to 104.9 nautical miles (186 to 195 kilometers) in length and are laid in water, for a part of the route, about 1,000 fathoms in depth.

The design of the submarine cables and the associated equipment differs from previous systems because of the service which they are required to furnish, the depth of water in which the cables are laid, and the length of the cables. As is usual with submarine-cable systems laid in deep-sea sections the cables are of the single-core type generally used for submarine telegraphy. In this type the return circuit is through the sea. These cables, however, are modified in this respect to make them more suitable for telephone and carrier use. The cables are wrapped with copper tapes surrounding the insulation of the single-core conductor, which forms an uninsulated return conductor.

In order to make efficient transmission possible, the cables are loaded; that is, the inductance of the circuit is increased by the use of a wrapping of iron 0.008 inch (0.2 millimeter) in diameter directly around the conductor.

The conductor itself consists of a round copper wire 0.115 inch (0.292 millimeter) in diameter surrounded by five copper tapes, each 0.077 inch (1.96 millimeters) wide and 0.9125 inch (0.32 millimeter) thick. This conductor has a weight of 350 pounds (159 kilograms) per nautical mile (1.853 kilometers, 6,080 feet). The resistance of this core is specified not to exceed 3.52 ohms per nautical mile at 75° Fahrenheit (24° centigrade).

The insulation consists of a special treatment of the conductor with Chatterton's compound and a covering of gutta-percha. The mixture is applied in three layers, thus forming a core. This insulating material has a weight of 315 pounds (143 kilograms) per nautical mile and has a thickness of approximately 0.135 inch (3.4 millimeters). Because of the loading and the high frequencies for which the cable is used, low alternating-current conductance was specified for the dielectric, requiring the use of a special gutta-percha mixture.

The continuous loading material consists of a single close layer of iron wire 0.008 inch (0.2 millimeter) in diameter wrapped directly around the copper conductor.

CS Colonia laying the cable
Illustration by Charles Rosner from a 1945
IT&T magazine advertisement

Return Conductor. Copper tapes have been utilized as stated, wrapped around the outside of the core to diminish the losses caused by the sea return. A copper tape 1 inch (2.54 centimeters) wide and 0.004 inch (0.1 millimeter) thick is applied directly upon the core with a short enough lay to provide safe overlap. Upon this tape are laid two heavier copper tapes, each 0.625 inch (1.59 centimeters) wide and 0.022 inch (0.56 millimeters) thick. These two heavier tapes are applied with a much longer lay, and are laid side by side with the edges not quite touching. All the copper tapes mentioned weigh approximately 850 pounds (390 kilograms) per nautical mile, and provide a return conductor with a direct current resistance in the laid cables of approximately 1.05 ohms per nautical mile. The thin copper tape that is first applied directly on the core has an additional value in affording protection against injury to the core caused by the toredo worm.

In Fig. 41 is shown the appearance of the cables taken from photographs of specimens of the several types.

Tom Perera's web site has more photographs of this
Type A cable that he recovered off the Florida coast in 1997. However, the note on his site that this is a 19th century telegraph cable needs to be updated, as his sample is clearly the 1921 telephone cable described here.

Sea Earth. It is the practice in submarine-cable construction to provide two cores near the shore lines so that the return circuit is carried out to some distance seaward before connecting to earth. These cables have been constructed in this manner. In the terminals of the cables the return, or sea-earth, core has the same central conductor having thin copper tape, but without loading iron or heavy copper tapes. The sea-earth conductors are electrically welded to the conductive copper tape on the main core.

AT&T promotional brochure for the new service

Cable Armor. It is essential that the cables be provided with adequate exterior protection and possess sufficient tensile strength. The size and number of armored wires must be in accordance with the location, which is determined after a survey of the proposed, bed for the cables. This survey includes consideration of depth, the kind of bed, and the sea currents.

Before the armor wires are applied to the core or cores they are wrapped with tanned jute yarn in one or more layers, which form a bedding for the armor wires. In those portions of the shore-end cable which have two cores, the cores are laid up together with a relatively long lay and with tanned jute in the interstices between them, before applying the jute bedding for the armor wire.

The heaviest armor is placed at the shore ends, where the water is shallow and most likely to be injured. This armor has a diameter of 0.3 inch (7.6 millimeters). In contrast, the deep-water cable has an armor protection of 0.104 inch (2.6 millimeters). There are also intermediate portions of the cable having armored wires 0.192 inch (4.9 millimeters) in diameter. The latter armored wires are of specially constructed springy steel for the purpose of giving the cable great tensile strength. The armor at the shore ends is of heavier iron wire of soft material. All the armored wires are galvanized in addition to being coated with a preservative compound.

The deep-sea portions of the cable are further treated in addition to the foregoing by being wrapped with an impregnated fabric tape, which protects the wire and makes the cable more flexible by keeping the armor wires separated. At the extreme shore ends a short length is made with each core covered with a close-fitting tube of lead to protect it from light and air, which would tend to cause deterioration of the gutta-percha in those portions which may extend out of the water.

The lengths of the three cables are somewhat different because each one follows a different route. The heaviest type of armoring is generally provided to extend from the shore to a location in the sea where the depth is approximately 100 fathoms (183 meters). Another cable extends to a depth of about 250 to 300 fathoms (457 to 549 meters). Other sections of cable are of the deep-sea type.

These cables carry the heavily armored cable to a point considerably beyond the Havana terminal because of a greater graduated increase in depth of water at the Key West terminal.

The two-core, or twin, cable utilized as the return core for the sea ground is carried out much farther at the Key West end because of the gradual increase of water depth as already mentioned.

In Fig. 40 some of the details of the types and designs of cable are shown. The over-all diameter of the largest shore end cable is approximately 2.4 inches (6.1 centimeters) and that of the deep-sea type approximately 1.2 inch (3 centimeters).

See the 1920 Brazil - Uruguay - Argentina telegraph cable, also made by Telcon, for photographs of this type of cable.

Source: CASPER, Louis, Telephone and Telegraph Cables, Scranton, 1928, International Textbook Company

See also this explanation of sea earths from DE GIULI, Italo, Submarine Telegraphy - A Practical Manual, London, 1932, Sir Isaac Pitman & Sons, Ltd.

A technical paper on this cable was presented at the 10th Midwinter Convention of the A.I.E.E., New York, N.Y., February 15-17, 1922.

Thanks to Tom Perera for the 1921 cable sample shown at the top of the page.

For further information on the history of the Key West - Havana telephone cables see this article by J. Gregory Griffin.

Last revised: 18 May, 2023

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