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    Re: Vernier explanation
    From: Peter Monta
    Date: 2022 Jun 22, 21:15 +0000


    Unfortunately
    this instrument can no longer give very precise readings, since the threaded
    shaft of the vernier adjuster is slightly bent so that the vernier scale can
    only move through about a fifth of its original travel before the shaft jams
    solid.

    That is unfortunate.  It seems likely that an instrument repairer or machinist could set that to rights, though, with careful bending in a press or vise.  The threads on the tangent screw are not critical to accuracy---that comes from the arc and vernier---but of course precautions should be taken not to damage them.

    ...
    (Please correct this as required if I am mistaken.)

    All correct.


     On the Blair instrument, by
    contrast, the vernier is engraved 0 to 10 (are these minutes, or are they tenths
    of a degree?)

    Minutes.


    , each unit being subdivided into six, and the 0–10 range spans
    exactly twenty degrees on the main scale; thus each half-unit on the vernier is
    precisely equal to one degree on the main scale and there is no staggering or
    offsetting between the two scales

    There is staggering, but smaller than the more common verniers subdividing by 10, so it's perhaps not easily seen at first glance.  So this is an ordinary vernier, just as you describe above.  The vernier is expanded by a factor of 2 in width, perhaps to make reading easier or perhaps for mechanical reasons to give a larger bearing area to the end of the index arm.


    The engraving on the scales (still done by hand,
    I believe) is extraordinarily delicate, each degree on the main scale being ruled
    off in six ten-minute divisions which my tired old eyes can barely resolve
    unaided.”

    The engraving would have been done by machine, a so-called "dividing engine", since Ramsden's day.  It's interesting, though, that the "furniture" on the scale, such as the slightly longer ticks for degrees and half-degrees and the "boxing" (horizontal bridging-lines; I don't know the proper term) are kept.  That must complicate the dividing engines somewhat.  It reminds me of the considerable mechanical circuitry on Babbage's difference engine to typeset the tables just right on the output stereotype plates.

    Cheers,
    Peter

       
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