Transatlantic Telegraph Cable

[nullstring]

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?

[helius]

Assuming the use of copper for the conductors, with a ρ of 1.68 x 10^-8 Ω·m, and a density of 8.96 g/cm³:
According to the article https://en.wikipedia.org/wiki/Transatlantic_telegraph_cable the 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.

[Red Squirrel]

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!

[Brumby]

According to Wiki, ocean going steamships started hitting the high seas from circa 1820.

[Gromitt]

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

[james_s]

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.

[tggzzz]

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!

[duak]

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,

[amyk]

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,

From http://en.wikipedia.org/wiki/Transatlantic_telegraph_cable#Communication_speeds

Initially messages were sent by an operator sending Morse code. The reception was very bad on the 1858 cable, and it took two minutes to transmit just one character (a single letter or a single number), a rate of about 0.1 words per minute.

An 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.