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    The first micrometer sextant.
    From: George Huxtable
    Date: 2005 Aug 1, 11:49 +0100

    The first micrometer sextant.
    Several weeks ago now, Alex Eremenko asked which maker was responsible for
    introducing the first sextant to provide a micrometer drum for readout,
    rather than a Vernier scale. At the time, the consensus seemed to be that
    the introduction of the micrometer came in the 1920s, the makers being
    Heath or Plath, but the priority was unclear. Information was available
    from Peter bIfland's "Taking the Stars", other texts about sextants being
    curiously coy about the matter.
    Ifland refers to the introduction of an unclampable  tangent-screw for fine
    ADJUSTMENT (but not read-out) in Heath's Hezzanith range in 1909.
    Jean Randier's book "Marine navigating instruments", translated from the
    French, 1980, shows a catalogue by the Hamburg firm of C Plath. The
    catalogue is labelled "Ausgabe V" but seems to carry no date. Randier says
    it's "about 1902".
    That catalogue shows a quintant (which, surely, counts as a sextant for
    this purpose) labelled "Vermessungsquintant mit Tromelablesung", which I
    take to mean "surveying quintant". Indeed, quintants were the instruments
    of choice for marine surveyors, over many years. A quintant was just like a
    sextant except that it had a wider arc, to cover an angular range of 144
    degrees (a fifth of a circle, hence the name quintant) instead of the 120
    degrees of a sextant. The geometry of the mirrors had to be adapted
    somewhat to maintain a decent field-of-view in the index mirror.
    Judging by its picture, this quintant clearly possessed all the features of
    the modern micrometer sextant. The only matter in question, then, is
    Randier's rather uncertain dating of that catalogue to be "about 1902".
    Does anyone have more definite information of the date of that catalogue,
    or the date of that instrument?
    That same catalogue shows a sextant which appears from its picture to be an
    entirely conventional instrument with Vernier scale. So Plath's
    introduction of the new micrometer technology seems to have been confined
    to the more exotic (and presumably more expensive) quintant, and not yet
    applied to the workhorse sextant.
    Herbert Prinz has kindly informed me that-
    >Plath published a brief history of the company at the occasion of its
    >100th birthday anniversary:
    >    F.W. Schaafhausen, H. Hoffmann, P. Kaltenbach, _C. Plath. 1862 -
    > 1962_, Verlag Hanseatischer Merkur, Hamburg 1962.
    > From this publication, we learn (p. 107) that Plath introduced their
    > "Trommelsextant" (sextant with micrometer drum) in 1908. ("... entwickelt
    > und herausgebracht ...", they do not claim its invention). In different
    > context (p. 30) it is explained that the improvement was made possible by
    > new technical equipment, in particular machines that could cut worm-gear
    > with sufficient precision. There is no mentioning of a surveying quintant
    > or sextant in this text.
    >Plath was probably the first company that offered a marine sextant with
    >micrometer drum as a standard product (besides the one with Vernier, which
    >it did not totally replace for quite a while).
    So it seems that Plath were indeed the first to provide a true micrometer
    instrument in their quintant (about 1902 and before 1908) and sextant in
    1908. Is anyone aware of such an instrument, by another maker, in that period?
    On a related matter, Ifland's book states-
    "The earliest known application of the drum micrometer to navigation
    instruments is found on a sextant by Jesse Ramsden, ca. 1787, now in the
    National Maritime Museum in Greenwich." I've contacted Peter Ifland, who
    tells me that this information is drawn from Alan Stimson, who was in
    charge of instruments at the NMM, and who wrote a paper-
    "The influence of the Royal Observatory at Greenwich upon the design of 17th
    and 18th century angle-measuring instruments at sea "   A[lan] Stimson.,
    1976. - pp. 123-130  from: Vistas in Astronomy, 1976, Vol. 20.
    Stimson stated "Micrometer tangent screws had been fitted to astronomical
    instruments since the 17th century but its first application to a sextant
    appears to be on a Ramsden instrument of c.1780 (Fig. 18.3) although they
    did not come into general use until about 1920."
    That illustration, fig 18.3, is captioned "A 12-inch sextant signed
    'Ramsden, London'., serial number 724, c.1783.This sextant is fitted with
    the earliest known micrometer drum. NMM reference: S:.27." The photograph
    isn't very clear, butappears to me to show an adjusting screw which carries
    a nut so that when it's turned the screw/nut acts as a sort-of adjustable
    strut. its length varying linearly with the number of turns. There is
    indeed a "drum", to show the fractional turns. This strut is pinned to the
    index arm, at one end, and to a movable "shoe", which can be clamped to
    different positions along the arc, at the other., so that the length of the
    strut is a chord of a circle which passes through those pins.
    This is not very different from the fine-adjustment screw that was commonly
    fitted to Vernier sextants, from the very earliest models. For that purpose
    a clamp-shoe and adjustment-screw were provided in exactly the same way,
    but no provision was made to use the turns and partial-turns of the screw
    to provide any information about the position of the index arm; just to
    provide a smooth controllable motion. Instead, the Vernier scale was relied
    on for the index-arm position information. Even the Ramsden sextant which
    carried the "micrometer drum" retained its Vernier scale.
    In my own view that "micrometer drum" is not a  "tangent screw" at all,
    though that is the term Stimson used for it. A tangent screw (which has no
    connection with the trig. tan function) is so called because it is
    tangential to the arc as it moves around, so that at any position on the
    arc, the same rotation of the screw will cause the same movement of the
    index arm.
    In the instrument described by Stimson, and also in many astronomical
    instruments back to the 17th century, the micrometer screw works quite
    differently from such a "tangent screw". Firstly, it's rather short, so can
    be used to vary the index arm position over a range of a few degrees only.
    For larger movements, the shoe has to be unclamped and repositioned at a
    new spot. Because the strut is a chord of a circle, its change in length is
    proportional to the sin of half the angular change about some preset value,
    which depends on the position of the shoe. For that reason, it can only be
    roughly linear, and restricted to a limited angular range. A correction
    table could be provided to allow for that.
    The worst weakness of such an arrangement is that the fine measurement by
    the micrometer screw is restricted to small differences in angle, that are
    within the range of the that screw. If the shoe position has to be reset,
    then the accuracy of the observation falls back to be the accuracy of
    reading the Vernier. For some astronomical purposes, that may not be a
    severe handicap; if, for example a star position was being measured with
    respect to some other reference star, not very many degrees away.
    For nautical observations using a sextant, however, the only reference
    point available is the index-check observation, near zero degrees on the
    scale, and all observations must be referred to that zero point. In which
    case, the micrometer scale would add nothing to the overall accuracy of the
    observation, which would be dependent entirely on the precision of reading
    the Vernier. In my view, that is the reason why, after Ramsden's attempt at
    a micrometer sextant, the idea was dropped until about 120 years later,
    when machine-tool technology allowed a true tangent-screw to be applied to
    the whole arc.
    My view is that Ramsden's experiment was an abortive attempt, years ahead
    of its time and the necessary technology, to create a more precise sextant.
    Although it could be described, perhaps, as the first micrometer sextant,
    it was NOT a tangent micrometer, and became a dead-end in the development
    of the sextant. True tangent micrometers did not appear until the early
    20th century.
    It's clear that a visit to Greenwich, to see the Ramsden instrument in the
    flesh, is called for, and I will go there when I can. I've just spoken with
    Gloria Clifton, the museum's head, who tells me that the instrument is not
    on view but can be brought out from storage, which at least would give me a
    chance to pick it up and handle it; not so simple when it's in a display case.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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