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    Re: telegraphic longitude article
    From: Brooke Clarke
    Date: 2003 Dec 26, 13:59 -0800

    Hi Fred:
    
    I'm not knowledgeable about the specifics of the telegraph system, but I
    would think that the drum recorder could be used to characterize the
    relay (repeater) delay quite easily.  Then knowing the exact route of
    the line sum all of these.  I would expect the delay in a good relay to
    be less than 20 milliseconds.
    
    Happy Holidays,
    
    Brooke Clarke, N6GCE
    http://www.PRC68.com
    
    Fred Hebard wrote:
    
    > Brooke,
    >
    > Thanks.  I was wondering about propagation delays engendered by relays.
    >  My imperfect understanding is that these were used every hundred miles
    > or so, and were the big breakthrough Morse obtained from Henry that
    > permitted a practical telegraph system.  But it's unclear to me the
    > separation needed between relay stations and their switching time.
    >
    > Fred
    >
    > On Dec 26, 2003, at 2:27 PM, Brooke Clarke wrote:
    >
    >> Hi:
    >>
    >> My reading is that the key element was replacing ears trying to
    >> interpolate between one second ticks and the use of a drum recorder
    >> with
    >> a tick recorded once per second where interpolation could be used to
    >> determine the time of star crossings that were also marked on the drum
    >> by an observer pressing a manual key while looking through a 48"
    >> meridian transit.
    >>
    >> 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.
    >>
    >> Happy Holidays,
    >>
    >> Brooke Clarke, N6GCE
    >> http://www.PRC68.com
    >>
    >> Fred Hebard wrote:
    >>
    >>> I have read this article carefully, and methods for determining the
    >>> propagation delay in the circuit are not readily apparent to me.  If
    >>> an
    >>> astronomic phenomenon that occurred simultaneously at both sites were
    >>> measured, the delay could be determined, it appears to me.  Perhaps
    >>> occultations of Jupiter's moons might be free of parallax effects?
    >>> Also, it would seem for the measurement of a simultaneous effect that
    >>> the sites might need to be connected by two circuits.
    >>>
    >>> On Dec 24, 2003, at 2:32 PM, Fred Hebard wrote:
    >>>
    >>>> Thanks Paul!  I had been wondering how they got around propagation
    >>>> delays.
    >>>>
    >>>> On Dec 24, 2003, at 2:14 PM, Paul Hirose wrote:
    >>>>
    >>>>> Professional Surveyor magazine has an online article about the early
    >>>>> use of the telegraph for longitude determinations.
    >>>>>
    >>>>> http://www.profsurv.com/ps_scripts/article.idc?id=1147
    >>>>>
    >>>>> Alexander Bache, head of the U.S. Coast Survey, was quick to realize
    >>>>> the possibilities. He organized an experiment which measured the
    >>>>> longitude difference between Washington and Philadelphia by means of
    >>>>> telegraph in 1846.
    >>>>>
    >>>>> By the mid-1850s, the technique had become routine. Chronographs
    >>>>> recorded the electrical impulses of the observer's hand switch on a
    >>>>> paper-covered rotating drum as stars crossed the meridian at both
    >>>>> observatories. Also recorded were 1-second pulses from break-circuit
    >>>>> chronometers at both ends of the telegraph line. With this data,
    >>>>> surveyors could eliminate clock offsets and propagation delays.
    >>>>>
    >>>>> Telegraphic longitudes were a huge improvement over the Coast
    >>>>> Survey's
    >>>>> former longitude methods: lunar culminations, lunar occultations,
    >>>>> and
    >>>>> chronometer transportation. (Before the trans-Atlantic cable was
    >>>>> laid,
    >>>>> the Survey made more than 1200 chronometer exchanges with England.)
    >>>>>
    >>>>
    >>>
    >>>
    >>> ----------------------------------------------------------------------
    >>> --
    >>> Frederick V. Hebard, PhD                      Email:
    >>> mailto:Fred{at}acf.org
    >>> Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
    >>> American Chestnut Foundation                  Phone: (276) 944-4631
    >>> 14005 Glenbrook Ave.                          Fax: (276) 944-0934
    >>> Meadowview, VA 24361
    >>>
    >>>
    >>
    >
    >
    
    
    

       
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