An Irish-Scottish mathematician in 1854 put a tiny mirror on a suspended magnet; his invention helped send electric signals through an undersea cable and connect Europe with America

In 1854, Irish-Scottish mathematician and physicist William Thomson turned his attention to a problem that stood between the world and a communications revolution: how could an electric signal travel fast enough through a telegraph cable stretching thousands of miles beneath the Atlantic Ocean?

Thomson, who would later become Lord Kelvin, had already made his mark in mathematics, thermodynamics and physics. But his work on the Atlantic telegraph pushed him from theoretical science into practical engineering.

As engineers prepared to connect Europe and North America by submarine cable, Thomson realized that an electrical signal did not simply race through a long wire at a fixed speed. Instead, the signal became spread out and delayed as it traveled through a long, insulated cable. His calculations showed that the retardation increased with the square of the cable's length, a finding that became known as the “law of squares.”


The implications were enormous. A cable stretching roughly 2,000 miles across the Atlantic could not simply be treated as a giant version of an ordinary telegraph wire. Its dimensions, copper conductor and insulation had to be carefully considered if messages were to travel at a useful speed.

Thomson argued that the telegraph needed an extremely sensitive receiving instrument capable of detecting the tiny currents arriving after their journey beneath the ocean. He developed a mirror galvanometer, replacing a heavy moving needle with an extraordinarily light suspended magnet and mirror. A beam of light reflected from the tiny mirror onto a scale transformed minute movements of the magnet into visible signals.

The instrument became one of Thomson's most important contributions to submarine telegraphy. It was sensitive enough to handle very small currents and also valuable for testing the cable itself. Thomson later developed a marine version specifically suited for use aboard ships.

But getting the cable across the Atlantic proved anything but straightforward. The first expedition in 1857 ended in failure after the cable broke while being paid out in deep water. Thomson had already warned about the challenges of cable construction and laying, and he also worked on machinery to regulate the cable's tension as it descended from the ship.

The following year brought another attempt, and another series of disasters. The expedition encountered a violent Atlantic storm, the cable was damaged, and repeated attempts to lay it failed. After one breakdown, the ships returned to Queenstown, with the project appearing close to collapse.

A final attempt began in July 1858. The ships met in the middle of the Atlantic, spliced the cable and began paying it out toward opposite shores. The operation was painfully tense as it had to be watched constantly, while Thomson's marine galvanometer provided crucial information about the electrical condition of the line.

Then, on August 5, 1858, the cable finally reached land at both ends. For the first time, a submarine telegraph cable had successfully connected Europe and North America across the Atlantic. Thomson sent and received the first electrical signals through the completed cable, helping demonstrate that the enormous undertaking had succeeded.

The achievement triggered celebrations on both sides of the Atlantic. Yet the first cable's usefulness was short-lived, and signaling problems soon emerged. The initial system was painfully slow, demonstrating once again that successfully laying a cable was only part of the challenge. Thomson's sensitive instruments, however, had already shown their value, and his work became deeply connected with the subsequent development of submarine telegraphy.