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Jan Kalivoda wrote-

>If I understood George Huxtable well, both places A and B worked with time
>pulses sent through the line only by the chronometer in A and marked their
>meridian passages against A-pulses (obtained by telegraph in B). The shift
>of the time of a meridian passage of the same star from A to B against
>A-pulses gave the longitude difference between A and B without any need of
>knowing the real signal delay in the wire.
>
>After the accurate longitude difference of B to Greenwich (or any other
>place A) had been known, one could compare his chronometers in B with the
>time in A by meridian passages at B without blocking the telegraph line.

==============

I summarise Chauvenet's description (over several pages) of the procedure
to be as follows-

There was an agreed "get ready" signal which could be made by either
operator, at which point both would release the weight that turned the
paper-drum that the recording pen worked on, producing a helical plot. In
some instruments this would continue for, say, 15 minutes, allowing a
number of stars to be timed from the same end. The single clock, at one end
or the other (not both) would break the current  on the common circuit for
a short moment at 1-second intervals, which would show up as a blip on both
chronographs. In some implementations, there was a special coding on the
clock to miss out one pulse as the second-hand passed zero, as Paul Hirose
suggests.

Let's say the sequence was initiated by a signal from operator A, because a
star was just going to transit at his location. Then as the star passed the
crosswires (of which there were several) in his eyepiece, he would press a
key each time. Each key-press would break the common circuit, but for a
longer tme than the clock pulses, so they could be readily distinguished,
as they showed up on the paper drum, at both ends. So at each end, a paper
plot resulted showing the relative timings between the clock and the star
timing marks for the star passing A. Each operator would write down the
date and approx. time and star name on the record.

Then as the star passed overhead for B, the same thing would happen,
initiated by B. Again, both chronographs would show a plot of the blips as
the star passed B's crosswires, together with the timing marks from the
common-clock.

This would be done for many stars on many nights, with the transit
instrument sometimes reversed on its mountings. If there was also a precise
clock available at B, for half the observations that would be used as the
common time-reference, instead of the clock at A.

Later, the chronograph plots from both ends would be brought together for
analysis. This allowed any delay in the electrical signal between the two
ends, and any difference between the mechanical responses of the two
pen-systems, to be allowed for.

Chauvenet includes sample traces (in vol. II) to show what the recording
looked like, on different chronograph instruments.

There's a matter that has been omitted so far. It all depends on the two
observers looking at the stars passing crosswires and pressing a key, and
on their reaction times being the same, to high accuracy. This was ensured
by exchanging the operators between the two ends, part way through the
campaign.

By such careful attention to detail, time diffences to 20 milliseconds
could be achieved, which I make to be somewhat less than 30 feet in
longitude difference (depending on latitude). Quite a technical achievement
for 1857, in the days long before electronics, in my view. The man behind
it was B A Gould, of the US Coast Survey.

===============

Ken James added-

>OK, I am sure a no nothing here, but...why could they not have just put the
>recording station electrically/physically halfway between the two stations
>and record the pulses from both stations, then it would be easy to get a
>time difference, which is all that matters, who cares about "absolute" time
>right then? You can measure and interpret later with no need to record
>actual time except at ONE station.

That seems a fair suggestion to me, but consider this:

The telegraph stations were set up in centres of civilisation, where
possible, with somewhere for operators to inhabit and obtain supplies of
essentials such as food and battery cells. The halfway point couldn't be
chosen for a suitable location. It may well have been out in hostile or
barren or inaccessible territory, with local tribes to whom the telegraph
meant no more than a source of copper or of glass insulators. It would not
have been economical (I suggest) to establish telegraph huts at such
unpromising locations simply for the purpose of determining longitudes.

However, I admit to knowing little about telegraphy in general, or about
its early history in the US in particular. One question I would like to ask
is- What was the diameter (or AWG) and the height above ground, of the wire
used in those early US telegraph systems, roughly speaking? I presume that
it was always copper wire.

George.



================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

   

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