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Putting those words into a search engine produces many interesting
hits. Take a look at this paper presented at an IEEE conference by
Trudy Bell. She goes into the specific techniques in some detail.

http://www.ieee.org/organizations/history_center/cht_papers/Bell.pdf

And this online version of the 1911 Encyclopaedia Britannica has an
article with surprising technical depth. It's a bit rough due to
misreads by the optical character reader used to transcribe the text.
However, it's possible to make out the method used to cancel the
effect of signal transit time over the telegraph link.

http://26.1911encyclopedia.org/T/TI/TIME_STANDARD.htm

Those two links are just from the first page of hits; there may be
even better stuff out there. I haven't looked further yet.


Coincidentally, I have "The Victorian Internet" (Tom Standage, 1998)
checked out of a library. The high voltage telegraphy George is
wondering about was the brainchild of Dr. Edward Whitehouse. Tycoon
Cyrus Field hired him as chief electrician on the Atlantic cable
project. Unfortunately, Whitehouse was a medical doctor who was merely
a self-taught hobbyist when it came to telegraphy.

Although the cable worked at first, it couldn't take the voltage from
Whitehouse's big induction coils. After a month the gutta-percha
insulation broke down.

It was the great scientist William Thomson (later known as Lord
Kelvin) who showed that low voltage and an extremely sensitive current
detector were the right approach. Standage says: "The death blow was
finally dealt to Whitehouse's high-voltage theories by the noted
engineer Josiah Latimer Clark, who had the two cables connected
back-to-back and successfully sent a signal around the whole circuit
-- from Ireland to Newfoundland and back -- using a tiny battery and
Thomson's mirror galvanometer as the detector."

   

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