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Brooke Clarke wrote-

>In this scenario with a 1,000 mile separation between stations the
>propagation delay would be on the order of 5 milliseconds which is
>probably in the noise compared to the human reaction time variation in
>pressing the key.  But could be calculated based on the known
>propagation constant for the open wire type lines then in use.

Response from George.

We have considered this matter before on Nav-l, but I can't recall the
threadname.

Brooke seems to be assuming that the velocity of the signals travelling
down the wire is something like the velocity of light. That would be the
case for a conductor with a defined inductance per unit length and capacity
per unit length, terminated by its characteristic impedance, which is
defined by the ratio of those quantities. In the case of those early
telegraphs, they were not terminated that way, in order to obtain
sufficient voltage at the receiving end. Also, the high series resistance
of the wire, much greater than its characteristic impedance, played an
important part. After a step-voltage was applied at the sending end, that
step would reflect backwards and forwards between the two ends, growing all
the time, until it reached a detectable threshold value at the receiving
end.

So the transmission time was MUCH greater than you would calculate using
the velocity of light, and depended significantly on the sending voltage
and the sensitivity of the receiver.

Gould, of the US Coast Survey in 1857, found that the effective signal
velocity along a 300 mile wire was 7792 miles per second, or only 1/24 of
the velocity of light (it's in Chauvenet, vol 1, page 349).

I doubt if there were relays involved in the telegraphs for that purpose at
that time.  Observer A would tap a key when a star crossed (one of many)
hairs in his transit telescope, and then a current pulse would flow down
the wire. At both ends A and B of the wire would be a chronograph, which
was a recording galvanometer, which drew a trace of the resulting
current-pulses. A single master-clock, somewhere in the system, added
timing pulses to the trace, at the two ends. Times were recorded at both
ends when a star passed over observer A, and again when the same star
passed over observer B.

This technique allowed the transit time of the elactrical signals to be
cancelled out.

By averaging many observations of many stars, a precision of time
measurement of 0.02 seconds could be achieved. Rather good, without
electronics, wasn't it?

George


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