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150 Years of Transatlantic Telegraphy

January 5, 2017

Optical fibers spanning the Atlantic carry the Internet between the major population centers of the world. The multitude of Atlantic fiberoptic cables now in place are technological marvels. Some of these have a data rate far exceeding a terabit/second with a latency under 100 milliseconds. A standard DVD optical disk of the type I use to back up files on my computer has a capacity slightly less than 0.005 terabytes.

Transatlantic data communication was first accomplished a few decades after the invention and improvement of the telegraph by Samuel Morse and many others in the 1830s. The first transatlantic telegraph cables were placed in 1857 and 1858, but they did not work as planned. The idea that an undersea cable, suitable toughened for its environment, would work as well as its terrestrial counterparts was fundamentally flawed.

Air, which electrically approximates a vacuum to a large extent, has the lowest possible relative permittivity, also known as the dielectric constant, with a value of 1.00. Water has a very high dielectric constant at low, telegraph frequencies (see figure), and seawater has nearly an identical value, so these undersea cables had a high associated capacitance. That capacitance, combined with the electrical resistance of the cables, resulted in a very low data rate and high signal loss.

Relative permittivity of water

The relative permittivity of pure water as a function of frequency. The response of seawater is not much different. Graphed using Inkscape from data in ref. 1.[1] The data below 300 MHz are extrapolated)


Signalling was possible only when the cables were driven by extremely high currents, and this resulted in their early demise. Oliver Heaviside, a maverick but knowledgeable physicist and electrical engineer of that era, explained all this by a simplification of Maxwell's equations.

More importantly, Heaviside devised a solution to the problem; namely, adding inductors, called loading coils, along the transmission line to balance the capacitance. Another physicist, William Thomson (Lord Kelvin) proposed that a reduction in cable resistance through use of a larger diameter conductor, and thicker cable insulation to reduce capacitance, would prove beneficial.

Figure caption

Oliver Heaviside, circa 1900.

One of Heaviside's discoveries, the Heaviside layer, a layer of Earth's ionosphere, is used as a metaphor for Heaven in "Cats," the Andrew Lloyd Webber musical.

(Wikimedia Commons image, modified for artistic effect.)


This first transatlantic cable was laid by two ships, the USS Niagara and the HMS Agamemnon, each of which each carried half the cable.[2] They met in mid-ocean, the two cables were spliced together, and they laid the cable to the opposite shores.[2] The seven conductor cable was made from copper wires triple-coated with the natural latex, gutta-percha, clad with tarred hemp.[2] Eighteen iron wires, wound in a close spiral at the peripherey, added mechanical strength.[2]

The cable laying was completed in August, 1858, but the cable quickly deteriorated, and it was useless by September 18, 1858.[2] Various factors, including the American Civil War, discouraged another attempt until 1865, when that cable broke during placement.[2] Another attempt, this time successful, was made the next year by the SS Great Eastern.

The SS Great Eastern, a vessel of nearly 18 ton's weight and 211 meters length, was the largest vessel at that time.[2] It sailed from Valentia Island, Ireland, to Trinity Bay, Newfoundland, Canada, laying about 4,000 kilometers (2485 miles) of cable.[2] Engineers had learned from previous failures, and the new cable was far more rugged, weighing 1,014 kilograms per kilometer.[2] The copper wires were three times larger in cross-section than those of the 1858 attempt, and they were coated with an improved insulator consisting of Chatterton's compound with gutta-percha. A spiral of steel wires added stength, just as in the previous cables.[2]

1958 Atlantic Cable US postage stamp by George Giusti

1858, or 1866?

It depends on your viewpoint.

(1958 US postage stamp by George Giusti, Via Wikimedia Commons.)


Final connection was made at Heart's Content, Newfoundland, on July 27, 1866. The Great Eastern then returned to the Atlantic, where engineers successfully retrieved the broken ends of the 1865 cable and spliced them together, giving the world two transatlantic cables in a single year.[2]

Since the capital cost of these cables was large, it was expensive to send a transatlantic message. The inital price was $10/word, with a ten word minimum ($100).[2] A $100 is equivalent to $1,500 in today's money. Because of this high price, only 5% of the potential data bandwidth was used, so the price was reduced to about half, and revenues increased.[2] Still, only wealthy customers, such as newspapers, used the service, and it led to the formation of United Press International that shared stories among many newspapers.[2]

As mentioned in my primary reference to the 1865 cable, an article by Robert Colburn, a research coordinator at the IEEE History Center,[2] the transatlantic cable has been recognized by two IEEE Milestones. The first milestone was the Heart's Content terminal, recognized in 1985. The second milestone, in 2000, recognized the terminal sites at Ireland, Valentia Island, Ballinskelligs, and Waterville.[2]

submarine optical cable

Cross-section of a modern submarine cable, as described in US Patent No. 4,278,835. (Modified Wikimedia Commons image by Oona Räisänen.)


References:

  1. Martin Chaplin, "Water and Microwaves," London South Bank University Web Site, December 18, 2015.
  2. Robert Colburn, "First Successful Transatlantic Telegraph Cable Celebrates 150th Anniversary," The Institute, IEEE, July 11, 2017.
  3. Lynden A. Jackson, "Submarine communication cable including optical fibres within an electrically conductive tube, US Patent No. 4,278,835, July 14, 1981.

Permanent Link to this article

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