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    Re: telegraphic longitude article
    From: Fred Hebard
    Date: 2003 Dec 26, 21:00 -0500

    Brooke,
    
    Yes, Lunars can be done without a chronometer, but a watch for carrying
    local time forward would be helpful, unless local time were determined
    during the shooting of the lunar, which was not usually the case, in my
    understanding.
    
    Fred Hebard
    
    On Dec 26, 2003, at 6:59 PM, Brooke Clarke wrote:
    
    > Hi George:
    >
    > Thanks for that clarification.
    >
    > While reading a number of papers about the Transit Satellite
    > navigation system I learned that the main method being proposed while
    > Harrison was working on his chronometers was what's now called
    > Lunars.  I don't know anything about Lunars, but surmise that they can
    > be done without a chronometer, is that the case?
    >
    > Happy Holidays,
    >
    > Brooke Clarke, N6GCE
    > http://www.PRC68.com
    >
    > George Huxtable wrote:
    >
    > 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
    >
    >
    > ================================================================
    > 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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