If you look at a long level there are usually two "bubbles", side by
side ... one concave up and the other concave down. I have always read
the one that is concave up in any application. No one at Home Depot or
Lowes home improvement stores seems to know the difference. Would you
please explain the difference, and which to use?
Thanks for your input,
The two bubbles are for convenience. Use the one with the concave down
for the position you are holding the level. You would use one if
leveling something on the floor, the other if holding the level against
Nonsense. You always use the one with the convex side up (or concave down, if
you prefer), regardless of where you are taking your measurements. There are
two vials so that you can use either side of the level. Makes no difference
whether you're checking a floor or a ceiling, you *always* use the upper vial.
The vials are slightly convex for two reasons: (1) so that the bubble will
move slowly and gradually to the center as the level approaches level, and (2)
to *keep* it centered. But this works only if you're using it properly: convex
Doug Miller (alphageek at milmac dot com)
The difference is that if you read the one concave up, like letter "U" then
the bubble goes to one end or the other. Lifting or lowering an end of the
level, and the bubble stays in the same place. [this is less useful]
If you read the one that is concave down (convex up) like the letter "n"
then the bubble is in the center if the level is level. Or off to one side,
if it's off level. [this is very useful]
Since you're reading the useless one, I presume people wonder why you're
building crooked stuff all the time?
Now you're being silly. The Equator is essentially self-leveling since
everything runs uphill from there. It might look different on a globe,
but it's really at the bottom of a trough. The poles as expected are
just the opposite - you just have to worry about plumb. If you don't
believe me, just take a look at an Eskimo's igloo. That's what happens
when you try to find level at one of the poles.
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