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RAF Mk.IX BM bubble sextant, cased

RAF Mk.IX BM bubble sextant, cased

Code: M24228

SOLD

Stores reference 6B/313. This instrument is intended for use on aircraft for the purpose of obtaining the altitude of the sun or other celestial body and was carried on all multi-manned aircraft including wartime bombers.  It is manufactured with a black painted metal case with rubber rimmed eyepieces.  A black painted drum at the front houses a clockwork automatic averaging attachment.  The Mark IX sextant is built in two halves, the plane of junction being approximately down the centre, and this plane serves as the reference plane of the instrument and is the plane of the line of sight. The attachment is secured to the front by screws entering the right half of the sextant case. The left half of the sextant carries the bubble and its collimating system which together form the artificial horizon; it also carries the left handle which contains the dry battery or plug adaptor and the lighting control switch attached to the upper part of the handle. The right half of the sextant carries the sextant proper, consisting of two mirrors together with the gear for measuring their rotation, sunshades, the altitude scales and the scale lighting lamp in the right handle. The scales on the right half of the sextant show the altitude of any single observation. The clockwork driven attachment is connected to the measuring gear in the right half of the sextant and gives the average of the altitude shown on the right half of the sextant at 60 intervals of time evenly spaced over a period of approximately 2 minutes, during which the observer follows the object to be observed in the sextant, maintaining coincidence with the bubble to the best of his ability. A swivel clip is permanently affixed to the top of the instrument to enable it to be fitted to the suspension arm in the astrodome. The transport case, which is also intended for use as a stowage case, contains two spare lamp bulbs, Ref. 5L/2274, space for two dry batteries, Ref. 5A/2067, and a plug adaptor.  This example is 1944 dated with later inspection marks into the early 1950s.  It is somewhat soiled/marked but appears to be complete and in good overall condition.  It comes with the bakelite transit/storage case which is also in good order with leather carry handle.  The case contains four spare bulbs and a loom/plug adaptor.  Two plastic tubes are positioned where the batteries would fit.  We have no idea whether this works but cosmetically at least, in good order.  Stock code M24228.

 

The instrument is intended for use on aircraft for the purpose of obtaining the altitude of the sun or other celestial body.

The Mark IX sextant is built in two halves, the plane of junction being approximately down the centre, and this plane serves as the reference plane of the instrument and is the plane of the line of sight. The attachment is secured to the front by screws entering the right half of the sextant case.

The left half of the sextant carries the bubble and its collimating system which together form the artificial horizon; it also carries the left handle which contains the dry battery or plug adaptor and the lighting control switch attached to the upper part of the handle.

The right half of the sextant carries the sextant proper, consisting of two mirrors together with the gear for measuring their rotation, sunshades, the altitude scales and the scale lighting lamp in the right handle. The scales on the right half of the sextant show the altitude of any single observation. The clockwork driven attachment is connected to the measuring gear in the right half

of the sextant and gives the average of the altitude shown on the right half of the sextant at 60 intervals of time evenly spaced over a period of approximately 2 minutes, during which the observer follows the object to be observed in the sextant, maintaining coincidence with the bubble to the best of his ability.

A swivel clip is permanently affixed to the top of the instrument to enable it to be fitted to the suspension arm in the astrodome.

The transport case, which is also intended for use as a stowage case, contains two spare lamp bulbs, Ref. 5L/2274, space for two dry batteries, Ref. 5A/2067, and a plug adaptor, Re

A normal sextant is unsuitable for use in an aircraft because the horizon is not level with the observer as in a ship, so the bubble sextant was developed for aeronautical use in the 1920s. It was very important in WWII. The automatic averaging atachment was meant to ensure that unbiassed measurements were made while the bubble and observer wobbled randomly, so that the error would reduce as the square root of the number of observations. Sixty measurements were averaged as the observer tried to keep the bubble and a star in coincidence for a two minute period.