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    Re: telegraphic longitude article
    From: George Huxtable
    Date: 2003 Dec 27, 00:04 +0000

    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
    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
    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?
    contact George Huxtable by email at george---.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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