- #1
Alfred Cann
- 82
- 4
I'm having difficulty reproducing an experiment I remember from high school. It's basically a glass bottle with a rubber stopper through which runs a glass tube, bent to form a horizontal portion. A drop of water sits in the horizontal portion. When the external air pressure changes, the air in the bottle expands or compresses as indicated by movement of the drop. I remember the whole class trooping up the stairs with our physics teacher, from basement to top floor, and the drop clearly moved.
I have a 1.5 liter bottle with a 2-hole stopper. One hole has a straight piece of 2 mm tubing extending a few cm above the stopper. The tubing in the other hole is bent into a 3/4-turn spiral leading to a horizontal section of about10 cm. (The curl is a trap to make it less likely that the drop will fall into the bottle.) After the drop is positioned properly, the vertical tube is plugged with a bit of caulk.
Initially, I had trouble with the drop shrinking and tending to move toward the bottle. I theorized it might be evaporating into the bottle, and keeping a little water on the bottom of the bottle seemed to fix that.
Experiments
1. Warming the bottle with my hands produces a gratifying motion of the drop away from the bottle, proving that any leaks are not too bad.
2. Changing elevation produces no discernible motion, although the calculated motion for 8 ft of elevation is 14 cm!
3. Tilting the assembly so that the 'horizontal' tubing is vertical makes the drop move about 2.5 cm, while the calculated motion is ~ 10 cm! I also observe that the drop moves very slowly, suggesting high friction force against the glass. The menisci are very pronounced, suggesting that capillary forces have a major effect. But I don't see how that can destroy the barometer function.
Calculations
Air pressure gradient at sea level is 0.012 %/m.
8 ft = 2.44 m = 0.029 %.
0.029 % of 1.5 liter = 0.44 ml = 440 cu mm.
Cross section of 2 mm bore = 3.14 mm
Predicted displacement = 440/3.14 = 141 mm ~ 14 cm.
Pressure exerted by a 10 m column of water ~ 1 Atm.
Pressure exerted by a 2 mm long drop = 0.0002 Atm = 0.02 %
That should cause a displacement of 9.7 cm.
I have a 1.5 liter bottle with a 2-hole stopper. One hole has a straight piece of 2 mm tubing extending a few cm above the stopper. The tubing in the other hole is bent into a 3/4-turn spiral leading to a horizontal section of about10 cm. (The curl is a trap to make it less likely that the drop will fall into the bottle.) After the drop is positioned properly, the vertical tube is plugged with a bit of caulk.
Initially, I had trouble with the drop shrinking and tending to move toward the bottle. I theorized it might be evaporating into the bottle, and keeping a little water on the bottom of the bottle seemed to fix that.
Experiments
1. Warming the bottle with my hands produces a gratifying motion of the drop away from the bottle, proving that any leaks are not too bad.
2. Changing elevation produces no discernible motion, although the calculated motion for 8 ft of elevation is 14 cm!
3. Tilting the assembly so that the 'horizontal' tubing is vertical makes the drop move about 2.5 cm, while the calculated motion is ~ 10 cm! I also observe that the drop moves very slowly, suggesting high friction force against the glass. The menisci are very pronounced, suggesting that capillary forces have a major effect. But I don't see how that can destroy the barometer function.
Calculations
Air pressure gradient at sea level is 0.012 %/m.
8 ft = 2.44 m = 0.029 %.
0.029 % of 1.5 liter = 0.44 ml = 440 cu mm.
Cross section of 2 mm bore = 3.14 mm
Predicted displacement = 440/3.14 = 141 mm ~ 14 cm.
Pressure exerted by a 10 m column of water ~ 1 Atm.
Pressure exerted by a 2 mm long drop = 0.0002 Atm = 0.02 %
That should cause a displacement of 9.7 cm.
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