AHC: Earliest plausible Trans-Atlantic undersea cable?

I'm assuming this means "given an Industrial Revolution occuring roughly as OTL", as otherwise I could come up with something involving Phoenician steamships in 1000 BC.

First, of course, the telegraph needs to be invented: the first telegraph that could work over long distances and underwater was basically developed in 1836 OTL. With some small POD's you could probably push that back to 1800. In OTL it took 30 years to get to a successful (for more than a few weeks) transatlantic telegraph: let's say the British keep hold of the US, and have a very good reason to want to communicate quickly across the sea, so we push back the timeline to 20 years after.

Thus, 1820 (we also assume no Napoleonic wars to distract from the project).
 
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Not before a ship big enough to carry the cable drum has been built, IOTL it was the commercially completely unsuccessful ocean liner Great Eastern, that was converted to hold the gargantuan drum with the Transatlantic cable no other ship of that time could house, and even this was only possible due to the fact that the line operating the Great Eastern wanted to get rid of her as fast as possible to cut the losses each new trip of this ship caused them and sold this by far biggest ship of its' time - it had more than 3 times the gross tonnage and displacement of the most powerful warship of its' time, the HMS Warrior - for a mere 25.000 £, only 5% of the 500.000 £ construction costs.

Without this bargain the competitor of Cyrus West Field's Atlantic Telegraph Company, building a telegraph line through Canada and Alaska as well as Siberia necessitating only the crossing of the merely 85 km wide and 30 to 50 m deep Bering Strait by an undersea cable, might have won the race to first connect Europe and America by telegraph and might thus have even further delayed a working Transatlantic cable. Then there's also the necessity to develop an insulation for the cable able to withstand the adversities of the ocean (IOTL the first cable laid wasn't and stopped working within a few weeks).
 
Not before a ship big enough to carry the cable drum has been built, IOTL it was the commercially completely unsuccessful ocean liner Great Eastern, that was converted to hold the gargantuan drum with the Transatlantic cable no other ship of that time could house, and even this was only possible due to the fact that the line operating the Great Eastern wanted to get rid of her as fast as possible to cut the losses each new trip of this ship caused them and sold this by far biggest ship of its' time - it had more than 3 times the gross tonnage and displacement of the most powerful warship of its' time, the HMS Warrior - for a mere 25.000 £, only 5% of the 500.000 £ construction costs.

Without this bargain the competitor of Cyrus West Field's Atlantic Telegraph Company, building a telegraph line through Canada and Alaska as well as Siberia necessitating only the crossing of the merely 85 km wide and 30 to 50 m deep Bering Strait by an undersea cable, might have won the race to first connect Europe and America by telegraph and might thus have even further delayed a working Transatlantic cable. Then there's also the necessity to develop an insulation for the cable able to withstand the adversities of the ocean (IOTL the first cable laid wasn't and stopped working within a few weeks).

They also didnt understand the electrical properties of long cables. The first cable had a data rate of one character per ten seconds. Not ten per second, one tenth per second. The guys in charge then upped the voltage, hoping that would improve data rates, and burnt the cable out.

Otl may not be the very earliest, but there were significant advances that had to happen to make it practicable.
 
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