Just Add Water?

If a wire could carry messages over land, could an insulated wire do the same under water? In practice, things turned out to be more complicated:

• The longer a circuit is, the weaker the signal. This is due to an electrical property called resistance. (Think of it as a kind of friction.) Underwater circuits can be thousands of miles long.
  
  
• When you put insulation between two conductors (wire, water), an electrical property called capacitance comes into play. Capacitance tends to hold on to electricity. This slows the signal and distorts it - an unpleasant surprise for the men who built the first cables.

These problems and others had to be solved before underwater cables could work.

	In addition to the problems caused by mixing water and electricity, underwater cables had to survive corrosion, barnacles, burrowing worms, and the occasional shark bite. This cable was made for a proposed route between Siberia and Alaska, but the line was never laid.
Underwater cable for Bering Strait, 1866 National Museum of American History, from Isabelle Field Judson

	The great Scottish physicist William Thomson (later Lord Kelvin) (1824-1907) made underwater cables possible by solving many of the problems. He was aboard ship for all of the early transatlantic expeditions, and he designed sensitive receiving equipment to detect feeble and distorted telegraph pulses.
William Thomson (Lord Kelvin), 1859 From Agnes Gardner, Kelvin the Man, 1925 Smithsonian Institution Libraries

Letter from William Thomson to American cable promoter Cyrus Field, 1862 National Museum of American History, from Isabelle Field Judson

As a physicist, William Thomson combined brilliant mathematical analysis with hands-on practicality. His description of electrical action in a cable was difficult even for some physicists to follow, but he also invented devices that made the system work.
William Thomson, "On the Theory of the Electric Telegraph" (left, 1855) and "On Practical Methods for Rapid Signaling by the Electric Telegraph" (right, 1856), Proceedings of the Royal Society of London Smithsonian Institution Libraries

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