Re: What is an aneroid barometer?

An aneroid barometer is a barometer that uses a closed container rather than 
a mercury column to measure air pressure. It works because air pressure 
always tries to balance itself. When you blow up a balloon, you are creating 
high pressure inside it, so it expands until the air pressure inside and 
outside the balloon is the same. You can do the opposite with a pop can, 
though it's a bit more difficult. If you put your mouth over the pop can 
opening and suck, you can actually make the can crumple as the force of the 
higher pressure air outside the can tries to make the air pressure inside 
the can the same. (I don't recommend this experiment unless you take 
precautions against cutting your lip on the very sharp edge of the opening!)

An aneroid barometer is a sealed metal container. When the air pressure 
falls, it expands a little, just like the balloon, and when the air pressure 
rises, it collapses a little, just like the pop can. By attaching a pointer 
to the container, you can measure the rise and fall of air pressure.

For a simple explanation and diagram of an aneroid barometer, visit

 http://129.252.37.27/erth_sci/Atmos/Aneroid.htm

There is a slightly more detailed explanation at this site:

 http://www.mid-c.com/manmar/The_Aner.htm

You can also try making your own aneroid barometer. Take an empty tuna can 
and stretch some balloon rubber tightly over the open end. Seal it really 
well around the edge with duct tape or something similar, so no air will 
leak in or out. Then take a long straw, tape one end to the middle of the 
rubber cover, and let the other end dangle out over the edge of the can. 
Then put your barometer in a place where you can easily tell when the end of 
the straw goes up or down (for example, tape a piece of graph paper to the 
wall and put the end of the straw really close but not quite touching the 
paper). If you mark the position of the top of the straw from day to day, 
you should notice it goes up and down with changes in the weather. When it's 
cloudy and cool, the straw should drop ever so slightly. On hot, sunny days, 
the straw should be higher up on the graph paper. 

Happy barometry!