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Edgar Brooks
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« Reply #2 on: June 14, 2010, 09:55:29 AM » |
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The P8 was superceded by the P10-P12 compasses, which can still be obtained from Sirs, who are based in Kent. The tubes, on the spider, were not filled with radium paint, but radium powder, so should be handled with extreme caution, since inhaling it, from a broken tube, would be very injurious to health. Post-war, the powder was replaced by "fluo" powder, which reacted to fluorescent light, rather than simply glowing in the dark. The interior paint (black for a/c compasses, white for landing compasses) is a special alcohol-proof type, which can be removed by cellulose thinners. The tubes were attached to the spider with small blobs of the paint; there should also be a small tube attached to a pointer somewhere on the edge of the bowl (which was aligned with the cetre-line of the a/c.)
From your description, I can't tell if you have the whole thing, since the P compasses consist of a bowl, which sits on cone-shaped brass springs inside a metal casing, and has a rotating circular bezel set on top of the whole thing. The bezel is marked with the full 360 degrees of the compass, with N, E, W, S, engraved into squares, at their relevant positions. A pair of wires (four on later versions) are laid E-W under the glass of the bezel. The wires, over some of their length, and the bezel's cardinal points were painted with radium paint, which, by now, is probably turning to dust, and could also be very dangerous to your lungs.
The liquid is Industrial Methylated Spirit (IMS)99, aka known simply as "alcohol," which is now used neat, but usually had about 10% water added, during wartime. Having been splashed with the stuff, I can reassure you that it tastes foul, and is poisonous, so no attempt, to drink it, should be made. The glass has a chamfered edge, and should have a surrounding O ring, made of 1/8" (3mm, these days) rubber, held in place, and squeezed by the outer brass ring (secured by 6BA countersunk brass screws.) The bung, in the filler neck, should have a fibre washer fitted to it; normally, the whole item is immersed in a quantity of alcohol, and de-aerated at the same time as the alcohol inside the bowl. The spider should have a pivot fitted in its centre, which sits on a sapphire "jewel" in the vertical post set in the middle of the bowl. Balance of the spider, in the fluid, is achived by judicious bending of the "spare" four arms of the spider, and application of quantities of paint where necessary. The handling difficulties are made worse by the paint's propensity to flake off, if, once soaked with alcohol, it's allowed to dry out, so speed is essential. The compass was designed for use anywhere in the world, so could cope with any angle of dip induced by the Poles (around a maximum of 13 degrees, if memory serves.) This means that the further North, or South, you go, the lower one edge of the spider would go; this means that, here in the U.K., the compass has to have a 7 degree (or thereabouts) Northern dip built-in.
A.M. = Air Ministry, and the crown signifies military (Crown Property.) 6A/726 is the stores reference; some postwar landing compasses were 6B/34, 6C/2024 & 6C/2027, for example. 18845 is the serial no., but I don't know the significance of the "B." Carrying boxes were normally wood, using a mixture of glue, and brass screws; in the base would be a false "floor," with three cut-outs to accept the three lugs which were used for fitting the compass to the a/c. The hinged lid would normally have a couple of pieces of wood, with felt attached, which would just lightly bear down on the rotating ring.
Hope that helps a little.
Edgar
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