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My recollection is that micrometer drum sextant was developed by Heath,
shortly after World War I.  I don't have documentation for this
recollection, however.

Fred

On Jun 9, 2005, at 11:30 AM, George Huxtable wrote:

> Alex asked-
>
>> George,
>> Do you know by chance when exactly and who
>> made micrometer drum common?
>> (I suspect that this could be Heath of London
>> sometimes in 1930-s. Maybe you know more precisely?)
>
> No, sorry, I don't. It's not my field, really. Though I have quite a
> few
> books on the history of navigation, and one (by Cotter) specifically on
> "The History of the Mariner's Sextant", none gives the answer to Alex's
> question.
>
> I think it probably happened in stages.
>
> Right from the start, there was a fine-adjustment screw, which could be
> clamped to the index arm at any setting, and used to move it
> deliberately
> and controllably; but not with a motion such that one turn always
> corresponded to the same change in angle. And such a device would only
> work
> over a limited arc of a few degrees (perhaps 10). As the Sun's altitude
> changed, after a time you would come to the end of the limited
> adjustment,
> and have to unclamp it and reset.
>
> Then, some date in the early 1900s, came the "endless-tangent" screw,
> in
> which a worm, which could be readily disengaged for quick-motion,
> engaged
> against a machined track which was cut along the full length of the
> arc. I
> think that development probably came first from Heath. But it was NOT a
> micrometer, in that no markings were put around the rim of the
> adjustment
> knod, and there was no attempt to cut the teeth of the rack so
> precisely
> that the turns of the knob were exactly a degree apart, so they
> couldn't be
> used to measure the angle. Instead, angles were read using a Vernier
> against the arc, just as before.
>
> The final development, of the drum sextant, came when every tooth of
> the
> rack was cut so precisely that the turns and fractional turns of the
> drum
> corresponded exactly to the angular movement of the index arm, along
> the
> whole arc. It was the ultimate expression of the machinist's art at the
> time. But which firm was first responsible, and when, my books don't
> say.
> My guess it would be either Hughes or Plath, but it would be
> interesting to
> discover if any listmember knows the real answer.
>
> ======================
>  Alex continued-
>
>> In fact, micrometer drum was proposed in XVIII century
>> possibly by Halley or even earlier. I can check when I
>> come back to the US in August and will have access to my
>> papers. I am curious why this invention had to wait until
>> 1930-s to become common.
>
> I have my doubts about that. I'm aware that Halley put forward a way of
> moving the angle of a mirror, using a sort of pantograph arrangement
> with a
> linear nut-and-screw across its diagonal. And then, presumably, using
> some
> sort of trig table to convert that to an angle.
>
> What the micrometer had to wait for was mass-production machining with
> sufficient precision. There was no difficulty in proposing the
> principle;
> it was realising it in practice that was difficult.
>
>> The reason is unlikely to be in some advanced wormscrew
>> technology because a wormscrew was used in the arc
>> division machines since XVIII century. So why did not they use
>> it in sextants if the idea was known and technology available?
>
> You just needed the one "dividing engine", of large diameter, to
> engrave
> lots of instruments. To do what Alex suggests would require that
> technology
> to be miniaturised and mass-produced. That was the difficult bit.
>
> George.
>
> ================================================================
> contact George Huxtable by email at george@huxtable.u-net.com, by
> phone at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ================================================================
>

   

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