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The Great Transatlantic Cable | Article

Inventing the Mirror Galvanometer

Cable Galvanometer.jpg
Burndy Library

A chance "eureka moment" by a Scottish physicist solved a problem that had plagued the transatlantic cable -- and made a lord out of the man who had the insight.

Young Physicist
Son of a mathematics professor at Glasgow University, William Thomson entered university at age 11, studied physics in Paris, and was appointed to the natural philosophy chair at Glasgow at the ripe old age of 22. There he created the world's first physics laboratory, allowing students to conduct experiments rather than just study theory. Author of more than 300 papers, Thomson presented the first statement of the Second Law of Thermodynamics. Fortunately for the cable project, Thomson also accepted an invitation to join the board of directors of the Atlantic Telegraph Company.

Improving Weak Signals
One of the fundamental problems of transmitting a signal through a lengthy submarine cable was that the electrical current tended to be very low (as little as 1/100,000th of a standard light bulb), making any signals sent difficult to detect. A.T.C. Chief Electrician Edward Whitehouse thought this problem could only be solved by sending larger doses of electric current down the line. But Thomson had a different approach. He thought the best response was to devise a device that could read faint signals. The galvanometer, first invented in 1802, was a means of detecting electric current. It consisted of a needle that was deflected by the magnetic field that electric current created; a stronger current produced a larger deflection. But the galvanometers of the day couldn't detect the weak signals that came through a long underwater cable.

Idea for a Detector
One day Thomson was in his study, standing at a desk and twirling his monocle. He observed that the sunshine coming in the window was reflecting off the monocle, and the spot of light it created was zooming around the room. Suddenly, Thomson realized what he had to do. He directed his assistant to attach a tiny glass reflector to an iron wire. When Thomson focused a kerosene flame on the mirror, it showed even the slightest movement of the wire. This device, which Thomson dubbed the mirror galvanometer, could detect signals one thousand times fainter than other receivers.

Lord Kelvin
Thanks to Thomson's invention, the weak currents through the submarine cable could now be easily read. Whereas Whitehouse's high-voltage induction coils had burnt out the insulation of the first cable, Thomson's mirror galvanometer required only a little battery and enabled operators to read the Morse Code coming through the cable at ten times the old rate. It was a crowning achievement for the man whom Arthur C. Clarke would later describe as combining the talents of Albert Einstein and Thomas Edison. The invention of the mirror galvanometer inspired poetry and led to Thomson being knighted Lord Kelvin, the first scientist to receive such an honor. And Kelvin's name lives on; one year after his death, the temperature unit used by scientists was renamed the Kelvin in his honor.

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