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    Re: Follow-up to Jared's post, Re: levels
    From: Jared Sherman
    Date: 2003 Jul 15, 13:07 -0400

    Marvin-
     
     Wasn't it Galileo who said "Never the less, it moves." ? As best I can tell, 
    the glass vials that are commonly used as spirit levels (not siphon levels) 
    are always round tubes, straight along their lengths. I have no idea how the 
    mechanics of it works, beyond the suspicions I mentioned. If the tube was 
    curved, I would think the mechanics of building/aligning/assembling it tould 
    make the inexpensive ones impossible to market, although obviously there are 
    some very curved tubes, such as the clinometers that use a rolling ball 
    bearing instead of a bubble.
    
    Consider if you will, the liquid filled glass "pipe". If it is perfectly flat 
    and level, and you introduce "just a little" air, the air will go to the top 
    and spread in a thin line along the very top of the cylinder--which will also 
    force it to expand along the length of the cylinder, where the "top" is 
    running at the highest level, extending along the length of the tub before it 
    can extend "down" the walls. Carefully admit more air, and the bubble becomes 
    longer and longer, eventually reaching both ends of the tube. At this point, 
    the bubble must grow fatter since it can no longer extend along the length of 
    the tube and it must spread wider. Release some air (add back some fluid) and 
    at a certain point, you can make the bubble, say, 9.5" long in a 10" long 
    tube. Now, as you tilt the tube, the bubble will "crawl" towards the high 
    end. All you need to do is mark along both ends, in the final 1/4" where the 
    bubble should never be. By observing the intrusion into that final 1/4!
    " you will
    be seeing the bubble shifting as the tube goes out of level, in the direction 
    of its long axis. And if the interior walls of the tube are perfectly 
    uniform, the bubble--held and shaped by surface tension and an equillibrium 
    of "density" compared to the liquid and pulled by the surface tension--should 
    hold itself centered in the tube, I think.
     You can of course shake a spirit level, breaking the bubble, and creating two 
    bubbles and a false reading now. but once they rejoin, I think surface 
    tension and similar forces will force that bubble to center itself--in 
    theory, becoming infinitely thin and long until it hits both ends of the 
    tube. in practice, never becomgin infinitely thin because there are forces 
    working to make it uniform and round, effectively "pulling in" the ends, 
    equally.
    
    I'm not at all sure--that's why I gave up on the spirit level and went on to a siphon level.
    
    << I think that Starrett's Precision Machinist Level is 12" long, at least. It 
    is stated to have 10 second accuracy.>>
     I've only gotten one reply, from a vendor that sells bubble and tube levels 
    for telescope stands. At $10 he's obvisouly not in the high end, but he says 
    "45 minutes per 1/10th inch" and I've asked him to clarify what that means, I 
    think it means flat to within 45 minutes per the 1/10th inch that the bubble 
    moves.
    
    
    

       
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