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From: George Huxtable (no email)
Date: Thu Jun 02 2005 - 13:11:34 EDT
I'll have a go at correcting some recent misleading statements about
telegraphic cables, made under the heading "Time Sights".
Alex said-
>I can suggest another reason why time transmission though
>a transatlantic (or other very long) cable could not be acceptable.
>In those XIX century cables,
>the signals were substantially spread in time
>when transmitted.
>For example, in the very first transatlantic cable,
>a short message of few words had to be transmitted
>for several hours. A sharp impuls you send from one end
>arrived as a very long wave.
>So reliable transmission of a time signal could be
>impossible.
It's true, as Alex said, that "a sharp impulse you send from one end
arrived as a very long wave". But the time-scales that he refers to are
quite misleading. The first cable, in 1858, only lasted a few days before
its performance deteriorated so as to make in quite unusable. During that
period, the receipt of messages was bedevilled by an increasing level of
intermittent faults, presumably the result of the gradual ingress of
seawater. With the crude high-voltage source employed at one end and the
crude signal-detector employed at the other, even the simplest messages
degenerated into long sequences of "please repeat", "what?" "please send
slower". The detected signals were just too hard to disentangle from the
noise. That's how Alex's "several hours" accumulated, just as Lu Abel has
explained. In 1866 two successful cables were installed, within a few
weeks, and without those attendant problems.
I don't know how long the interval was between the pressing of the "send"
key and the detection of the signal at the other end. It depends on the
voltage and drive-impedance of the source and the sensitivity of the
detector, and the noise level, but a delay of a few seconds seems to me
most likely. Such a delay would be amenable to time-correction using the
method that Paul Hirose has described.
Lu continued-
""Spreading" of electrical signals takes place when a medium (such as a
cable) can not transmit the high frequency portion of a signal. An
on-off pulse, such as a telegraphic character, theoretically includes
signal components of very high (indeed, infinite) frequencies. But the
speed of transmission is still the speed of light. Thus a nice sharp
pulse transmitted on one end of the cable might arrive as a smeared-out,
possibly unrecognizable mess -- but it would travel across the Atlantic
(and therefore arrive) in a fraction of a second."
Well, it's true that a coax cable, used in the manner they usually are
today, transmits signals at the velocity of light. Not the velocity of
light in vacuum, of 3 x 10-to-the-8 metres per second, but a bit less,
relating to the speed of electromagnetic waves in its insulation material.
In polyethylene, as an example, its roughly 2 x 10-to-the-8. (It's similar
to the way light is slowed when it passes through a medium such as glass or
water). But such usage requires that the cable is driven at one end by its
"characteristic impedance" and is connected to a detector at the other end
with that same impedance. For most coax cables that impedance is of the
order of a few tens of ohms, such as TV downlead, which is usually 50-ohm
coax. That characteristic impedance doesn't depend on the length, or
overall resistance, of the cable. If such a cable could be "terminated" in
that way, then a transatlantic cable of say 4000 km would have a signal
delay of about 20 milliseconds.
But it just wasn't possible for a transatlantic cable to be operated in
that manner. Instead, the rise of the signal at the receiving end was
dependent on the gradual charging of the total capacitance of the whole
length of the cable, through the total resistance of the whole length of
the cable. That "CR time constant" is what dominated the transmission times
and the sending rates, and increased considerably with length of cable.
Finally, Lu states-
"David Thompson was the chief engineer (or scientist) on the second
cable. "
No! It was William Thomson, later Lord Kelvin, about the greatest PRACTICAL
scientist there's ever been, the inventor of the Kelvin (Admiralty pattern)
compass and a host of other marine devices, and the first man to properly
understand how an iron vessel's compass should be corrected. In fact, he
was scientist aboard the first, unsuccessful, attempts in 1857 and 58, as
well, but on those occasions he had to implement another man's faulty
designs.
My information on cable matters comes from "Voice across the Sea", by
Arthur C Clarke, a well-known name now, but perhaps not when he wrote the
first edition in 1958.
Fred Hebard asked-
"Is there any mention of this in the older texts, such as Chauvenet,
where time sights were done at geographic markers set by a professional
astronomer?"
I think the American Coastal Survey, under Gould, had these telegraphic /
astronomic techniques rather well sewn up long before the transatlantic
telegraph arrived, and well ahead of the rest of the world. Chauvenet
refers to Gould's work in 1853 to establish the time-difference (and
therefore longitude-difference) between Seaton Station (Washington) and
Raleigh Station in North Carolina, using star transits, as being 6 min
32.99 sec, with an error of ± 0.02 seconds, which I estimate to be to a
precision of about 10 metres! Pretty good going, in 1853...
George.
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contact George Huxtable by email at , by phone at
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