Spitfire compass

I bought one of these a few weeks ago, only in a lot worse condition!

Radium 226 (the most stable isotope of radium) is the element used in painting luminous instrument dials (and compass grid rings and parallel wires.)
Having worked at Smiths in the instrument assembly shop; the process was to mix Radium powder with distilled water, and a binding agent. and paint the graduations of the dial as required. Fresh Radium 226 mixed compound had a pale custard colour. The stuff you find these days on instrument faces is usually no longer luminous; this is due to the fluorescence of the zinc sulfide fluorescent medium being worn out by the radiation from the radium.
The old stuff can be a dull, dirty green; but more usually, various shades of biscuit-brown. Occasionally this latter colour has black flecks. This is where the paint is beginning to flake into dust due to oxidisation.
Most luminous instruments were originally fitted with a high lead-content glass.This tended to drastically reduce the radiation and probably explains why so many RAF pilots had no ill effects at all from sitting behind a bank of these instruments for years.
The danger comes when an instrument has a cracked glass, replaced glass; or no glass at all. These should be immediately bagged and sealed as inhalation of dust will do you absolutely no good at all.
(Another reason why the “I’ll just suck and blow on the pitot and static to see if the pointer moves on this instrument” is not the most sensible thing to do.)
The authorities stripped most ATC squadron stocks of their Radium instruments in the Eighties.
At the old CARG workshop at RAF Innsworth, we had frequent Radiological inspections of our stocks of instruments, which were held in a controlled access room for dispositions to other BAPC groups, with only the occasional infraction due to damaged instruments.
However, on one inspection it was found that our Auster AOP9 had red-hot readings emanating from the emergency door handles and surrounding area… so you never can tell just by looking!

That doesn’t mean that its not still hot, though!

Tested a variety of instruments last year with the Geiger counter. The P8 compass was among the most active ones with 5.3 mSv/h. Was hardly luminous anymore too…

The P8M differed from the standard P8 by having four luminous cross-hairs… (M for modified with four, full length luminous cross hairs)… rather than the standard P8 luminous “T” (Two long; two short.)
It looks as though this example has a replacement glass cover ring. (The screw slots look a little chewed.)
Problem with luminosity is that it fades with age… and this one being getting on for 70 years old, means that you can shine whatever you like at it and you’ll see hardly any reaction.
That doesn’t mean that its not still hot, though!

Yes, there’s a rumor that P8M versions aren’t hot. Plenty hot enough for me to give it away to my watchmaker friend and kiss (not literally) it goodbye. I picked up a meter as a precaution. They say “normal” background is around 0.10 mSv/h and I got 4.5 right up to it….and about .30 mSv/h 2 feet away. The thing is also not airtight. Good with no kids around on a bookshelf but not cool enough for me. Good riddance!

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No, the stuff degrades quite quickly.

When I first started collecting wartime instruments in the 1970s, it wasn’t unusual to get quite a good glow off some of them in the dark but even then most had lost the luminescence entirely. I seem to recall that Boost Guages lasted quite well for some reason!

I think I read that useful life during WW2 could be as little as 6 months before they needed retreating.

The switch to fluorescent instrument lighting had nothing to do with health concerns, just efficiency.

Cardinal points, every tenth degree, the 45º marks, the grid wires and arms of the spider are all still “hot”.

Thanks AM. I started this thread. I put the compass up to a good strong light for a short time, turned the lights out and…… nothing. Ok, i wasn’t expecting Blackpool illuminations but thought i’d see something glowing. Does this mean it could have been painted over with ordinary paint sometime in it’s history?

Tony.

Sorry. Got my info from an old (now 87) instrument basher. I asked him about the Radium /flourescent/metal/plastic spider etc and was told, with great disdain, ‘Oh. You weren’t aware of the plastic. I thought you would have at least known that!) Silly boy me.

Thanks aircraftclocks, I think that rules out P8M = P8 (Marine).

I can’t agree with Stan’s suggestion that a P8M necessarily = Fluorescent paint though as all of the P8Ms I’ve looked at in the last day or so (Google search, Ebay and the one in the garage) still have luminous paint, including the one that started this thread.

Cardinal points, every tenth degree, the 45º marks, the grid wires and arms of the spider are all still “hot”.

I’m pointing this out lest anyone out there in the real world happens to stumble upon this thread and is lulled into thinking a P8M compass isn’t radioactive.

According to A.P.1275B, Vol. I, Sect. III, Chap. 2 (A.L. No.33 January, 1945), it has this to say about the P.8 and P.8M compasses:

17. The P.8 compass is similar to the P.4 compass except that it is modified in certain details of its construction, moulded plastic parts being substituted for metal wherever possible. The bowl is made of metal on account of the incorporation of a bellows, but the floating ring and the grid ring are of moulded plastic. P.8M differs inasmuch as it has increased magnetic moment and E-W parallel steering. The three compasses P.8, P.8M and P.11 have the same purpose and function, and the provision for fitting them to aircraft is the same.

Ive got two P4’s Stan, one like new in a box! And yes you are right its actualy a P11, well spotted!

Sorry to say you have a bit of a “bitzer” there The spider and the bezel are P11 and the verge ring ( marked P8M) is ,surprise surprise, from a P8. P8s were originally Radium and were later modified (the M part) with flourescent paint. If you have a look at the compass in the dark and if it glows then it has an original P11 insides. If it requires UV light then it will have been modified and probably fitted with a P8 verge ring. The P8 has only the parallel grid wires, the P11 has the Tee grid wires. Apologies if this all seems to muddy the waters a bit.
The P—- line of compasses were fitted to a multitude of aircraft types and came in various sizes and by various makers. The Huson Mk 111 was a smaller one of 97 mm diameter, through 104mm for P8/P11 and the large P4A which is 167mm dia. (BTW if anyone needs a P4A I have three overhauled examples to sell.)

Yes, P8M = P8 modified to P11 standard perhaps.

I have no reference for this unfortunately, but I thought that the “M” referred to modified, as Alan mentions and was added when the lubber lines went from straight across the glass to the “T” type seen in BH Tony’s picture. This I believe was done to mitigate against pilots flying a reciprocal course.

Wish I knew where I heard/read this.

Robert, is that an original compass tray or new build? It’s very nice either way.

Seasons Greetings

Michael

As has been said P8’s were fitted to many different A/C and other applications so not most likely “Spitfire”. Nice piece in its own right and not everything HAS to come from a Spitfire!!!

Probably War Department as opposed to Air Ministry.

Have noticed that some compasses have the A.M. plate with crown, and others have the sort of Arrow marking on the plate, was the A.M one the wartime one and the arrow plate the post war type I wonder?:confused:

http://spitfireforums.com/index.php?topic=393.0

http://forum.keypublishing.com/showthread.php?t=83360

P8 was also used in British tanks up to about late 1943. They were binnacle mounted between the drivers legs.

Hi Tony,

In the absence of anything on the body of the compass, its Air Ministry Ref. would probably have been on the missing/never fitted plate.

If there’s nothing on the wooden box, I’m tempted to suggest this might not be an A.M. compass at all. Perhaps produced for cilvilan use?

This isn’t gospel, just a hunch and I hope it doesn’t spoil your enjoyment of it too much.

On a broader note, there are far too many of these instruments advertised for sale as “Spitfire compass” – they were fitted to many different types, of course.

And maybe not even intended for fitment in an aeroplane at all – if we could get confirmation of what P8M actually indicates, we might be able to make a bit of worthwhile progress on this topic.

Perhaps someone could post a picture showing this and where.

There are loads of P8 Compass photos on the net if you use Google search.