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> It is worth bearing in mind that calibration certificates for micrometer
> sextants give the errors only for whole numbers of degrees, every fifteen
> degrees for NPL certificates, and every twenty degrees for Heath and Co
> certificates. In doing so, they are in effect calibrating only the rack and
> centring error. It isn't really all that safe to interpolate, as the sources
> of error are complex and the errors do not usually vary predicatably over the
> measurement range of the sextant. Ideally, makers should perhaps also
> calibrate the worm and its centring errors through a whole rotation. Noone
> would consider that an engineer's micrometer had been properly calibrated if
> this had not been done, as well as checking at intervals of, say, 2.5
> millimetres throughout the measurement range.

I have that deju vu feeling all over again ;-)  A few years ago Frank
proposed a "hallway" test for drum eccentricity that (with a few
modifications to make it more elegant/efficient for me--nothing works
correctly right out of the box;) works extremely well.

George mentioned, if I understood correctly, it would be swell if errors
along the arc were smooth curves, implying IMHO they are not.  It is my
experience errors along the arc are rather erratic, and a test every 15
degrees is perhaps less reliable than testing the quality of every n product
that comes off an assembly line.  Even spot QA testing is probably better
than current international standards that give a statistical probability
that a unit functions to specs ;-)

Bill B.


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