Lesson 1 of 81857–18666 min
The Atlantic telegraph
On 5 August 1858 two warships, HMS Agamemnon and the US frigate Niagara, reached opposite shores of the Atlantic. They had met mid-ocean, spliced their halves of a cable together and steamed apart, paying it out behind them. For the first time a wire ran from Ireland to Newfoundland.
Days later Queen Victoria sent President Buchanan a message of 98 words. It took about sixteen and a half hours to get across. By 18 September the cable was dead, after carrying a few hundred messages.
It took eight more years, a lost cable and the largest ship in the world to make the link permanent.
Why the 1858 cable was so slow
A telegraph sends pulses of current: on and off, short and long. Over a few kilometres of overhead wire, a pulse arrives looking much like the one that left. Over thousands of kilometres of cable lying in seawater, it does not. The signal weakens with distance, which engineers call attenuation, and each sharp pulse is smeared into a slow swell, so pulses sent too quickly blur into one another. The only way to keep them apart was to send slowly, which is why 98 words took most of a day.
Reading such faint signals needed a new instrument. William Thomson, later Lord Kelvin, designed the mirror galvanometer: a tiny magnet glued to a small mirror, hung inside a coil. A whisper of current turned the mirror, and a beam of light reflected from it swung along a scale, so a clerk could read the message as movements of a spot of light. The Science Museum, which holds one, describes it as the only instrument sensitive enough to read the first transatlantic messages reliably.
Sender's key → Cable: Battery pulse
The operator closes the key and current flows into the cable at Valentia.Failure, then success
The 1858 cable had faults in its copper core and insulation from the start. As it deteriorated, chief electrician Wildman Whitehouse raised the voltage to push signals through, which hastened its end. The lesson was lasting: a weak line is not fixed by shouting down it. The fix was a better cable and a more sensitive receiver.
Brunel's Great Eastern, converted into a cable ship, tried in 1865 and lost the cable about 600 miles from Newfoundland. On 13 July 1866 she sailed from Valentia again and on 27 July landed a new cable at Heart's Content, Newfoundland. She then went back, grappled the lost 1865 cable from the seabed, spliced it and finished it. By early September 1866 two cables were working across the Atlantic, and the station at Heart's Content stayed in service until 1965.
Key terms
- Attenuation
- Loss of signal strength over distance, measured in decibels.
- Distortion
- Change in the shape of a signal, such as a sharp pulse being smeared out.
- Galvanometer
- An instrument that detects small electric currents.
- Repeater
- A device that receives a weakened signal and sends it on at full strength.
- Throughput
- The data actually delivered per second, always at or below the link's bandwidth.
Check what you learned
1.What hastened the failure of the 1858 Atlantic cable?
2.Which ship laid the first lasting Atlantic cable in 1866?
3.A signal arrives too weak to read at the end of a long cable. What is that called?
Practise it for the exam
- Network+ · Networking ConceptsQuestions on this topic
- Network+ · Network TroubleshootingQuestions on this topic
Sources
- Milestones: Landing of the Transatlantic Cable, 1866 — IEEE (Engineering and Technology History Wiki)
- A History of the Atlantic Cable & Undersea Communications (Cookson history) — atlantic-cable.com
- Mirror galvanometer for the transatlantic telegraph, 1858 — Science Museum Group