Transatlantic Telegraph Cable - Page 1
Forum section: electronics / general

author: nullstring
author_rank: Newbie
timestamp: May 18, 2018, 01:33:48 am
content: Strange the stuff you watch on YouTube at 2am. I’ve just watched 2 videos about the first transatlantic telegraph cable laid in 1865. This got me thinking, as signal degradation was a huge problem, what would the line resistance have been in that first cable?

author: helius
author_rank: Super Contributor
timestamp: May 18, 2018, 02:12:45 am
content: Assuming the use of copper for the conductors, with a ρ of 1.68 x 10-8Ω·m, and a density of 8.96 g/cm3:According to the articlehttps://en.wikipedia.org/wiki/Transatlantic_telegraph_cablethe cables were 4300 km long, made of 7 conductors totalling 73 kg/km, which is 73 g/m or 0.104 g/cm for each conductor. The cross-section A of the conductors was 0.104 g/cm over 8.96 g/cm3, or 0.0116 cm2. Using Pouillet's law, we find R = ρ · ℓ / A, or 62.1 kΩ.However, the real problem seems to have been the imbalance between the inductance and capacitance of the cable.

author: Red Squirrel
author_rank: Super Contributor
timestamp: May 18, 2018, 02:04:35 pm
content: That's impressive that they were able to do that way back then without modern tech. Like there would not have been GPS or anything like that to navigate the most efficient route. Just that alone would make it harder. Did they have powered ships back then or only sail boats? That would be another challenge!

author: Brumby
author_rank: Unknown
timestamp: May 18, 2018, 02:13:10 pm
content: According to Wiki, ocean going steamships started hitting the high seas from circa 1820.

author: Gromitt
author_rank: Regular Contributor
timestamp: May 18, 2018, 06:36:45 pm
content: Also according to Wikipedia, the transatlantic cable was laid by SS Great Eastern, the largest ship built at the time.https://en.wikipedia.org/wiki/SS_Great_Eastern#Cable_laying

author: james_s
author_rank: Super Contributor
timestamp: May 18, 2018, 06:57:32 pm
content: Wow, that ship was purchased for £16,000 as scrap, that was a hell of a lot of money in the late 1800's, I guess there would have been a lot of bronze and brass and whatnot though.

author: tggzzz
author_rank: Super Contributor
timestamp: May 18, 2018, 08:04:42 pm
content: There's a lot of solid information in Arthur C Clarke's "Voice Across the Sea", written in 1958 just after the first trans-Atlantic voice circuits were deployed.Ob question: why did the last trans-Atlantic coax cable (TAT-7) have an impedance of (IIRC) 61.8ohms? There used to be a section of it at my lab, and I really wished I'd cut off a piece as a memento!

author: duak
author_rank: Super Contributor
timestamp: May 19, 2018, 01:39:55 am
content: One fellow I know has some of the equipment that was used in one of the later 19th century cables in his collection of archaotechnik. It may have been on one of the shorter Newfoundland to the mainland cables though.With a series resistance of some 60 odd K, the time constant must be several seconds if not minutes. However, if it's also a transmission line it should be possible to circumvent this somewhat. I understand the receiver was a mirror galvanometer that was operated in a darkened room. If memory serves, the data rate was about 1/60 baud. Since Morse code has a varying symbol length the net data rate was less than that, It'd take a few minutes to get a word. Anyone know for sure?Cheers,

author: amyk
author_rank: Super Contributor
timestamp: May 19, 2018, 05:20:27 am
content: Quote from: duak on May 19, 2018, 01:39:55 amFromhttp://en.wikipedia.org/wiki/Transatlantic_telegraph_cable#Communication_speedsQuoteAn interesting contrast to the common notion that signals travel "at near the speed of light" --- we're used to seeing things like turning on a switch and seeing the device connected to it powered up instantly, and think that wave propagation effects are only relevant for very high speed signaling (into the MHz range if not higher), but I guess the long distance made the effects visible in terms of human time scales.