page_title: Transatlantic telegraph cable
section_title: Thomson's mirror galvanometer
hierarchical_section_title: Transatlantic telegraph cable / Failure of the first cable / Thomson's mirror galvanometer
caption_attribution_description: Sir William Thomson, Lord Kelvin, (1824-1907) was Professor of Natural Philosophy at Glasgow University, and was well known for his work in many branches of physics, including electricity. The first transatlantic telegraph cable was layed in 1858, reducing the time it took for messages to cross from days or months to hours. The success of the system relied on an instrument that could detect the tiny electrical signals that had passed so far under the sea. Thomson's galvanometer was the only instrument sensitive enough to detect reliably the first transatlantic telegraph messages. Made by J White.
context_page_description: Transatlantic telegraph cables were undersea cables running under the Atlantic Ocean used for telegraph communications. Telegraphy is now an obsolete form of communication and the cables have long since been decommissioned, but telephone and data are still carried on other transatlantic telecommunications cables. The first cable was laid across the floor of the Atlantic from Telegraph Field, Foilhommerum Bay, Valentia Island in western Ireland to Heart's Content in eastern Newfoundland. The first communications occurred August 16, 1858, reducing the communication time between North America and Europe from ten days—the time it took to deliver a message by ship—to a matter of minutes.
  Cyrus West Field and the Atlantic Telegraph Company were behind the construction of the first transatlantic telegraph cable. The project began in 1854 and was completed in 1858. The cable functioned for only three weeks, but it was the first such project to yield practical results. The first official telegram to pass between two continents was a letter of congratulations from Queen Victoria of the United Kingdom to President of the United States James Buchanan on August 16.
context_section_description: After his experience on the 1857 voyage, Thomson realised that a better method of detecting the telegraph signal was required.  He spent the time waiting for a new voyage inventing his mirror galvanometer.  This was an extremely sensitive instrument, much better than any existing equipment.  He requested £2,000 from the board to build several of them, but was only given £500 for a prototype, and permission to trial it on the next voyage.  The mirror galvanometer was extremely good at detecting the positive, or negative edge of a telegraph pulse respectively representing a Morse 'dash' and a Morse 'dot' (this was the standard system on submarine cables – both pulse types were the same length, unlike overland telegraph).  Thomson believed he could use this instrument with the low voltages from regular telegraph equipment even over the vast length of the Atlantic cable.  He successfully tested the instrument on 2,700 miles of cable in underwater storage at Plymouth.
  The mirror galvanometer proved to be yet another point of contention.  Whitehouse wanted to work the cable with a very different scheme.  He wanted to drive the line with a massive high-voltage induction coil, producing several thousand volts, so that enough current was available to drive standard electromechanical printing telegraphs used on inland telegraphs.  Thomson's instrument had to be read by hand.  It was not capable of printing, but much later he invented the syphon recorder for the second transatlantic attempt in 1866.  The decision to start mid-Atlantic combined with Whitehouse dropping out of another voyage left Thomson, on board Agememnon sailing towards Ireland, with a free hand to use his equipment without Whitehouse's interference.  Although Thomson had the status of a mere advisor to engineer C. W. de Sauty, it was not long before all electrical decisions were deferred to him.  Whitehouse in Valentia remained out of contact until the ship reached Ireland and landed the cable.
  Around this time, the board started to have doubts over Whitehouse's generally negative attitude.  Not only did he repeatedly clash with Thomson, but he was also critical of Field, and his repeated refusal to carry out his primary duty as chief electrician on board ship made a very bad impression.  With the removal of Morse, Whitehouse had lost his only ally on the board, but at this time no action was taken.
caption: Thomson's mirror galvanometer
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