So, how exactly was Torricelli's barameter experiment conducted?

[Speaker_for_the_Dead]

In absolute secrecy.

[Ringo]

From The first barometer:

A glass tube, sealed at one end, is filled with mercury. Then a glass dish is partially filled with mercury.
Cover the open end of the tube with a finger, invert it, and plunge it into the mercury in the dish.
Mercury will then run out of the tube into the dish until the weight of the column of mercury in the tube exactly balances the outside air pressure.
Under standard conditions at sea level, the height will be 29.92 inch or 76 centimetres.

[Ringo]

I like this one a little bit more:

Any instrument that measures air pressure is called a barometer. The first measurement of atmospheric pressure began with a simple experiment performed by Evangelista Torricelli in 1643. In his experiment, Torricelli immersed a tube, sealed at one end, into a container of mercury (see Figure 7d-2 below). Atmospheric pressure then forced the mercury up into the tube to a level that was considerably higher than the mercury in the container. Torricelli determined from this experiment that the pressure of the atmosphere is approximately 30 inches or 76 centimeters (one centimeter of mercury is equal to 13.3 millibars). He also noticed that height of the mercury varied with changes in outside weather conditions.

Are we doin’ homework?

[alice_in_wonderland]

From here:

In 1643 Torricelli proposed an experiment, later performed by his colleague Vincenzo Viviani, that demonstrated that atmospheric pressure determines the height to which a fluid will rise in a tube inverted over the same liquid. This concept led to the development of the barometer.

So, can anyone explain the set up of this experiment, and a bit of the science? I presume that he was using a set volume of liquid (Mercury, is specifically what I’m interested in), in a set size of tube, but how exactly would you execute this?

Thanks!

[Ringo]

Deja vu

[Ringo]

Deja vu

[alice_in_wonderland]

*Originally posted by Ringo *
**Are we doin’ homework? **

Good lord - I’m a 30 year old Psych major - no homework here.

Just a passing interest is all.

Thanks for the info, BTW. Given that my original post was missing - interestingly, there is a whole other thread of the same name - perhaps my OP is there…

[Padeye]

The barometer is pretty straightforward. First don’t get hung up on specific size of tube. The diameter will be important but don’t worry about that now.

First think of the atmosphere as an ocean of air pressing on everything in it. The pressure comes from the weight of all the air above it. The higher you go, the less air above to press down. We don’t percieve this pressure because there is pressur inside us pressing out just as much.

Picture a long test tube. To make the math simple this tube has a square cross section instead of round. The inside dimension is one inch so the cross section is one square inch.

We’ll submerge the tube in an open tank of liquid, water, mercury, whatever. We turn the tube open end down and begin to lift it out of the liquid. The tube stays full even though it’s above the level of the tank. The liquid can’t pull away from the top of the tube. You may think the top of the tube is pulling but in reality it’s the atmosphere pushing against the the liquid in the tank that keeps it there.

If you keep raising the test tube eventually a vacuum bubble will form at the top when the atmosphere can’t hold up the weight of the column of liquid in the tube. There is an interesting coincidence, the weight of the liquid in the column above the tank level is equal to the pressure of the atmosphere pressing on the cross section area of the tube.

Pretty cool huh. If the atmosphere pressure increases it will push the liquid up in the tube. The column is now heavier, corresponding to the pressure pushing it up. If you decrease atmosphere the column will fall.

[Nametag]

Close, but no cigar, Padeye. The weight of the mercury in the tube, divided by the cross-section area of the tube, is equal to the atmospheric pressure on the reservoir. The weight of mercury is also equal to the density of mercury times the volume of the mercury column (which is length times area). Do the math, and you’ll see that the height of the mercury column depends solely on the pressure, and is completely independent of any of the tube’s dimensions. That’s why pressure is measured in millimeters (or inches) of mercury.

[bibliophage]

I have merged the two copies of the thread because both had good responses.

[bibliophage]

The merging had the unintended consequence of changing the thread starter field from alice_in_wonderland (who actually started the thread) to Speaker for the Dead (who merely has the earliest actual post). I apologize for the confusion.