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    Re: Artificial horizons and mercury
    From: George Huxtable
    Date: 2003 Jul 19, 22:47 +0100

    A useful mailing about mercury from Paul Hirose included this-
    
    
    
    >This web page on handling mercury in the lab is pretty good:
    >
    >http://www.ilpi.com/safety/mercury.html
    
    =================
    
    I've looked at that web page and it is indeed useful. It refers to
    volatility and evaporation rate, and provides links about them, to other
    parts of the document. Although those concepts are defined there, there's
    no clue as to what the evaporation rate from mercury actually is. This is
    the vital quantity we need to know in order to assess the hazard from
    mercury vapour in the air near to its exposed surface. No doubt it depends
    rather strongly on temperature, and is likely to be dependent on air flow
    across the surface. Clearly, it's related in some way to the vapour
    pressure (which is well documented), but I rather doubt if one can be
    derived from the other.
    
    If any chemists among us (such as Peter Ifland) know whether such
    evaporation-rate data has ever been obtained, for mercury, and can find a
    reference or web address, then that would be very useful.
    
    ================
    
    Did anyone look hard at Clive Sutherland's mailing in 15 July, in which he said-
    
    
    >5) If you insist in using mercury, why not consider some kind of
    >containment.
    >My artificial horizon which you have seen consists of a hollow glass 90
    >deg prism made of 1/4 inch float glass pieces glued together with fish
    >tank adhesive, to make a totally enclosed bottle with a flat bottom, Two
    >small holes are drilled and plugged through the parallel sides for
    >filling.  Sights are made through the two 45 deg faces. The whole thing
    >is fixed on top of a tripod and only crude levelling is necessary. I
    >filled this device over 30 years ago and the mercury has not diminished
    >in the slightest.
    > The only drawback has been that the glass has tarnished with contact
    >with the mercury for some reason I don?t know why. Maybe the mercury is
    >amalgamating with the lead in the glass.
    
    ================
    
    I haven't actually seen, yet, this device of Clive's, but expect to take a
    look shortly.
    
    From Clive's description, it seems to me a brilliant concept, which
    overcomes almost all of the hazards of mercury. It relies on the flatness
    and optical quality of the two sloping windows of the "cloche" above the
    mercury. The surface of the base doesn't matter at all, as it's submerged
    under the mercury. Mercury is permanently contained within the hollow prism
    and never needs to be exposed or transferred, so there's no flask. The
    mercury surface is always protected against ruffling by wind. No precise
    levelling is required. What more could anyone ask?
    
    The only defect appears to be the reported tarnishing of the glass over the
    long term, where it's in contact with the liquid. This can not be a general
    problem with a glass-mercury surface; if it was, no old barometers would be
    working still. It needs some investigation.
    
    I would think about adding some sort of diaphragm or mini-condom in one of
    the triangular end-plates, to relieve internal pressure-changes brought on
    by high-temperture conditions.
    
    George
    
    ================================================================
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ================================================================
    
    
    

       
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