That book led me to Gutta Percha, the plastic-like coating on the wires used in these cables which was quite the innovation and made this all possible. Vulcanized rubber was the other option but performed poorly in cables and was harder to work with.

https://atlantic-cable.com/Article/GuttaPercha/

The above is a fascinating and depressing history of the Gutta Percha factory that made all these cables, after joining with the cable company that supplied the actual wires. There's an 1853 travelogue piece embedded here of an author visiting the factory, where he notes in the worst parts of the factory where boiling and heat are applied, it was staffed with boys who barely made more than a dollar a week. By boys I thought it was slang for young men then I realized 1850s England was heavily using child labor.

Those cables are the product of child labor, like much of the Victorian age's industrial and textile output. Children often made up significant portions of factory workforces, sometimes 25-50% in certain textile sectors, with many under 14. I wish the stories of child labor were better told and more prominent. This abuse and exploitation of children gets quite whitewashed during this age and its nice to see it acknowledged, albeit briefly.

At least in the UK the fact that the Victorians and others used a lot of child labour is well and widely known.

Blake wrote the poem The Chimney Sweeper about boys sold into the trade long before the 1850s and Elizabeth Barrett Browning published The Cry of the Children in Blackwood's magazine in 1843. Charles Kingsley used his The Water Babies to question child labour and England's treatment of the poor in general in 1862-3.

No one with any pretensions to knowledge of those times can claim not to know about child labour.