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    Re: micrometer and its origins
    From: George Huxtable
    Date: 2005 Jun 20, 08:00 +0100

    Robert Gainer quotes an article at
    http://home.earthlink.net/~nbrass1/cardart.htm which states
    "There is an example of the drum micrometer incorporated into a sextant made
    >by Ramsden that dates circa 1780. Before that it was incorporated on some
    >astronomical instruments dating back to the 17th century!"
    That's interesting, and I would like to discover more.
    But we need to be careful in discussing "micrometers" used in astronomy. It
    was a term used to describe several methods of measuring small angular
    distances between two close objects in the sky.
    For example, an engraved calibrated glass scale, placed at the internal
    focus of a telescope, to measure directly the separation of objects close
    enough to appear together in the field of view, was called a micrometer.
    So was the device in which the object lens was sawn in half across its
    diameter, with a screw adjustment to slide one half past the other, to
    bring two objects into alighnment.
    So was a device in which a telescope was clamped into position with the
    crosswires aligned on a star, and then a calibrated screw allowed it to be
    shifted by a precisely known small amount to align with another star.
    All these were intended for measuring small angular differences to high
    precision. When stars were being mapped, a few reference stars were
    positioned to high accuracy, and then offsets to neighbouring stars were
    observed. A similar technique was used later when photographic plates came in.
    Lewis and Clark even used the word "micrometer", erroneously, to describe
    the lens system that magnified the Vernier scale of their sextant, to allow
    it to be read more precisely.
    To be any sort of equivalent to the modern drum micrometer sextant, an
    instrument would need to be calibrated in whole-turns and fractional-turns
    of some sort of drum or wheel, in terms of the angle observed from the zero
    end of the scale, all the way to the other end at 90 or 120 degrees. It
    would call for a precision that was at least comparable with the Vernier
    instruments of the day, over that whole range. There might be no provision
    for quick-motion, and an observer might need to resort to counting whole
    turns; up to 120 of them, perhaps.
    It would be interesting  to learn how closely the Ramsden sextant of 1780,
    or Bird's adaptation of 1745, approached the definition of an effective
    drum micrometer instrument that I've suggested above.
    Perhaps they failed to do so, and that's why it took until the 20th century
    for micrometer instruments to displace the Vernier arrangement.
    Robert Gainer's contribution has indeed been interesting and useful. It
    leaves me wanting to know more still about the instruments he describes.
    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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