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    Member prismaticcompass's Avatar
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    Questions about Francis Barker M73 liquid prismatic compass



    Hi everyone,

    I recently acquired an M73 (above) from an auction website. I had used one while in the infantry 20 years ago and have been in love with it ever since. The numerous prismatic compass threads on this forum, along with other websites, have brought me up to speed with many of the technicalities of this great military design.

    Close examination of the particular copy in hand has however raised the following questions:

    1. When viewed directly from the top such that the hairline is aligned with the lubber line, the card pivot is slightly out of alignment with the hairline (image below). The tiny holes on the cover for tying the backup thread are also slightly out of alignment with the hairline. Do these misalignments indicate that the compass has been tampered with before, and how badly does it compromise accuracy?



    2. On this and some other compasses I've seen online, the thumb ring has a notch at its midpoint (image below). Is this a manufacturer marking indicating a particular batch? This particular compass is engraved on the back with "M73 Pat Appl" and nothing else. As is evident from the images it is the degree version.



    3. The tritium capsule on the card has been affixed with a crude lump of adhesive (image below). Was this a substandard replacement job? The tritium on this compass is no longer luminous.



    4. From the images above does the card look like real mother-of-pearl? It has the green-pink sheen and is translucent. I read somewhere that later prismatics used a substitute.

    I would be grateful to hear opinions and other esoteric trivia of the M73 and its relations. Thank you!

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    First, the tilted card. The axis hole in the card is slightly larger than the actual axis and this allows for the card to be moved slightly so that it will balance exactly square on the brass axis before it is clamped up. I wouldn't worry about it too much unless it becomes impossible to read the car through the prism when taking a bearing. There is a facility to focus the card readings but if you can't get it in focus..... Never noticed one where the line doesn't accurately align with the small holes you show. Will the error you show make a vast distance? I'd say no, not unless you are driving a great distance across the desert using compass stops as guidance. The notch on the thumb ring is used for aligning the compass with the grid lines on the map. That glue does look a bit scrappy but it won't make any difference to the operation of the compass. If you go along the Ayer Rajah Road where our big REME Base Workshop was, you'll find zillions of these radium dials buried where we just dumped them. No such thing as health and safety radium regimes in the 60's!!!!!

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    Contributing Member CINDERS's Avatar
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    Radium

    I guess you'd be easy to find in a dark room then Peter!

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    You're right! We used to use it without thought. In fact, in the 50's my dad bought me a small jar of luminous paint that was probably radium based, to paint the headlights of my dinky toys headlights. And I'd be nodding off to sleep with hundreds of these lights twinkling in the dark bedroom. We also used to put a dob of similar paint onto wrist watch dials, open anti-tank gun sights and compass dials. That and the fact that we had open tanks of boiling trichlorthylene vapour big and deep enough to put a complete Centurion tank engine and gearbox into. Alas, one of my uni professors (and the best mentor anyone could ever wish to meet) was one of those who flew Canberra bombers through the Christmas Island atomic bomb cloud to collect samples. He wasn't so lucky..... but I blame Australiaicon!

  7. Thank You to Peter Laidler For This Useful Post:


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    Contributing Member CINDERS's Avatar
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    That's K Peter to blame Australia and I am sorry for your loss guess it was in the pursuit of science and potential future conflicts and believe me I am not trying to start a full on discussion or trying to stir up an argument I am just presenting the facts about this stuff save to say our Govt showed scant regard for the citizens of this country in allowing nuclear tests on the main land and the Montebello Islands which affected its citizens and indigenous tribes in the area.
    There were 2 series of tests on our mainland involving 7 atomic device detonations by Englandicon Operation Buffalo yields in Kt - 15Kt, 1.5Kt, 3Kt & 10Kt and Operation Antler yields in Kt - .930Kt, 6Kt & 26.6Kt imagine the outcry if we detonated 7 nuclear devices in the old countries back yard..........

    Snippet from the article;

    A Department of Veterans' Affairs study concluded that "Overall, the doses received by Australianicon participants were small. ... Only 2% of participants received more than the current Australian annual dose limit for occupationally exposed persons (20 mSv)."[38] However, such findings are contested. Australian servicemen were ordered to: repeatedly fly through the mushroom clouds from atomic explosions, without protection; and to march into ground zero immediately after bomb detonation. Airborne drifts of radioactive material resulted in "radioactive rain" being dropped on Brisbane and Queensland country areas. A 1999 study for the British Nuclear Test Veterans Association found that 30 per cent of involved veterans had died, mostly in their fifties, from cancers.[39]

    Full article can be found here;
    British nuclear tests at Maralinga - Wikipedia
    Last edited by CINDERS; 02-20-2018 at 01:03 AM. Reason: spelling

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    I was only being flippant about blaming Australiaicon Cinders. I blame Prof H's death on old age!!!!! We won't hear from the thread author again. I don't know why they even ask the Q.

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    Peter, thank you very much for your insights. I did bring the compass for a spin in the mountains and it was really at home there.


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