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    Micrometer and its origins
    From: Robert Gainer
    Date: 2005 Jun 18, 23:34 +0000

    I was looking around the web today and found this material about the
    A short article on the web by Rod Cardoza and Edited by A. N. Stimson, Head
    of Navigation Section, Department of Astronomy and Navigation, National
    Maritime Museum, Greenwich, England, says,
    �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!�
    You can find this at http://home.earthlink.net/~nbrass1/cardart.htm
    Another interesting article about Ramsden is at
    This describes the invention of the circular dividing engine in 1773 by
    Jesse Ramsden.
    Then at http://www.brera.unimi.it/old/HEAVENS/MUSEO/Schede/app1.html,
    is the page titled, Instruments in the history of the Osservatorio
    Astronomico di Brera. This describes a micrometer and its origins in this
    The micrometer was developed by Bird and applied for the first time in 1745,
    on the request of Bradley, to a mural quadrant built by Graham, located at
    Greenwich. This consisted of a tangent screw which connected the end of the
    index arm which supported the vernier to the blockage shoe used to fix the
    index arm to the border. The step of the screw had to be extremely regular
    and the screw was fitted with a scaled disc at one end from which, by means
    of a fixed point of reference, the revolutions and fractions of revolution
    completed by the screw were read. In the initial analysis of the instrument,
    the correspondence between one revolution of the screw and the angular
    measurement was obtained. The observer had to move the telescope until he
    focused on the star. At this point the telescope was blocked in position by
    means of the shoe and the screw was rotated until it was exactly at the
    centre of the reticle. The position was then read on the scaled border, if
    necessary using the micrometer again to make a division of the vernier
    coincide with one on the border. The value corresponding to the fractions of
    revolution of the micrometer was added to the position read.�
    A lot more material is on the web describing sextants and there development,
    but I did not find any thing else about the origin of the micrometer.
    Robert Gainer
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