A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2010 Jan 9, 14:08 -0800
A little while ago, I answered some of George's questions in an illustrated post on my blog at www.sextantbook.com as "Evolution of sextant micrometers", so I won't repeat all of it here.
I too am puzzled by the twenty year gap between C Plath and the other two big sextant manufacturers, Heath and Co and Henry Hughes and Son. It cannot have been because of patents, since the rack and worm screw was, in the words of patent attorneys, "known art", described as far back as about 50 AD by Hero Of Alexandria. Certainly, no German patent has been traced, and Heath's patent was for the method of engaging and disengaging the worm, not for the principle of the worm and rack applied to the sextant.
The rack is cut using a hobbing machine, which generates the teeth of the rack, rather than the insufficiently accurate method of cutting the spaces between them one by one. According to LTC Rolt's "Tools for the Job", by 1909 there were 24 manufacturers of hobbing machines, though hob design was still advancing. However, hobbing machines were (and are) not cheap, and firms like Hughes and Co may well not have found it economical to buy one to create only a few hundred instruments a year. Nevertheless, by 1925 they were producing an intermediate form which had a rack and worm of 18 threads per inch, but no micrometer drum, as the radius of the rack was a little too large. Similarly, Heath and Co had the principle applied in their "Hezzanith endless tangent screw automatic clamp", but initially the pitch was much too small for the useful addition of a micrometer drum. By about 1930, Hughes had decreased the radius of the rack and Heath had increased the pitch of their teeth, so that one turn now moved the index arm half a degree and a micrometer drum could be added. (As an aside, a Hughes three-circle micrometer sextant in early 1945 cost 18 pounds and a Lumex 4 x 42 Galilean telescope could be had as an extra for four pounds, thirteen shilling and fourpence.)
As for accuracy, Plath's selling point was not on accuracy but on the convenience of setting and reading the instrument. Vernier instruments do not wear out, unless some vandal polishes the scale too vigorously, so they should hold whatever accuracy they had when manufactured. Many vernier sextants may have had verniers divided to ten seconds, but I very often, even with ideal lighting and magnification, find it difficult to decide which of two or even three divisions coincide.
As a survey of calibration certificates will show, the micrometer instruments were often much less accurate than good vernier ones, but were catching up by the mid 1950s. Wear shouldn't be a problem, as all the parts are lightly loaded and move slowly. I tested this recently on a very well-used Freiberger drum sextant from the 1960s and found no error greater than 9 seconds. The dividing of the rack may be very precise and accurate, but the cutting of the worm sometimes leaves much to be desired. Again, there are some details on my web site.
In the US in the 20th century, Brandis and Sons Inc. was probably the only manufacturer of importance up to about 1940. The Pioneer Instrument Company gained a controlling interest in 1922 and was itself swallowed by the Bendix Aviation Corporation in 1928. No sextants named Brandis were made after 1932, but just prior to this, they had produced a micrometer instrument, the Aeronautical Octant Mark I, in the form of a left handed octant. As Henry has described, huge numbers of the US BUShips Sextant Mark II were produced in WW II (a few had been made by Pioneer prior to this), by Pioneer Bendix, David White and Ajax Engineering. In their design, they clearly owed much to Brandis vernier precursors.
I wonder whether the delay was perhaps due to a kind of jingoism on the part of sailors. Britain's Royal and Merchant Navies dominated the seas, at least in numbers, with the German Merchant Navy being its only significant rival. British sailors would tend to buy British, so Heath and Hughes could perhaps afford to be complacent, while Carl Plath and its Bremen sales division, Cassens and Plath, had mainland Europe to play with.
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