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    Re: Intended High Resolution Chronometer
    From: Brad Morris
    Date: 2014 Feb 26, 21:05 -0500


    I agree that the simple way to deal with gear lash is to take a uni-directional approach to the sextant adjustment.  Always approach from the same side, so that the lost motion in the gear train has the same bias.  This insures that your readings are consistent, because they are indicated on the *opposite side of the gear train* from the index arm and mirror.  This practice (unidirectional approach) is required for micrometer sextants.

    Now consider the vernier sextant.  The vernier and mirror are both hard mounted to the index arm.  The angle indicated is consistent INDEPENDENT of the direction of sextant approach.  Gear lash is irrelevant, since the angle indicated and the mirror are on the same side of any gear.  Align the celestial body to the horizon from EITHER side, and the angle indicated is identical.  The gear plays no part in the indication of angle for a vernier sextant, unlike that found on a micrometer sextant.  This inherent advantage cannot be disposed of lightly.  The navigator is free to naturally twirl the adjustment until correct, without concern of approach direction due to gear lash!

    I will defer to your greater knowledge of gear hobbing and gear tooth formation.  While I did mention this factor in passing, your explanation is far superior.


    On Feb 26, 2014 6:52 PM, "Bill Morris" <engineer{at}clear.net.nz> wrote:

    Brad and Frank

    The problem with producing a practical micrometer sextant was not so much that of gear lash (back-lash, lost motion)as there are simple ways to deal with it. Rather it was that precision gear-cutting received a stimulus from the motor industry, which required gears that would run at high speed without the noise that accompanied unequal tooth spacing and inexact tooth form. The gear hobbing machine, which deals with both problems, had been developed to a sufficient accuracy by the turn of the century and Plath took advantage of this by about 1909. The less technologically advanced British firms took another 20 years to wake up.

    Frank mentioned platinum arcs. Late in the eighteenth century platinum was actually cheaper than gold, as no one could think of a use for this intractable metal. It was at first a by-product of alluvial gold recovery in Spanish America and the platinum was actually thrown away into the Bogota river by order of the Spanish government, to prevent it from being fraudulently used to adulterate gold.

    It took until 1804 for William Hyde Wollaston to produce ingots of platinum of sufficient size to be commercially useful and the metal was then incorporated into high class instruments, as it did not tarnish like the slightly cheaper silver and was harder than the more expensive gold. In a Troughton and Simms catalogue of about 1840, an 8 inch pillar sextant was 18 guineas with a silver arc, 20 guineas with a silver arc and 22 guineas when the arc was gold (a guinea was one pound one shilling).

    Bill Morris
    New Zealand
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