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Re: micrometer and its origins
From: George Huxtable
Date: 2005 Jun 20, 08:00 +0100
From: George Huxtable
Date: 2005 Jun 20, 08:00 +0100
Robert Gainer quotes an article at http://home.earthlink.net/~nbrass1/cardart.htm which states "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!" That's interesting, and I would like to discover more. But we need to be careful in discussing "micrometers" used in astronomy. It was a term used to describe several methods of measuring small angular distances between two close objects in the sky. For example, an engraved calibrated glass scale, placed at the internal focus of a telescope, to measure directly the separation of objects close enough to appear together in the field of view, was called a micrometer. So was the device in which the object lens was sawn in half across its diameter, with a screw adjustment to slide one half past the other, to bring two objects into alighnment. So was a device in which a telescope was clamped into position with the crosswires aligned on a star, and then a calibrated screw allowed it to be shifted by a precisely known small amount to align with another star. All these were intended for measuring small angular differences to high precision. When stars were being mapped, a few reference stars were positioned to high accuracy, and then offsets to neighbouring stars were observed. A similar technique was used later when photographic plates came in. Lewis and Clark even used the word "micrometer", erroneously, to describe the lens system that magnified the Vernier scale of their sextant, to allow it to be read more precisely. To be any sort of equivalent to the modern drum micrometer sextant, an instrument would need to be calibrated in whole-turns and fractional-turns of some sort of drum or wheel, in terms of the angle observed from the zero end of the scale, all the way to the other end at 90 or 120 degrees. It would call for a precision that was at least comparable with the Vernier instruments of the day, over that whole range. There might be no provision for quick-motion, and an observer might need to resort to counting whole turns; up to 120 of them, perhaps. It would be interesting to learn how closely the Ramsden sextant of 1780, or Bird's adaptation of 1745, approached the definition of an effective drum micrometer instrument that I've suggested above. Perhaps they failed to do so, and that's why it took until the 20th century for micrometer instruments to displace the Vernier arrangement. Robert Gainer's contribution has indeed been interesting and useful. It leaves me wanting to know more still about the instruments he describes. George. =============================================================== Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.