Good Morning
I have a P10 compass that has not got the usual AM and crown, it only has the broad arrow with 6A/1671
The serial number is 4976T The paint is typical RAF.
Worst case, it is not from a plane at all, perhaps a ship. Better case it’s post war and the AM & crown was dropped.
It is in excellent working condition. The alcohol has been drained, do you recommend refilling it? or is it likely to leak.
I am aware of the dangers to health if it is opened, so I have no intentions in doing so. Both glass sections are in place and in good order.
Finally, the lever on the side appears to do nothing, should it lock the shock mechanism?
By: Edgar Brooks - 25th May 2013 at 10:10
The lever is designed to lock the bowl, once the course has been set, to stop it vibrating, and turning. The alcohol is a damping fluid, to stop oscillations damaging the sapphire “jewel” set in the jewel post (difficult to see, since it’s partially hidden by the central mushroom of the “spider.”) Without the fluid, the pivot (screwed into the “spider”) can bounce up and down, scoring or chipping the sapphire, with consequent misreadings.
Post-war, many compasses were modified, so that the “glowing” interiors were replaced by items filled with a powder which needs fluorescent light to make them react, which makes them completely non-hazardous, but only a check with “flu” lamps and a geiger counter can be certain.
The main danger is in the grid ring, since the crosswires, and certain cardinal points, were normally painted with radioactive material, and were rarely (if ever) modified to the new standards. With the passing of years, the paint turns to dust, and drops onto the bowl’s glass, giving you the opportunity to breathe it in should you ever remove the ring. What the “hot” paint can do to your lungs I’ll leave to your imagination.
There are only two points of likely leakage, should you feel like refilling the compass. The main (bevelled) glass normally has a rubber seal between it and its brass retaining ring, which can be made from 1/8″ diameter rubber (and is a nightmare to replace accurately,) or a sticky two-part adhesive, which is a gooey mess to remove (and even more gooey to replace.) The filling point is on the side of the bowl, and only accessible by removing it, and the grid ring, from the base; the plug should have a fibre washer fitted onto it,which is normally de-aerated (as is the alcohol) before final filling. On the bottom of the bowl is a set of bellows, which are normally retracted slightly, while filling, to allow for any temperature (or height) induced contraction/expansion of the fluid. If you’re not planning on flying your compass, that’s something you can safely ignore.
If you do refill the compass, and the “spider” does not appear to be level, this is quite normal, since it has an inbuilt “angle of dip,” which varies according to your position on the earth’s surface; it is normally only level at the equator.
Post-war compasses were normally painted in Dark Admiralty Grey, or Light Admiralty Grey (not far removed from cockpit green,) and came under the control of the Admiralty Compass Observatory, Slough (now closed down.)
By: Mark12 - 25th May 2013 at 08:45
I cannot answer your question directly other than to say at some time in the distant past I was told that the Admiralty had some responsibility for the RAF compasses…posssibly accounting for the paint colour applied.
This site may interest.
http://www.compassmuseum.com/aero/aero.htm#AM
Mark
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May 25, 2013 at 8:20 am